Continuous Wave Sound Analyzer

Abstract

The gas analyzer uses a continuous acoustic signal passing through the conduit to determine the composition of the gas in the conduit. A transmitting transducer transmits an acoustic signal at a fixed frequency, and a second transducer receives the acoustic signal. The phase shift between the two signals corresponds to the speed of sound through the gas and is related to the gas composition. Electronic versions of these signals are processed by lowering, or dividing, the fixed frequency to extend the measurement range of the phase shift and enable the determination of the gas composition over an extended range. This gas analyzer is well suited to determining the composition of gases generated from air as gases of known composition and calibration point gases in ozone generation systems.

Description

[Technical field]

[0001] The present invention relates to a gas composition analyzer, and more particularly to a gas composition analyzer for ozone generation. Ozone is a highly activated form of oxygen that is often used for disinfection and water treatment. Due to its properties, ozone is typically generated on-site at the point of use. [Background technology]

[0002] Ozone can be generated in many ways, one of which is by ionization of oxygen using an electrical discharge to generate a plasma. The concentration of ozone, when generated by an electrical discharge, depends on many factors, including but not limited to the composition of the feed gas, the flow rate of the feed gas, the temperature of the ozone generating cell, the dimensions and material of the cell, and the power used to generate the plasma. The multiple factors that affect ozone production make it very difficult to predict ozone production with any accuracy. If control or information on ozone production is desired, it is necessary or desirable to monitor ozone production. For this purpose, an on-site analyzer is required.

[0003] Several different technologies are available for analyzers to measure the ozone concentration in gases. These include the use of ultraviolet absorption in gases, and have been found to include products from Oxidation Technologies, LLC (Inwood, Iowa, USA) and Teledyne API (San Diego, California, USA). This technology is effective, but is expensive to generate. Furthermore, it does not provide information about the composition of the feed gas going to the ozone generating cell. It would be desirable to know the composition of the feed gas, which may consist of dry air with an increased concentration of oxygen. Discharge ozone generators work more effectively with a high oxygen percentage. Thus, oxygen concentrators are sometimes used to increase the oxygen from 20.9% (atmospheric) to values ​​of over 90%. Discharge ozone generators appear to be significantly more efficient when a small amount of nitrogen is present in the feed gas. However, it can occur that too much nitrogen is removed from the feed gas, which reduces the efficiency of the cell. Such oxygen-enriched air is primarily composed of nitrogen, oxygen, and small amounts of argon. The nitrogen concentration can be estimated from the oxygen concentration by complementation.

[0004] [Problem to be solved by the invention] The use of sound velocity to estimate the ozone concentration in a gas is described in U.S. Pat. No. 5,644,070 (Gibboney). If the temperature of the feed gas, the sound velocity of the feed gas, the temperature of the gas as it leaves the ozone generator, and the sound velocity of the gas as it leaves the ozone generator are measured or known, the speed of the sound pulse in the gas is determined by measuring the delay over a known path length. Four measured or known variables are used to estimate the ozone concentration. However, with a resonant transducer, it is difficult to ascertain when the pulse begins and ends, since the pulse necessarily consists of multiple periods making it difficult to determine the exact arrival of the sound pulse. A further drawback is the complexity of the described system. The sound pulse requires a relatively long measurement path and therefore a conduit with a relatively large volume, which increases the volume of sample gas required. Scavenging pumps are costly and are used to move the feed gas to the ozone generating cell or the output gas from the cell. This complicates the measurement system. The pump must be made from materials that will not degrade over time in the presence of high concentrations of corrosive ozone.

[0005] The sound speed of a continuous sound wave is used to help determine the concentration of two gases, neither of which is ozone. In the present inventor's patents U.S. Pat. No. 6,202,468 and U.S. Pat. No. 6,520,001, systems are described that combine that technique with another. Two separate and unrelated physical parameters, paramagnetism and sound speed, are measured to determine the concentration of both oxygen and carbon dioxide in the respiratory gas. In this case, sound alone cannot be used to determine the concentration of either gas.

[0006] There is therefore a need for a low-cost analyzer that can measure both the oxygen concentration in the feed gas to an ozone generating cell and the ozone concentration at the cell outlet. Such an instrument can be used for evaluation of the generated ozone and for process control. For example, the oxygen concentration can be adjusted based on the output of the instrument to maintain a desired ozone concentration, and the cell power can be controlled based on the output of the instrument to maintain a desired ozone concentration. A single low-cost analyzer that handles both of these functions would be convenient and economical. It is an object of the present invention to perform both functions with a single reliable low-cost instrument. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 5,644,070 [Patent Document 2] U.S. Patent No. 6,202,468 [Patent Document 3] U.S. Patent No. 6,520,001 Summary of the Invention [Means for solving the problem]

[0008] The present invention provides an analyzer for one or more evolved gases from a first gas of known composition and speed of sound, each evolved gas having a varying concentration of a component, the analyzer comprising a first transducer for transmitting continuous acoustic waves in response to a fixed frequency signal source, a conduit acoustically connected to the first transducer, the conduit selectively receiving and holding samples of the first gas and the one or more evolved gases, a second transducer acoustically connected to the conduit opposite the first transducer arrangement, the second transducer receiving acoustic waves from the first transducer through the conduit and generating a second transducer signal in response to the received acoustic waves, and a second transducer for receiving the fixed frequency signal source signal and the second transducer signal. The system includes a processing device that determines a relative phase shift between a frequency source signal and a second transducer signal for a gas sample in the conduit, the relative phase shift corresponding to a difference between the speed of sound in one gas sample and the speed of sound in another gas sample, the processing device including circuitry for lowering the frequency of the received fixed frequency source signal and the second transducer signal to expand the measurement range of the relative phase shift, and a computing device that determines the speed of sound of one or more evolved gases from the first gas from a first gas of known composition and calculates the speed of sound of one or more evolved gases from the first gas based on the composition of a sample of the one or more evolved gases from the first gas.

[0009] The present invention also provides a method of operating an analyzer for one or more evolved gases from a first gas of known composition and speed of sound, each evolved gas having a concentration of a varying component, the method comprising the steps of transmitting a continuous acoustic wave with a first transducer in response to a fixed frequency electrical signal through a conduit holding a sample of the first gas or one or more evolved gases at a time, receiving the transmitted acoustic waves through the conduit with a second transducer and generating an electrical signal in response to the received acoustic waves, where a phase shift between the received and transmitted acoustic signals corresponds to the speed of sound in the gas sample, processing the fixed frequency electrical signal and the electrical signal generated in response to the received acoustic waves at a low frequency, and determining a range for measuring the phase shift. By extension, the method includes the steps of determining a relative phase shift between the frequency source signal and the second transducer signal for a gas sample in the conduit, the relative phase shift corresponding to a difference between the speed of sound in one gas sample and the speed of sound in another gas sample; determining the speed of sound of one or more gases emanating from the first gas from the known speed of sound of the first gas and from the relative phase shift of the first gas sample and the one or more gases emanating from the first gas in the conduit; and calculating the speed of sound of one or more gases emanating from the first gas based on the composition of the gas sample.

[0010] The invention further provides a method for determining the composition of one or more gases derived from a first gas of known composition and speed of sound, comprising the steps of transmitting a continuous sound wave at a fixed frequency through a conduit, where a phase difference between the sound waves entering the conduit and exiting the conduit corresponds to the speed of sound of the gas in the conduit, processing an electrical signal corresponding to the continuous sound wave entering the conduit and exiting the conduit at a reduced frequency to extend the measurement range of the phase shift, varying the gas in the conduit between the first gas and one or more evolved gases, determining the speed of sound of the first gas of known composition and the one or more evolved gases over the extended range from the relative phase shift of the first gas of known composition and the one or more evolved gases, where the relative phase shift corresponds to the difference between the speed of sound in one gas sample and the other gas sample, and calculating the composition of the one or more evolved gases from the first gas.

[0011] Other objects, features and advantages of the present invention will become apparent from consideration of the following detailed description and the accompanying drawings, in which like reference designations refer to similar features throughout the drawings. [Brief description of the drawings]

[0012] [Figure 1] 1 illustrates a general configuration of an analyzer in an ozone generation system according to an embodiment of the present invention. [Diagram 2] 2 shows a fixed frequency signal going to a source transducer and a signal from a receiving transducer of the analyzer device of FIG. 1. [Diagram 3] 3 illustrates how the measurement range of the phase shift increases as the frequency of the signal in FIG. 2 going to the source transducer and from the receiving transducer of the analyzer device decreases, in accordance with one embodiment of the present invention. [Figure 4]Figure 4A shows a simplified circuit for the phase shift detector of Figure 1. Figure 4B shows an example transmitting transducer signal and a receiving transducer signal, an output signal from the phase shift detector of Figure 4A, and a signal after the output of the phase detector has passed through the low pass filter of Figure 1. Figure 4C shows the output of the low pass filter for different phase shifts. Figure 4D shows the output of the low pass filter for different phase shifts for baseline adjustment. [Diagram 5] 1 shows a flow chart of the operation of an analyzer device in the operation of an ozone generation system according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] As described below, the present invention provides a method for measuring ozone concentration with high resolution and accuracy, is relatively independent of the oxygen concentration of the feed gas and is relatively independent of temperature, and is simple and low cost to build.

[0014] FIG. 1 shows a generalized diagram of a portion of an ozone generation system with a continuous sonic analyzer device according to an embodiment of the present invention. It should be noted that the diagram is representative and the elements in the diagram are not drawn to scale. The system includes an ozone generation block 300 and a continuous sonic analyzer device 100 formed by a transducer / valve block 400 and a controller / analysis block 200. The ozone generation block 300 receives compressed air from a source (not shown) and directs the generated gas containing ozone to a process, i.e., a specific application of the ozone. The compressed air (gas flow is indicated by the broad arrows in the diagram) is received by a concentrator 301 which increases the oxygen ratio of the generated gas. The concentrator 301 may be a pressure swing absorption device, and the gas from the concentrator 301 is passed to an ozone generator 302, typically to a discharge cell. The gas output from the ozone generator 302 is sent to an inlet pressure regulator 303 which controls the pressure of the gas and ozone sent to the process, the application using the generated ozone.

[0015] The analyzer device 100 determines the relative sound speed of gas at different locations in the ozone generation block 300 and includes a transducer / valve block 400 and a controller / analysis block 200. The transducer / valve block 400 processes gas samples from the different locations and the controller / analysis block 200 controls the operation of the transducer / valve block 400 and analyzes the output from the transducer / valve block 400. The transducer / valve block 400 has a first input valve 403 that receives compressed air from a source to the concentrator 301, a second input valve 402 that receives output gas from the concentrator 301 to the ozone generator 302, and a third input valve 401 that receives output gas from the ozone generator 302 to the inlet pressure regulator 303. The outputs of the valves 401-403 are connected to a first transducer arrangement 404, the output of which is connected to a gas conduit 405, which is connected to a second transducer arrangement 406. The transducer arrangement 404 transmits continuous sound waves through the conduit 405 to the receiving transducer arrangement 406, and the gas in the conduit 405 (and the transmitting transducer arrangement 404 and the receiving transducer arrangement 406) determines the relative speed of sound. The gas output of the receiving transducer arrangement 406 is connected to a disruptor 407, which removes ozone from the sampled gas before releasing the gas into the atmosphere.

[0016] Generally, a reference phase is read for a first gas of known composition (usually air), then unknown second and third gases are introduced causing corresponding changes in the phase shift, and from these changes in phase shift and the known sound speed of the first gas, the sound speeds of the second and third gases are calculated.

[0017] Ozone is highly corrosive, and care is taken in the selection of components that come into contact with ozone. The transducer assemblies 404 and 406 are made from aluminum, which forms a tough coating of aluminum oxide, and the conduit 405 is made from polytetrafluoroethylene (PTFE) tubing that is resistant to ozone. The conduit 405 is also temperature controlled to maintain the temperature of the gas within the conduit at a desired temperature, and has a relatively large thermal mass to stabilize the temperature of the conduit. Preferably, the length L of the conduit 405 is long enough to give the device good sensitivity and to prevent standing wave artifacts, yet short enough to provide a small volume, convenient to manufacture, and unambiguous measurement of the phase shift. From the standpoint of sensitivity and standing waves, a preferred path length L may be about 24 wavelengths, although other path lengths may be used. However, a path length of about 24 wavelengths may result in ambiguous readings due to excessive changes in phase shift as the speed of sound changes with changes in the gas composition. For example, the phase shift caused by replacing air with oxygen is approximately 0.9 wavelengths. The phase shift caused by replacing air with a mixture of ozone and oxygen can be as much as two wavelengths. A method of mitigating this problem using frequency division is a feature of the present invention and is described below. This method allows the path length to be selected based on a desired tradeoff of the factors mentioned above, without worrying about ambiguous results from the phase detector.

[0018] The controller block 200 of the analyzer device 100 has an oscillator device 201 which drives a first transducer device 404 in a transducer / valve block 400. A first counter 205 receives a control signal from a controller 208 and a drive signal from the oscillator 201. An amplifier 202 in the controller block 200 receives the electrical output of a second transducer device 406. The output of the amplifier 202 is received by a comparator 203 which shapes the amplified signal into a square wave. The output of the comparator is sent to a second counter 204 which also receives a control signal from the controller 208. The outputs of the counters 204 and 205 are both sent to a phase detector 206 which is sent to the controller 208 through a low pass filter 207.

[0019] The present invention utilizes the speed of sound in a gas to determine the ozone concentration in the gas. The molecular weight of ozone is higher than that of oxygen, so sound in ozone is much slower than sound in oxygen. Similarly, the average molecular weight of oxygen is higher than that of air, so sound in oxygen is slower than sound in air. For gases with only two components, if the speed of sound in each component is different from the speed of sound in the other component, then the measured speed of sound through the gas is characterized by the ratio of the two components. This is true even if the components themselves comprise a mixture of two or more gases. A discussion of this can be found in the aforementioned US Pat. Nos. 5,993,333 and 5,993,363.

[0020] The speed of sound in a gas is known by the known properties of the first gas introduced and the corresponding change in phase shift. This change is experienced by successive sound waves originating from a source transducer as the waves travel with the subsequent gas to the receiving transducer. FIG. 2 shows the sound wave train from the transmitting transducer 404 as a series of square waves indicating the digital nature of the signals and the circuitry that processes the signals. Analog signals and circuitry may also be used. The rising edges of the waves are shown as solid vertical bars to serve as reference points to aid the reader's understanding. Similarly, another series of square waves received by a second transducer 406 is shown with the rising edge of each sound wave received by a second transducer 405 as a solid bar. The arrows indicate the phase shift reflecting the time interval of a particular wavefront traveling from the source transducer 404 to the receiving transducer 405. It should be clear that the longer the time interval or the slower the speed of the sound waves through the gas medium, the greater the phase shift.

[0021] Due to the nature of the continuous wave to reliably determine the amount of shift, the phase shift should be kept within a limited range. For example, it is difficult to determine whether the phase shift is x or x+i*360° (where i is an integer). Therefore, in this example, only phase shifts less than 360° should be performed. However, this severely limits the range of velocities that can be determined. Note that the limited phase range also depends on the circuitry used to determine the phase shift, and the circuitry may further limit the phase shift, such as from 0° to 180°. In any case, the present invention expands the range of velocities that can be determined, as described below.

[0022] While the source transducer is driven at a fixed frequency, the frequency of the source signal is reduced for processing. FIG. 3 shows an exemplary reduction in frequency of 66%, or in other words, dividing the frequency by 3 by deleting two of the three vertical bars. The deletion is indicated by replacing the solid bars (showing the rising edges) with closed bars. With the reduction in frequency, the range of phase shifts (and therefore the range of sound velocities) that can be determined increases accordingly. That is, assuming the phase limit is 0-360°, by reducing the frequency by 3, the range of phase shifts can be increased by 3, thereby increasing the range to 0-1080°. By reducing the frequency, the range of phase shift measurements increases proportionally, allowing measurements to be made with high accuracy without the drawbacks of short path lengths of conduits. Typically, short path lengths can cause signal artifacts due to standing waves or residual standing waves in the conduit. Reducing the frequency avoids these problems.

[0023] A simple representation of the electronic circuit in Fig. 4A illustrates the measurement of the aforementioned phase shift. Two square wave data streams, A representing the signal of the transmitting transducer (404 in Fig. 1) and B representing the signal of the receiving transducer (406 in Fig. 1). These data streams are input to an Exclusive-OR logic gate (part of the phase detector 206 circuit). The output C of the gate is input to a low pass filter (207 in Fig. 1), which has an output D. Fig. 4B shows the relationship between the A signal, the B signal and the C signal at the output of the Exclusive-OR gate. In addition to the Exclusive-OR gate, a NEX-OR gate may also be used in the phase detector 206.

[0024] FIG. 4B also shows the D signal at the output of the low pass filter 207. The almost-changing signal of the exclusive-or gate is filtered by the low pass filter 207 to show the "average" value of the output signal. For example, when the A and B signals are completely out of phase with each other (i.e., the phase difference is 180°), the filtered phase difference (i.e., phase shift), signal, output D, is maximum, and when the A and B signals are completely in phase with each other (i.e., the phase difference is 0°), the output D is minimum, i.e., zero. When the A and B signals are "half" out of phase, i.e., half in phase, the phase difference is 90°, and the output D is halfway between the maximum and minimum values, i.e., half the maximum. FIG. 4C shows how this phase detector value, output D, varies with the phase difference between signals A and B. Here the phase difference is shown as values ​​less than -540° to values ​​greater than 540°. The negative and positive values ​​indicate whether the A or B signal leads the B or A signal. As stated above, the A signal lags the B signal. Figure 4C further illustrates graphically why the phase shift should be within limits. In this figure, to avoid ambiguity in determining the phase shift from the value of output D, the phase shift should be limited to within 180°.

[0025] When the frequency of the A and B signals is reduced, the output D of the low pass filter 207 changes. In the example of FIG. 4D, counters (205 and 204 in FIG. 1) are inserted in the data stream of the A and B digital signals to reduce the frequency by a divisor of 8 (N=8 for the counter). The divisor N is arbitrary and is chosen for the designer's convenience. The output D is correspondingly expanded. The period of the output D is not 360° (as shown in FIG. 4C), but 2880°, which is eight times larger. That is, the signal of the output D repeats every 2880°. In this example, the phase shift is expanded to a range of 1440°, which is half of 2880°. This is a much larger range than the 180° limit of the signal without frequency reduction.

[0026] Thus, the expanded range of output D, which is the signal of the filtered phase shift, is easily realized by a digital counter, for example counters 205 and 204 in FIG. 1. Using the above example of N=8, the counter for the A signal data stream is set to zero and then started. When the A counter reaches a certain value, for example 1, then the counter for the B signal data stream is set to zero and started. This ensures that the starting value of the phase detector when N=8 is non-negative. Thus, output D is 0 to 0.125 (assuming the maximum output of D is 1.0). 0.125 is 1 / 8. Now, if the phase difference between the A and B signals increases due to different gas samples in the conduit 405 (see FIG. 1), output D also increases linearly over a range of 1260° (1440°*(1-(1 / 8)) up to a maximum value. Note again that the counter divisor N and the corresponding expansion of the phase shift measurement are arbitrary.

[0027] The two frequency down counters 205 and 204 make the phase uncertain. In the above divide by 8 case, there are eight possible phase relationships depending on the counting relationship of the two counters. If the first counter 205 has a count N, the second counter 204 can have any count of the value (N+n) mod 8, where n is equal to any integer in the range 0 to 7. If all waveforms are properly counted, the relationship will be maintained indefinitely. Controlling the number n provides an opportunity to adjust the baseline with a resolution of 1 / 8 of full scale. In the example immediately above, the receiving transducer signal initially starts in a "second" relationship to the transmitting transducer signal, i.e., (N+n) mod 8=1. By controlling n so that (N+n) mod 8=0, the baseline is adjusted so that the phase measurement range is maximized and the determination of the gas composition is maximized. This is a feature of the present invention.

[0028] For the above phase shift detection, the electronic circuit of the controller block 200 in the analyzer device 100 is implemented by digital circuits according to one embodiment of the present invention. The oscillator block 201 generates a signal at a fixed frequency. In this embodiment, the frequency is 40 KHz. The oscillator block signal drives the transmitting transducer 404 and is divided in frequency by 8, in other words, the counter 205 reduces or decreases the frequency of the signal by a factor of 8. The output of the counter 205 is received by the phase detector block 206.

[0029] The output of the receiving transducer 206 is amplified by the amplifier 202 and then processed by a comparison logic circuit (block 203) to become a square wave. A counter 204 divides the signal frequency by 8. The output of the counter 204 is also received by a phase detector block 206. By operating an exclusive-or gate or an exclusive-nor gate, the output of the phase detector 206 varies between two power levels of the logic gate, for example from 0 to 5 volts. A low pass filter 207 removes the AC component of the output signal.

[0030] Control of the continuous sound wave analyzer device 100 is provided by a control device 208 in the controller / analysis block 200. In this embodiment, the control device 208 is essentially a programmed microprocessor or microcontroller having memory. Among other contents, the memory stores the value from the filtered phase detector 206. Control lines from the device 208 extend to each of the valves 401-403 and the counters 205 and 204. The device 208 also receives the phase shift value from the output of the low pass filter 207. A display 209 is connected to the control device 208 and provides a visual interface for operating the analyzer device 100.

[0031] Under the control of the controller 208, the analyzer apparatus 100 with an extended range of phase shifts determines the speed of sound in multiple gases and the composition of the gases. Although the following description refers to ozone generation and the system of FIG. 1, the analyzer apparatus should not be considered as so limited. Briefly, ambient air is introduced into the conduit and the valve is closed. The filtered phase detector output is read and recorded for ambient air. Then, a feed gas, which is oxygen enriched air, is introduced into the conduit and the valve is closed. The filtered phase detector output is read and recorded for the feed gas. Finally, an ozone-containing gas is introduced into the conduit and the valve is closed. The filtered phase detector output is read and recorded for the ozone-containing gas.

[0032] FIG. 5 shows the process flow of the operation just described. The steps of the process flow are generalized in that the gases are labeled A, B, and indicate that more gases evolving from the first gas may be included in the process flow. After the system is initialized as indicated by the dashed arrow 501, step 502 initializes an index valve # to zero. Step 503 then opens the valve indicated by valve # and pumps the gas selected by the open valve into the conduit. (In the ozone generation system of FIG. 1, index valve #=0 corresponds to 403, index valve #=1 corresponds to 402, and index valve #=2 corresponds to 401). Step 504 closes the valve. Step 505 checks whether index valve # is zero. If the index is zero, the counter is reset and step 507 reads and records the filtered phase detector output value. If the index is not zero, the test of step 505 proceeds to step 507. After step 507, step 508 checks whether index valve # is equal to 2. If not, then step 509 increments index valve # by 1 and the process returns to step 503. The steps are repeated until index valve # is equal to 2, and the process ends with step 510.

[0033] From the recorded phase shift values, the controller 208 analyzes the data and determines the sound speed and composition of the gas. A comparative approach is used. With the sound speed and composition of the first gas already known, dehumidified air is used as a reference to determine the sound speed and composition of the gas evolving from the first gas (air). In particular, the sound speed of the dehumidified air at the set point temperature of the temperature controlled conduit is known and the sound speed of the air is used as a reference to calculate the sound speeds of the second gas (oxygen enriched feed gas) and the third gas (ozone enriched air) from the measured phase shift.

[0034] The gases are processed in the order of decreasing sound speed, i.e. air, oxygen-enriched air, oxygen-enriched air containing converted ozone. The enriched air originates from air, and the air with ozone originates from the enriched air. Each gas is denser than the previous gas, and the sound speed is reduced compared to the speed in the previous gas. The first gas (dry air) is treated as the reference gas since its composition is known. Its sound speed at a given temperature is also known. Although it is possible to vary the temperature in the manner described in the aforementioned US Pat. No. 5,999,333 (Gibboney), it is preferable to maintain the temperature of the gas at a set temperature. A temperature-controlled gas conduit, such as that shown in FIG. 1, has been found to be effective in maintaining the gas at a set temperature. Therefore, when calculating the sound speed in the oxygen-enriched air and the ozone-containing air, all gases are assumed to have the same temperature. To minimize errors, it is desirable to add thermal mass or control the environmental temperature. If the speed of sound is greatest in the reference first gas, the baseline can be adjusted as described above to expand the range of phase shifts for oxygen-enriched air and ozone-containing air to accommodate the composition of these gases.

[0035] Sound propagating through oxygen-enriched air arrives at the second transducer a little later than in air. The additional delay is measured in conjunction with the phase shift and the known length of the conduit L between the first and second transducers. This allows the speed of sound in the oxygen-enriched air to be determined as a function of the speed of sound in air and the additional phase shift. As mentioned above, there is a direct relationship between phase shift and delay. In particular, L=sound path length, S0=known speed of sound in air, and the delay D0 in air is a known quantity D0=(L / S0). The delay D1 in oxygen-enriched air is D0+Dx, where Dx is the additional delay due to the slower speed of sound in oxygen-enriched air, known from the additional phase shift in oxygen-enriched air. Thus, S1=L / D1=L / (D0+Dx). If L, D0 and Dx are known, S1 can be determined.

[0036] The speed of sound in ozone-laden air is measured in the same way: S0 = speed of sound in known air, S1 = speed of sound in oxygen-enriched air, S2 = speed of sound in ozone-laden air. The delay D2 in ozone-laden air is D0 + Dz, where Dz is the additional delay due to the slower speed of sound in ozone-laden air, known from the additional phase shift in ozone-laden air. Thus, S2 = L / D2 = L / (D0 + Dz). If L, D0 and Dz are known, S2 can be determined.

[0037] The speed of sound in air with an oxygen composition of 20.9% is known, as is the speed of sound in 100% oxygen. The speed of sound S1 of the oxygen-enriched gas to be measured should be between the two known sound speed values, as should the oxygen ratio that describes the mixture of the two gases, air and 100% oxygen. This ratio can be calculated as O2 ratio = (S1-S0) / (Sox-S0), where Sox is the speed of sound in 100% oxygen gas. This explanation avoids many complex factors. The theoretical speed of sound in a gas can be calculated from many models, and these models have many factors, including the Boltzmann constant, temperature, mass of the molecule, adiabatic constants (which are not the same for all gases under discussion), and so on, that undermine theoretical certainty. Fortunately, certain linearity assumptions provide reasonable approximations in the region of interest. Empirical scaling provides good results.

[0038] Therefore, O2 percentage = 79.1 x (O2 ratio x O2 scale) + 20.9 was found to be a very good approximation. Because this model is nearly accurate and small variations occur in reality, a scaling factor, O2 scale (approximately 1), is used for the O2 ratio.

[0039] Similarly, for ozone-containing air, the O3 ratio = O3 scale x 3.329 x (S1-S2) / (calculated speed of sound in pure ozone). The speed of sound in pure ozone is calculated because there is likely no way to empirically determine the speed of sound at room temperature due to the explosively unstable nature of such gases. The numbers used are calculations based on molecular weight, temperature, and adiabatic constants. The O3 scale is an empirical scaling adjustment with a value close to 1.

[0040] To determine the ozone concentration in ozone per cubic centimeter, the following formula can be used: grams of ozone per cm^3 = O3 ratio x 2142.8571

[0041] The correction factor arises from complications in measuring gas composition. Ozone generation may not depend on dilution of one gas by another. For example, if 1 mole of oxygen passes through a discharge ozone cell and 10% of the O2 is converted to O3, the number of molecules reduced by the conversion of O2 to O3 will result in 0.0666... ​​moles of ozone emerging from the cell. The total oxygen emerging is 0.9 moles. Thus, the total gas released is 0.9666... ​​moles, with the mole percentage of O3 being 6.9%. However, the speed of sound still has a one-to-one relationship to ozone concentration.

[0042] The situation is similar when the gas entering the discharge cell is composed of two or more components. For example, if the gas entering the cell is 90% O2 and 10% N2, measured in molar terms, then one mole of gas will be composed of 0.9 moles of O2 and 0.1 moles of N2. If the 10% oxygen is converted to O3, the gas exiting will be 0.06 moles of O3, 0.81 moles of O2, and 0.1 moles of N2, totaling 0.91 moles. The mole percentage of O2 is 6.2%. The sound speeds still have a one-to-one relationship, since each gas has a concentration that is a unique function of the ozone concentration, and therefore a unique sound speed corresponding to that concentration.

[0043] The change in the speed of sound depends on the change in the ozone concentration in the gas. In an ozone generation system, the oxygen content in air is usually increased before the generated gas is sent to the ozone generation cell. It is therefore good to know the composition of the gas entering the generation cell and after it has entered the cell. For example, air can be considered to be 78% N2, 21% O2, and 1% argon. The published speed of sound at 0°C passes through each of these component gases at 337 m / s, 316 m / s, and 307 m / s, respectively. By averaging the velocities of these gases in proportion to their proportions in air, the overall speed of sound in air was found to be 332 m / s. This speed compares well with the published speed of 331 m / s. A reasonable estimate of the speed of sound in ozone at 0°C is 249 m / s, but the instability of high concentrations of ozone makes this an unlikely direct measurement.

[0044] Below are examples with different concentrations of oxygen entering the discharge cell. In the first example, assume that the gas sample is an approximation of air, consisting of 0.8 moles of N2 and 0.2 moles of O2. According to the calculations above, the speed of sound in this mixture is 332.80 m / s. When this sample then passes through the ozone generating cell, some of the O2 is converted to O3, which reduces the total molar amount of gas. If we assume that 0.1 moles of O2 are converted to O3, the total output is 0.8 moles of N2, 0.1 moles of O2, and 0.0667 moles of O3, for a total of 0.967 moles. The mole percentage of O3 is 6.9%. The speed of sound in the gas mixture is 328.65 m / s, the change in speed of sound is -4.146 m / s, and the proportional change is -0.01245.

[0045] In contrast, if we assume that the gas sample consists of 1.0 mole of O2, i.e., that the sample is all oxygen, the speed of sound in this mixture is 316 m / s according to the calculation above. As the sample passes through the ozone generating cell, some of the O2 is converted to O3, decreasing the total molar amount of gas. As in the last case, we assume that 0.1 mole of O2 is converted to O3. The total output is 0.9 moles of O2 and 0.0666 moles of O3, for a total of 0.967 moles. The mole percentage of O3 is 6.9% as before. The speed of sound in the gas mixture is 311.28 m / s. The change in speed of sound is -4.720 m / s, with a proportional change of -0.01493.

[0046] It should be noted that the proportional change in speed of sound with respect to the percentage of oxygen in the feed gas entering the cell is greater for pure oxygen than for air. This can be seen in a heuristic way: if nitrogen is initially abundant, the percentage of nitrogen in the ozone-containing gas increases as oxygen is converted to ozone. The increased nitrogen has a relatively high speed of sound and tends to counteract the decrease in speed of sound due to the increasing percentage of ozone.

[0047] Returning to the conduit path length L, some practical constraints on the length of the conduit should be considered, as mentioned earlier. These constraints depend on the speed of sound in the gas being measured, the operating frequency, and the method for measuring or detecting the phase shift. For each gas, there is a corresponding speed of sound and a corresponding wavelength. Some gases have a maximum wavelength λmax, and some have a minimum wavelength λmin. If one wishes to limit the phase shift to 360 degrees, then L / λmin-L / λmax must be less than 1. That is,

[0048] L / λmin-L / λmax<1

[0049] This equation corresponds to a difference in the number of wavelengths contained within the conduit being less than 1. By manipulating the terms to determine the length of the conduit, we obtain the following equation:

[0050] L<λmin λmax / (λmin-λmax)

[0051] Similarly, if we wish to limit the phase shift to 180 degrees, then L / λmin-L / λmax must be less than ½, i.e. in this case:

[0052] L / λmin-L / λmax<1 / 2

[0053] That is, L<1 / 2·λmin·λmax / (λmin-λmax)

[0054] Some illustrative figures may illustrate these points. If we assume that the sound speed Sref of the reference gas (air) is 343 m / s or λref=0.858 cm, and the range of sound speeds ΔS of the other gases in the analyzer (oxygen-enriched air and ozone) is 290 m / s, with a fixed analyzer frequency of 40 KHz, then the maximum wavelength λmax (λref) is 0.858 cm and the minimum wavelength λmin is 0.725 cm. If we wish to limit the phase shift to 180 degrees, then L((1 / 0.725)-(1 / 0.858))<1 / 2, i.e. L<2.339 cm. Similarly, if we wish to limit the phase shift to 360 degrees, then L<4.677 cm. These figures correspond to 2.726 times the wavelength (λref) of the reference gas and 5.452 times the wavelength (λref) of the reference gas, respectively.

[0055] However, a very short conduit will cause artifacts due to standing waves, artifacts due to the acoustic contribution of the holes to the entrance and exit of the test gas, artifacts due to uncertainties in the phase relationship of the electrical signal to the acoustic signal, and artifacts due to electrical and / or acoustic noise. For these reasons, to minimize these artifacts, it is desirable for the conduit to have a length of at least 10 times the wavelength of sound in the reference gas. As described in the previous paragraph, conventional phase detectors impose strict constraints on the length of the conduit. For a 360 degree phase shift detector, the path length L of the conduit can only be 5.452 times the wavelength of the reference gas, and for a 180 degree phase shift detector, the path length can only be 2.726 times the wavelength of the reference gas.

[0056] However, by applying the above-mentioned frequency division technique to a particular phase shift detection method, the constraint on the path length L of the conduit can be removed, and L can be increased. For example, dividing the frequency by n (e.g., n=8) increases the maximum wavelength and the maximum length L by a factor of n=8. That is, the upper limit of the path length of the conduit becomes:

[0057] Depending on whether the phase shift detector is 360 degrees or 180 degrees, L < n·λmin·λmax / (λmin - λmax) or L < 1 / 2·n·λmin·λmax / (λmin - λmax)

[0058] On the other hand, even when using the frequency division technique, the upper limit of the conduit path length is not infinite. Also, when the conduit is long, it causes problems including signal attenuation, an increase in the sample volume, and a space-taking design. It is desirable to limit the conduit length L to about 30 times the wavelength. In this regard, the drawbacks of a long path length begin to become serious.

[0059] In a preferred embodiment, the length of the conduit corresponding to about 23 times the wavelength (λref) in the reference gas is selected, that is, L = about 23 * 0.858 cm. This creates a situation where the phase shift exceeds the limit of the 180° phase detector selected, but by using the described frequency division technique, the advantage of a relatively long signal path is maintained.

[0060] The described relatively simple and low-cost gas analyzer measures the ozone concentration in the ozone generation system with high resolution and high precision, and relatively independently of the oxygen concentration in the supply gas and relatively independently of the temperature. Further, it accurately measures the oxygen concentration in the gas supplied for ozone generation. This provides an inexpensive and productive method for generating ozone on-site and during use.

[0061] This description of the present invention is presented for purposes of illustration and explanation. It is not intended to be exhaustive or to limit the invention to the precise forms described, and many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. From this specification, those skilled in the art can make various modifications suitable for specific applications in various embodiments and utilize and implement the invention to its best advantage. The scope of the present invention is defined by the following claims.

Claims

1. 1. An analyzer for one or more generated gases generated from a first gas of known composition and sonic velocity, each of the one or more generated gases having a concentration of a component that has changed from the first gas, the analyzer comprising: a first transducer, the first transducer transmitting continuous acoustic waves in response to a fixed frequency signal source; a conduit acoustically connected to the first transducer, the conduit selectively receiving and holding the first gas or the one or more generated gases or a gas sample of one of the one or more generated gases; a second transducer acoustically coupled to the conduit opposite the first transducer, the second transducer receiving acoustic waves from the first transducer through the conduit and generating a second transducer signal in response to the received acoustic waves; a processing device that receives a fixed frequency source signal and the second transducer signal, the processing device determining a relative phase shift between the fixed frequency source signal and the second transducer signal for the gas sample in the conduit, the relative phase shift corresponding to a difference between a speed of sound in the gas sample in the conduit and a speed of sound in the first gas, the processing device including circuitry for reducing a frequency of the received fixed frequency source signal and the second transducer signal to increase a measurement range of the relative phase shift; a calculation device for determining the sound speed of each of the one or more evolved gases from the first gas of known composition and calculating the composition of the gas sample of each of the one or more evolved gases generated from the first gas as a reference; Equipped with The computing device calculates a concentration of a component that has changed from the first gas in the gas sample of each of the one or more evolved gases.

2. 2. The analyzer of claim 1, wherein the circuits for reducing the frequency of the fixed frequency source signal and the second transducer signal each comprise a frequency divider circuit.

3. The analyzer of claim 2 , wherein the divider circuit comprises a digital counter circuit.

4. 4. The analyzer of claim 3, wherein the divider circuit divides by a divisor having a value that is a power of two.

5. 5. The analyzer of claim 4, wherein the divide-by circuit divides by eight.

6. The analyzer of claim 1 , wherein the first gas comprises air.

7. 7. The analyzer of claim 6, wherein the first evolved gas comprises air with an increased concentration of oxygen.

8. 8. The analyzer of claim 7, wherein the second generated gas comprises air with an increased concentration of ozone.

9. 4. The analyzer of claim 3, wherein the digital counter circuit has a baseline that is adjusted to increase the measurement range of the phase shift and the calculation range of the gas composition.

10. 2. The analyzer of claim 1, wherein the concentration of the altered component in the sample of the one or more evolved gases is calculated primarily from the ratio of the altered component in the sample of the one or more evolved gases.

11. 11. The analyzer of claim 10, wherein the proportion of the altered component in the sample of the one or more evolved gases is calculated from a ratio having the determined speed of sound in the sample of the one or more evolved gases in the numerator and 100 percent of the speed of sound in the altered component in the denominator.

12. The analyzer of claim 10 , wherein calculating the concentration of the altered component in the sample of the one or more evolved gases includes modifying an empirical correction factor.

13. The analyzer of claim 1 , wherein the conduit is maintained at a selected temperature.

14. 14. The analyzer of claim 13, wherein the conduit comprises an ozone resistant tube, the ozone resistant tube being surrounded by a metal block that provides thermal mass to the conduit.

15. 15. The analyzer of claim 14, wherein the ozone resistant tube is formed of polytetrafluoroethylene (PTFE) and the metal block is formed of aluminum.

16. The conduit has a path length L from the first transducer to the second transducer, and L determines whether the processing unit limits the relative phase shift to 360 degrees; Or limit it to 180 degrees, respectively. L < λmin・λmax / (λmin - λmax) or L < 1 / 2・λmin・λmax / (λmin - λmax) 3. The analyzer of claim 2, wherein the constraint that λmin is a minimum wavelength of the first gas and the one or more gases and λmax is a maximum wavelength of the first gas and the one or more gases is removed by the frequency divider circuit.

17. The conduits are arranged to be either 360 degrees or 180 degrees depending on whether the processor limits the relative phase shift to 360 degrees or 180 degrees, respectively. L < nλmin・λmax / (λmin - λmax) or L < 1 / 2・nλmin・λmax / (λmin - λmax) 17. The analyzer of claim 16, having a path length L where n comprises a divisor of the divider circuit.

18. 20. The analyzer of claim 17, wherein the path length L is greater than 10 times the wavelength of the first gas.

19. 20. The analyzer of claim 18, wherein the path length L is less than 30 times the wavelength of the first gas.

20. 1. A method of operating an analyzer for one or more generated gases generated from a first gas of known composition and sonic velocity, each of the one or more generated gases having a concentration of a component that has changed from the first gas, the method comprising: transmitting a continuous acoustic wave with a first transducer in response to a fixed frequency electrical signal through a conduit holding the first gas, or the one or more generated gases, or a gas sample of one of the one or more generated gases at a time; receiving with a second transducer the acoustic waves transmitted through the conduit and generating an electrical signal in response to the received acoustic waves, wherein a phase shift between the received and transmitted acoustic signals corresponds to a speed of sound in the gas sample within the conduit; processing the fixed frequency electrical signal and an electrical signal generated in response to the received acoustic waves at a low frequency; and extending the phase shift measurement range to determine a relative phase shift between the fixed frequency source signal and a second transducer signal for a gas sample in the conduit, the relative phase shift corresponding to a difference between the speed of sound in the gas sample and the speed of sound in the first gas in the conduit; determining a speed of sound for each of the one or more evolved gases from a known speed of sound for the first gas and from the gas sample of the first gas and a relative phase shift for each of the one or more evolved gases in the conduit; calculating a composition of the gas sample of the one or more evolved gases from the first gas as a reference; Including, The method, wherein the calculating step includes calculating the concentration of a component that has changed from the first gas in the gas sample of each of the one or more evolved gases.

21. 21. The method of claim 20, wherein the processing step includes reducing the frequency of the fixed frequency electrical signal and the electrical signal generated in response to the received acoustic waves.

22. 22. The method of claim 21, wherein the step of decreasing the frequency comprises dividing the frequency by a divisor having a value that is a power of two.

23. 23. The method of claim 22, wherein the step of decreasing the frequency comprises dividing the frequency by eight.

24. 21. The method of claim 20, wherein the first gas comprises air, the first emanating gas comprises air with an increased concentration of oxygen, and the second emanating gas comprises air with an increased concentration of ozone.

25. 21. The method of claim 20, wherein calculating the changed component comprises calculating the concentration of the changed component in the sample of the one or more evolved gases primarily from a ratio of the changed component in the sample of the one or more evolved gases.

26. 26. The method of claim 25, wherein calculating the changed component comprises calculating the proportion of the component that has changed in the sample of the one or more evolved gases from a ratio having in the numerator the determined speed of sound in the sample of the one or more evolved gases and in the denominator 100 percent of the speed of sound in the changed component.

27. 26. The method of claim 25, wherein calculating the altered components comprises modifying the ratios of the altered components in the samples of the one or more evolved gases with an empirical correction factor.

28. 21. The method of claim 20, further comprising the step of maintaining the conduit at a selected temperature.

29. 21. The method of claim 20, further comprising providing a conduit having a path length greater than 10 times the wavelength of sound in air.

30. 1. A method for determining a composition of one or more evolved gases generated from a first gas of known composition and sonic velocity, the method comprising: transmitting a continuous sound wave at a fixed frequency through a conduit; receiving acoustic waves transmitted through the conduit and generating an electrical signal in response to the received acoustic waves, wherein a phase shift between the acoustic waves entering the conduit and exiting the conduit corresponds to a speed of sound in the gas sample within the conduit; extending the measurement range of the phase shift by processing at a low frequency electrical signals corresponding to the continuous sound waves entering and exiting the conduit; alternating the gas sample in the conduit between the first gas and each of the one or more evolved gases; determining a speed of sound of each of the one or more evolved gases over an extended range from a relative phase shift of each of the first gas of known composition and the one or more evolved gases and a speed of sound of the first gas of known composition, the relative phase shift corresponding to a difference between the speed of sound in each of the one or more evolved gases and the speed of sound in the first gas; calculating a composition of each of the one or more evolved gases from the first gas as a reference; Including, the processing step includes reducing a frequency of the continuous sound waves entering and exiting the conduit; each of the one or more evolved gases has a concentration of a component that is changed from the first gas; The method, wherein the calculating step includes calculating the concentration of a component that has changed from the first gas in the gas sample of each of the one or more evolved gases.

31. 31. The method of claim 30, wherein the step of decreasing the frequency comprises dividing the frequency by a divisor having a value that is a power of two.

32. 32. The method of claim 31, wherein the step of decreasing the frequency comprises dividing the frequency by a divisor equal to eight.

33. 31. The method of claim 30, wherein the first gas comprises air, the first emanating gas comprises air with an increased concentration of oxygen, and the second emanating gas comprises air with an increased concentration of ozone.

34. 31. The method of claim 30, wherein calculating the changed component comprises calculating the concentration of the changed component in the sample of the one or more evolved gases primarily from a ratio of the changed component in the sample of the one or more evolved gases.

35. 35. The method of claim 34, wherein calculating the altered component comprises calculating the proportion of the component that is altered in the sample of the one or more evolved gases from a ratio having in the numerator the determined speed of sound in the sample of the one or more evolved gases and in the denominator 100 percent of the speed of sound in the altered component.

36. 35. The method of claim 34, wherein calculating the altered components comprises modifying the ratios of the altered components in the samples of the one or more evolved gases with an empirical correction factor.

37. 31. The method of claim 30, further comprising the step of maintaining the conduit at a selected temperature.

38. 31. The method of claim 30, further comprising providing a conduit having a path length greater than 10 times the wavelength of sound in air.

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