Continuous Wave Acoustic Wave Analyzer

Abstract

Gas analyzers use a continuous acoustic signal passing through a conduit to determine the composition of the gas in the conduit. A transmitting 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's composition. These signals are electronically processed by reducing or dividing a fixed frequency, which increases the range of phase shift measurement and allows for determination of a wider range of gas composition. In ozone generation systems, gas analyzers are ideal for determining the composition of gases derived from a gas of known composition and air as a calibration point.

Description

[0001] describe Background of the Invention

[0003] The present invention relates to gas composition analyzers, and more particularly to gas composition analyzers for generating ozone. Ozone is a highly reactive form of oxygen commonly used for disinfection and water treatment. Due to its properties, ozone is typically generated on-site and at the point of use.

[0004] Ozone can be generated in a variety of ways, one of which is by ionizing oxygen using an electrical discharge to produce a plasma. When ozone is generated by an electrical discharge, its concentration 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 generation cell, the size and material of the cell, and the electrical power used to generate the plasma. The various factors that affect ozone production make it difficult to predict ozone production with any accuracy. If it is necessary to control or understand the production of ozone, it is necessary or desirable to monitor the production of ozone. For this purpose, an on-site analyzer is required.

[0005] There are several different technologies available for use in analyzers to measure the concentration of ozone in a gas. These include utilizing the absorption of UV light in the gas, such as found in products from Oxidation Technologies, LLC, of ​​Inwood, Iowa, and Teledyne API, of San Diego, California. This technology is effective but can be expensive to produce. Furthermore, information on the composition of the feed gas to the ozone generating unit is not readily available. Knowledge of the feed gas composition is required, and it may include dry air with an increased oxygen concentration. Discharge-type ozone generators operate more efficiently with high oxygen levels. Therefore, oxygen concentrators are sometimes used to increase the oxygen content from 20.9% (ambient air) to values ​​exceeding 90%. For discharge-type ozone generators, the presence of a small amount of nitrogen in the feed gas appears to significantly improve efficiency. However, it is possible to remove too much nitrogen from the feed gas, thereby reducing the unit's efficiency. In this oxygen-enriched air, the main components are nitrogen, oxygen, and a small amount of argon. By complementation, the nitrogen concentration can be estimated from the oxygen concentration.

[0006] U.S. Patent No. 5,644,070 (Gibboney) describes the use of the velocity of sound to estimate the concentration of ozone in a gas. With the temperature of the feed gas, the velocity of sound of the feed gas, the temperature of the gas as it exits the ozone generator, and the velocity of sound as it exits the ozone generator measured or known, the velocity of the acoustic 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, the pulse necessarily consists of multiple cycles, which makes it difficult to accurately determine the arrival of the acoustic pulse; it is difficult to tell when the pulse begins and ends. Another disadvantage is that the described system is complex. The acoustic pulse requires a relatively long measurement path and, therefore, requires a relatively large conduit, which increases the volume of sample gas required. Expensive scavenging pumps are used to transport the feed gas to and from the ozone generating unit. This complicates the measurement system. The pump must be made of a material that does not degrade over time in the presence of high concentrations of corrosive ozone.

[0007] The velocity of sound in a continuous sound wave is used to help determine the concentration of two gases, neither of which is ozone. U.S. Patents Nos. 6,202,468 and 6,520,001, filed by the present inventors, describe a system in which this technology is combined with another technique. Two different and unrelated physical parameters (paramagnetism and velocity of sound) are measured to determine the concentrations of oxygen and carbon dioxide in respiratory gases. In this case, sound alone cannot determine the concentration of either gas.

[0008] Therefore, there is a need for a low-cost analyzer that can measure both the oxygen concentration in the feed gas to an ozone generation unit and the ozone concentration in the unit's output. Such an instrument can be used to evaluate the ozone generated and perform process control. For example, the oxygen concentration can be adjusted based on the instrument output to maintain a desired ozone concentration, and the unit power can be controlled based on the instrument output to maintain a desired ozone concentration. A single low-cost analyzer that handles both functions is both convenient and economical. The present invention is an object of this invention to perform both functions in a single, reliable, low-cost instrument. SUMMARY OF THE INVENTION

[0010] The present invention provides an analyzer for deriving one or more gases from a first gas of known composition and velocity, each derived gas having a varying concentration of a component. The analyzer comprises: a first transducer that drives a continuous acoustic wave in response to a fixed frequency signal source; a conduit that is acoustically connected to the first transducer and selectively receives and holds a sample having a first gas and one or more derived gases; a second transducer that is acoustically connected to the conduit opposite to the first transducer unit, the second transducer receiving the acoustic wave from the first transducer through the conduit and generating a second transducer signal in response to the received acoustic wave; a processing unit that receives the fixed frequency signal source signal and the second transducer signal, and that determines 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 in sound velocity of one gas sample relative to another gas sample, the processing unit including a circuit that reduces the frequency of the received fixed frequency source signal and the second transducer signal to expand a measurement range of the relative phase shift; and a calculation unit that determines the sound velocity of one or more gases derived from the first gas based on the first gas of known composition, and that calculates the composition of a sample of the one or more derived gases derived from the first gas as a reference.

[0011] The present invention also provides a method for operating an analyzer for deriving one or more gases from a first gas of known composition and sound velocity, each derived gas having a varying component concentration. The method comprises the following steps: driving a continuous acoustic wave through a conduit with a first transducer in response to a fixed-frequency electrical signal, the conduit holding a sample of the first gas or one or more derived gases at a time; receiving the acoustic wave driven through the conduit with a second transducer and generating an electrical signal in response to the received acoustic wave, wherein a relative phase shift between the received acoustic wave signal and the driven acoustic wave signal corresponds to a relative sound velocity in the gas sample; processing the fixed-frequency electrical signal and the electrical signal generated in response to the received acoustic wave at a reduced frequency to extend a measurement range of the relative phase shift; determining the sound velocity of the first gas of known composition and the one or more gases derived from the first gas within the extended range based on the relative phase shift between the first gas of known composition and a gas sample of the one or more gases derived from the first gas in the conduit; and calculating the composition of the sample of the one or more gases derived from the first gas as a reference.

[0012] The present invention also provides a method for determining the composition of one or more gases derived from a first gas of known composition and sound velocity. The method comprises the following steps: driving a continuous acoustic wave at a fixed frequency through a conduit, wherein the phase difference between the acoustic wave entering the conduit and exiting the conduit corresponds to the sound velocity of the gas in the conduit; processing electronic signals corresponding to the continuous acoustic wave entering and exiting the conduit at a reduced frequency to expand the measurement range of the phase shift; varying the gas in the conduit between the first gas and one or more derived gases; determining the sound velocity of the first gas of known composition and one or more gases derived from the first gas over the expanded range based on the relative phase shift of the gases in the first gas of known composition and the one or more derived gases, wherein the relative phase shift corresponds to the difference in sound velocity of one gas sample relative to another gas sample; and calculating the composition of the one or more derived gases derived from the first gas as a reference.

[0013] Other objects, features and advantages of the present invention will become apparent by consideration of the following detailed description and accompanying drawings, in which like reference numerals represent like features throughout the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The figure shows the overall structure of an analyzer in an ozone generating system according to an embodiment of the present invention.

[0016] Figure 2 represents the fixed frequency signal to the source transducer and the Figure 1 The analyzer unit receives the signal from the transducer.

[0017] Figure 3 The embodiment according to the present invention is shown Figure 2 How the reduced frequency of the signals to the source transducer and the receiving transducer from the analyzer unit expands the measurement range of the phase shift.

[0018] Figure 4A Shows Figure 1 Simplified circuit of phase shift detector; Figure 4B shows exemplary transmit and receive transducer signals, represented by Figure 4A The output signal generated by the phase shift detector in the phase detector and the output of the phase detector have passed Figure 1 The signal produced after the low-pass filter in ; Figure 4C shows the output of the low-pass filter for different phase shifts; and Figure 4D The output of the low-pass filter with baseline adjustment for different phase shifts is shown.

[0019] Figure 5 A flowchart illustrating the operation of the analyzer unit when the ozone generating system is running according to an embodiment of the present invention is shown.

[0020] Detailed Description of the Invention

[0021] As described below, the present invention provides a method for measuring ozone concentration with high resolution and accuracy. The ozone concentration is measured relatively independently of the oxygen concentration in the feed gas and independent of temperature. The analyzer is also simple in construction and low in cost.

[0022] Figure 1 A general view of a portion of an ozone generation system with a continuous acoustic wave analyzer unit according to one embodiment of the present invention is shown. It should be noted that the drawings are representative and that elements in the drawings are not drawn to scale. The system includes an ozone generation block 300 and a continuous acoustic wave analyzer unit 100 formed by a transducer / valve block 400 and a controller / analyzer block 200. The ozone generation block 300 receives compressed air from a source (not shown) and delivers the generated gas, including ozone, to a process, i.e., a specific application of ozone. The compressed air (the air flow is represented by widened arrows in the figure) is received by a concentrator 301, which increases the percentage of oxygen in the generated gas. The gas from the concentrator 301 is delivered to an ozone generator 302, typically a discharge unit, which may be a swing pressure absorption device. 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 delivered to the process (the application using the generated ozone).

[0023] The analyzer unit 100 determines the relative sound velocity of gas at different locations of the ozone generator block 300 and includes a transducer / valve block 400 and a controller / analysis block 200. The transducer / valve block 400 processes gas samples from 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 the output gas from the concentrator 301 to the ozone generator 302; and a third input valve 401 that receives the output gas from the ozone generator 302 to the inlet pressure regulator 303. The outputs of valves 401-403 are connected to a first transducer unit 404, the output of which is connected to a gas conduit 405, which in turn is connected to a second transducer unit 406. Transducer unit 404 transmits sound in the form of a continuous wave through conduit 405 to a receiving transducer unit 406 to determine the relative speed of sound of the gas passing through conduit 405 (and the transmitting transducer unit 404 and the receiving transducer unit 406). The gas output of receiving transducer unit 406 is connected to a destruct unit 407, which eliminates ozone from the sampled gas before releasing the gas into the ambient air.

[0024] Typically, a reference phase reading is taken of a first gas of known composition (usually ambient air), and then unknown second and third gases are introduced, producing corresponding phase shift changes, and the sound speeds of the second and third gases are calculated based on these phase shift changes and the known sound speed of the first gas.

[0025] Ozone is highly corrosive, and great care must be taken in selecting components that come into contact with it. Transducer units 404 and 406 are formed from aluminum with a tough aluminum oxide coating, and conduit 405 is formed from ozone-resistant polytetrafluoroethylene (PTFE) tubing. Conduit 405 is also temperature-controlled to maintain the desired gas temperature within the conduit and has a relatively large thermal mass to stabilize the conduit temperature. The length L of conduit 405 is preferably long enough to provide good device sensitivity and prevent artifacts caused by standing waves, but short enough to minimize size, facilitate manufacturing, and ensure unambiguous phase shift measurements. From a sensitivity and standing wave perspective, a preferred path length L is approximately 24 wavelengths, although other path lengths are possible. However, because the speed of sound varies with gas composition, the phase shift varies excessively, and a path length of 24 wavelengths may result in ambiguous readings. 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 alleviating 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 compromise of the factors mentioned, regardless of ambiguous results from the phase detector.

[0026] The controller block 200 of the analyzer unit 100 includes an oscillator unit 201, which drives a first transducer unit 404 in the transducer / valve block 400. A first counter 205 receives a control signal from a control unit 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 unit 406. The output of the amplifier 202 is received by a comparator 203, which shapes the amplified signal into a square wave. The comparator output is sent to a second counter 204, which also receives a control signal from the control unit 208. The outputs of both counters 204 and 205 are sent to a phase detector 206, and the output of the phase detector is transmitted to the control unit 208 via a low-pass filter 207.

[0027] The present invention uses the speed of sound in a gas to determine the concentration of ozone in the gas. Because ozone has a higher molecular weight relative to oxygen, sound in ozone is much slower than sound in oxygen. Similarly, because oxygen has a higher average molecular weight relative to air, sound in oxygen is slower than sound in air. For a gas with only two components, and if the speed of sound of each component differs from one another, 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 contain a mixture of more than one gas. Discussion of this can be found in previously cited U.S. Patents Nos. 6,202,468 and 6,520,001.

[0028] The speed of sound in the gas is determined from the known properties of the first gas introduced and the corresponding change in phase shift that successive sound waves originating from the source transducer undergo as the wave propagates with the subsequent gas to the receiving transducer. Figure 2 The sequence of acoustic waves from transmitting transducer 404 is shown as a series of square waves, indicating the circuitry used to process the signals and the digital nature of the signals. Analog signals and circuitry could also be used. The rising edges of the waves are shown as solid vertical lines as reference points to aid the reader's understanding. Similarly, another series of square waves received by second transducer 406 is illustrated as solid rising edges of each acoustic wave received by second transducer 406. The arrows indicate the phase shift, which reflects the time interval for a particular wavefront to propagate from source transducer 404 to receiving transducer 406. Clearly, the longer the time interval, or the slower the acoustic wave's velocity through the gaseous medium, the greater the phase shift.

[0029] Due to the nature of continuous waves, phase shifts must be kept within a limited range to confidently determine the offset. For example, it can be difficult to determine whether the phase shift is x or x+i*360°, where i is an integer. So, in this example, only phase shifts less than 360° should be used. However, this severely limits the range of speeds that can be determined. It should be noted that this limited phase range also depends on the circuitry used to determine the phase shift, which may further limit the phase shift to, for example, 0° to 180°. In any case, the present invention expands the range of speeds that can be determined, as explained below.

[0030] When the source transducer is driven at a fixed frequency, the frequency of the source signal is reduced for processing. Figure 3 This example shows a 66% reduction in frequency, or, stated another way, dividing the frequency by 3 eliminates two of the three vertical lines. This elimination is represented by replacing the solid line (representing the rising edge) with a barred bar. As the frequency is reduced, the range of phase shifts (and therefore the sound velocity range) that can be determined increases accordingly. That is, assuming a phase limit of 0-360°, reducing the frequency by a factor of 3 allows the phase shift range to expand by a factor of 3, to a range of 0-1080°. Reducing the frequency proportionally increases the range of phase shift measurements and allows for high-precision measurements without the drawbacks of short-path-length conduits. Short path lengths typically introduce signal artifacts due to standing waves or residual standing waves in the conduit. Reducing the frequency avoids these problems.

[0031] Figure 4A The simple representation of the electronic circuit in Figure 1 demonstrates the phase shift measurement as described previously. Two square wave data streams (A represents the transmitting transducer ( Figure 1 404) of the signal, and B represents the receiving transducer ( Figure 1The signal (406) in FIG) is input to an exclusive-OR logic gate (part of the circuit of phase detector 206). The output C of the gate is input to a low-pass filter ( Figure 1 207 in ). Figure 4B The relationship between the A signal and the B signal, as well as the output signal C of the XOR gate are shown. In addition to the XOR gate, an Exclusive-NOR gate can also be used for the phase detector 206.

[0032] Figure 4B Also shown is the output D signal of the low-pass filter 207, through which the signal with the largest variation from the XOR gate is filtered to reflect the "average" value of the output signal. For example, if the A and B signals are completely out of phase with each other (i.e., a phase difference of 180°), the filtered phase difference or phase shift signal, i.e., output D, is at its maximum value. If the A and B signals are completely in phase with each other (i.e., a phase difference of 0°), output D is at its minimum value, i.e., zero. If the A and B signals are "half" out of phase, or half in phase, i.e., a phase difference of 90°, output D is halfway between the maximum and minimum values, i.e., half the maximum value. Figure 4C Figure 2 shows how the phase detector value (output D) varies depending on the phase difference between signals A and B. The phase difference is shown here as values ​​from less than -540° to greater than 540°. Negative and positive values ​​indicate whether the A signal or the B signal leads the B signal or the A signal. As mentioned above, the A signal lags the B signal. Figure 4C The following diagram further illustrates why the phase shift should be within limits. In this diagram, the phase shift should be limited to 180° to avoid ambiguity when determining the phase shift based on the output D value.

[0033] When the frequencies of the A signal and the B signal are reduced, the output D of the low-pass filter 207 is changed. Figure 4D In the example, the counter ( Figure 1 205 and 204 in the figure are inserted into the data stream of the A digital signal and the B digital signal to reduce the frequency by a divisor of 8 (N=8 of the counter). The divisor N is arbitrary and is chosen to suit the convenience of the designer. The output D is expanded accordingly. The output D period is not a 360° period (such as Figure 4C Instead of being eight times larger (as shown in the figure), the phase shift is now 2880°, meaning the output D signal repeats every 2880°. In this example, the phase shift range is expanded to 1440°, or half of 2880°. This is significantly greater than the 180° limit in the absence of a down-converted signal.

[0034] Therefore, the extended range of the filtered phase-shifted signal, ie the output D, can be easily realized by a digital counter, for example by Figure 1The counters 205 and 204 in FIG are implemented. Using the example above of N=8, the counter of the A signal data stream is set to zero and then started. When the A counter reaches a certain value, for example, 1, the counter of the B signal data stream is set to zero and started. This ensures a non-negative starting value for the phase detector in the case of N=8. Therefore, the output D is between zero and 0.125 (assuming the maximum output D is 1.0). The number 0.125 is 1 / 8. If the phase difference between the A and B signals is now due to the conduit 405 (see FIG Figure 1 ) increases with different gas samples in the , the output D also increases linearly over the range of 1260°(1440°*(1-(1 / 8)) to a maximum value. Again, it should be noted that the counter divisor N and the corresponding expansion of the phase shift measurement are arbitrary.

[0035] The two down counters 205 and 204 introduce phase uncertainty. In the divide by 8 case described above, there are 8 possible phase relationships depending on the counting relationship of the two counters. If the first counter 205 has a count of N, then the second counter 204 can have any one of the values ​​of (N+n) mod 8, where n is equal to any integer in the range of 0 to 7. If each waveform is counted correctly, then this relationship will hold indefinitely. Control of the number n provides the opportunity to adjust the baseline with a resolution of 1 / 8 of the full scale. In the example above, the received transducer signal is initially transmitted at a "2" value relative to the transmitted transducer signal. nd "Starting with the relationship, (N+n) mod 8 = 1. By controlling n so that (N+n) mod 8 = 0, the baseline is adjusted, thereby extending the phase measurement range to its maximum extent and maximizing the determination of the gas composition, and is a feature of the present invention.

[0036] For the above-described phase shift detection, according to an embodiment of the present invention, the electronic circuitry of the controller block 200 of the analyzer unit 100 is implemented using digital circuitry. The oscillator block 201 generates a fixed-frequency signal. In this embodiment, the frequency is 40 kHz. The oscillator block signal drives the transmitting transducer 404, and the frequency is divided by 8. In other words, the counter 205 steps down or reduces the signal frequency by a factor of 8. The output of the counter 205 is received by the phase detector block 206.

[0037] After being amplified by amplifier 202, the output of receiving transducer 406 is processed into a square wave by comparison logic (block 203). Counter 204 divides the signal frequency by 8. The output of counter 204 is also received by phase detector block 206. Through the operation of an XOR gate or an XNOR gate, the output of phase detector 206 varies between two power levels (e.g., 0 volts and 5 volts) of the logic gate. Low-pass filter 207 removes the AC component of the output signal.

[0038] Control of the continuous acoustic wave analyzer unit 100 is performed by a control unit 208 in the controller / analysis block 200. In this embodiment, the control unit 208 is essentially a programmed microprocessor or microcontroller with memory. The memory stores, among other things, the value from the filtered phase detector 206. Control lines from the unit 208 extend to each of the valves 401-403 and the counters 205 and 204. The unit 208 also receives the phase shift value from the output of the low-pass filter 207. A display 209 is connected to the control unit 208 and provides a visual interface for the operation of the analyzer unit 100.

[0039] Under the control of the control unit 208, the analyzer unit 100 with an expanded phase shift range determines the speed of sound and gas composition in a variety of gases. The following description relates to the generation and Figure 1 The system in FIG, but the analyzer unit should not be considered so limited. Briefly, ambient air is introduced into the conduit and the valve is closed. The output of the filter phase detector is read and recorded for the ambient air. The feed gas (oxygen-enriched air) is then introduced into the conduit and the valve is closed. The output of the filter phase detector is read and recorded for the feed gas. Finally, ozone-bearing gas is introduced into the conduit and the valve is closed. The output of the filter phase detector is read and recorded for the ozone-bearing gas.

[0040] Figure 5 The process flow for the operation just described is shown. The steps of the process flow are summarized with the gases labeled A, B, and indicating that more gases derived from the initial gas can be included in the process flow. After the system is initialized as shown by the dashed arrow 501, step 502 initializes the index Valve# to zero. The valve indicated by Valve# is then opened, and the gas selected by the opened valve is delivered to the conduit via step 503. (In Figure 1 In the ozone generating system in FIG, index Valve#=0 corresponds to valve 403, index Valve#=1 corresponds to valve 402, and index Valve#=2 corresponds to valve 401). Step 504 closes the valve. Step 505 tests whether the index Valve# is zero. If the index is 0, the counter is reset in step 506, and the output value of the filtered phase detector is read and recorded in step 507. If the index is not 0, the test of step 505 moves to step 507. After step 507, it is tested whether the index Valve# is equal to 2 by step 508. If not, step 509 increases the index Valve# by 1 and the process returns to step 503. These steps are repeated until the index Valve# is equal to 2 and the process ends in step 510.

[0041] Based on the recorded phase shift values, control unit 208 analyzes the data to determine the sound velocity and gas composition. A comparison technique is used. Given the sound velocity and composition of the first gas, dehumidified ambient air is used as a reference to determine the sound velocity and composition of gases derived from the first gas, i.e., the ambient air. Specifically, the sound velocity of the dehumidified ambient air at the set temperature of the temperature-controlled conduit is known and used as a reference to calculate the sound velocity of the second gas (oxygen-enriched feed gas) and the sound velocity of the third gas (oxygen-enriched air containing ozone) from the measured phase shift.

[0042] The gases are processed in order of decreasing speed of sound, i.e. ambient air, ambient air enriched with oxygen, and oxygen enriched air containing converted ozone. The enriched air is derived from the ambient air, and the ozone containing air is derived from the enriched air. Each gas is denser than the gas preceding it, and the speed of sound decreases relative to the speed of the preceding gas. The first gas (dry ambient air) is considered the reference gas because its composition is known. Its speed of sound at a given temperature is also known. Although the temperature can be varied in the manner described in aforementioned U.S. Patent No. 5,644,070 (Gibboney), it is preferred that the gas temperature be maintained at a set temperature. It has been found that Figure 1 The temperature-controlled gas conduits shown effectively maintain the gases at a set temperature. Therefore, when calculating the speed of sound in oxygen-enriched and ozone-containing air, all gases are assumed to have the same temperature. To minimize errors, either additional thermal mass or ambient temperature control is required. By adjusting the baseline as previously described, with reference to the maximum speed of sound in the first gas, the phase shift range for oxygen-enriched and ozone-containing air can be expanded to accommodate the composition of those gases.

[0043] Sound propagating in oxygen-enriched air arrives at the second transducer slightly later than in ambient air. The additional delay is measured based on the phase shift combined with the known length L of the conduit between the first and second transducers. This allows the speed of sound of the oxygen-enriched air to be determined based on the speed of sound of the ambient air and the additional phase shift. As mentioned above, there is a direct relationship between phase shift and delay. In particular, where L = sound path length and S0 = the known speed of sound in ambient air, then the delay D0 for ambient air is D0 = (L / S0), which is a known quantity. The delay D1 for oxygen-enriched air is D0 + Dx, where Dx is the additional delay due to the slower speed of sound in oxygen-enriched air and is known from the additional phase shift of oxygen-enriched air. Therefore S1 = L / D1 = L / (D0 + Dx). Given L, D0 and Dx, S1 is determined.

[0044] The speed of sound of ozone-laden air is measured in the same manner. With S0 = the known speed of sound in ambient air, S1 = the speed of sound in oxygen-enriched air, and S2 = the speed of sound in ozone-laden air. The delay D2 for ozone-laden air is D0 + Dz, where Dz is the additional delay due to the slower speed of sound in ozone-laden air and is known from the additional phase shift of ozone-laden air. Therefore, S2 = L / D2 = L / (D0 + Dz). Given L, D0, and Dz, S2 is determined.

[0045] The sound velocity of ambient air with an oxygen composition of 20.9% S0 is known, as is the sound velocity of 100% oxygen. The measured sound velocity S1 of the oxygen-enriched gas should fall between these two known sound velocity values, representing the ratio of oxygen in the mixture of the two gases, ambient air, and 100% oxygen. This ratio can be calculated as: O2 Ratio = (S1-S0) / (S ox -S0) where S ox is the speed of sound in 100% oxygen. This discussion avoids many complicating factors. The theoretical speed of sound in gases can be calculated from many models, which in turn involve many factors, including Boltzmann's constant, temperature, molecular mass, and the adiabatic constant (which is not the same for all gases in question), discouraging theoretical certainty. Fortunately, certain linear assumptions yield reasonable approximations in the region of interest. Empirical scaling produces good results.

[0046] It was therefore found that: O2 percentage = 79.1 x (O2 ratio x O2scale) + 20.9 is a very good approximation. Because this model is approximate and small variations will occur in the real world, a scaling factor O2scale (close to 1) is used for O2 ratio.

[0047] Similarly, for ozone-laden air: O3 scale = O3scale x 3.329 x (S1 - S2) / (calculated speed of sound in pure ozone). The speed of sound in pure ozone is calculated because, due to the explosively unstable nature of this gas, it may not be possible to empirically determine the speed of sound at ambient temperature. The number used is calculated based on molecular weight, temperature, and the adiabatic constant. O3scale is an empirical scaling adjustment with a value close to 1.

[0048] To determine the concentration of ozone in terms of the amount of ozone per cubic centimeter, the following equation can be used: 3 ) Grams of ozone = O3 ratio x 2142.8571.

[0049] The correction factor stems from the complexity of gas composition measurement. Ozone production may not depend on the 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, 0.666 moles of ozone are emitted from the cell. This is due to the reduction in the number of molecules produced from the conversion of O2 to O3. The total oxygen emitted is 0.9 moles. Therefore, the total gas emitted is 0.9666 moles, and the mole percentage of O3 is 6.9%. However, the speed of sound still has a 1:1 relationship with the ozone concentration.

[0050] The situation is similar if the gas entering the discharge cell is composed of more than one component. For example, if the gas entering the cell is 90% O₂ and 10% N₂, by mole, then 1 mole of gas is composed of 0.9 moles of O₂ and 0.1 moles of N₂. With 10% of the oxygen converted to O₃, the exiting gas is composed of 0.06 moles of O₃, 0.81 moles of O₂, and 0.1 moles of N₂, for a total of 0.97 moles. The mole percentage of O₂ is 6.2%. The sound velocities still have a 1:1 relationship, as the concentration of each gas is a unique function of the ozone concentration and, therefore, has a unique sound velocity corresponding to that concentration.

[0051] The change in sound velocity depends on the change in ozone concentration in the gas. In ozone generation systems, the oxygen content of the air is typically increased before the generated gas is fed to the ozone generation unit. Therefore, it is best to know the composition of the gas entering and exiting the generation unit. For example, air can be assumed to be 78% N₂, 21% O₂, and 1% argon. The published sound velocities for these respective component gases at 0°C are 337 m / s, 316 m / s, and 307 m / s, respectively. By averaging these velocities by their proportions in air, the total sound velocity in air is found to be 332 m / s. This is comparable to the published velocity of 331 m / s. A reasonable estimate for the sound velocity in ozone at 0°C is 249 m / s, but this is difficult to measure directly because ozone at high concentrations is unstable.

[0052] The following are illustrative examples of different oxygen concentrations entering the discharge cell. The first example assumes that the gas sample consists of 0.8 moles of N2 and 0.2 moles of O2 (approximately air). Based on the above calculations, the speed of sound in this mixture is 332.80 m / s. If this sample is then passed through an ozone generating unit, some of the O2 will be converted to O3, thereby reducing the total molar amount of the gas. Assuming 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 molar percentage of O3 is 6.9%. The speed of sound in the gas mixture is 328.65 m / s, and the change in sound speed is -4.146 m / s, with a proportional change of -0.01245.

[0053] In contrast, suppose the gas sample consists of 1.0 mole of O₂—that is, the sample is entirely oxygen. Based on the above calculation, the speed of sound in this mixture is 316 m / s. If the sample passes through an ozone generator, some of the O₂ will be converted to O₃, reducing the total molar mass of the gas. Assume, as in the previous case, 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 remains the same as before, 6.9%. The speed of sound in the gas mixture is 311.28 m / s. The change in sound speed is -4.720 m / s, and the change in ratio is -0.01493.

[0054] It should be noted that the change in sound velocity relative to the proportion of oxygen in the feed gas entering the unit is greater with pure oxygen than with air. This can be viewed as a heuristic. If there is a large amount of nitrogen at the beginning, the proportion of nitrogen in the ozone-containing gas will increase as the oxygen is converted to ozone. The increase in nitrogen, with its relatively high sound velocity, will tend to offset the decrease in sound velocity caused by the increase in the proportion of ozone.

[0055] When considering the return duct path length L, some practical constraints on the duct length should be considered, as previously mentioned. These constraints depend on the sound velocity of the gas being measured, the operating frequency, and the method of measuring or detecting the phase shift. For each gas, there is a corresponding sound velocity and a corresponding wavelength. In the gas, there is a maximum wavelength λ 最大 The gas and wavelength are the smallest λ 最小 If the phase shift is to be limited to 360 degrees, then L / λ 最小 -L / λ 最大 Must be less than 1, that is:

[0056] L / λ 最小 -L / λ 最大 <1.

[0057] This corresponds to a difference in the number of wavelengths contained in the conduit being less than 1. By manipulating these terms to determine the conduit length, we obtain:

[0058] L<λ 最小 ·λ 最大 / (λ 最小 -λ 最大 ).

[0059] Similarly, if the phase shift is limited to 180 degrees, then L / λ 最小 -L / λ 最大 must be less than 1 / 2, which in this case is

[0060] L / λ 最小 -L / λ 最大 <1 / 2; or

[0061] L<1 / 2λ 最小 ·λ 最大 / (λ 最小 -λ 最大 ).

[0062] Some exemplary figures may illustrate these points. Fixing the analyzer frequency to 40 kHz and assuming that the speed of sound of the reference gas (air) is S 参考 343m / s or λ 参考 =0.858 cm, and the sound velocity range ΔS of other gases in the analyzer (oxygen-enriched air and ozone) is 290 m / s, with a maximum wavelength λ 最大 (λ 参考 ) is 0.858 cm, and the minimum wavelength λ 最小 =0.725cm. If the phase shift is limited to 180 degrees, then L((1 / .725)-(1 / 0.858)) < 1 / 2, or L < 2.339cm. Similarly, if the phase shift is limited to 360 degrees, then L < 4.677cm. These numbers correspond to 2.726 wavelengths of the reference gas (λ 参考 ) and 5.452 wavelengths of reference gas (λ 参考 ).

[0063] However, very short conduits can introduce artifacts due to standing waves, acoustic contributions from the test gas entering and exiting the orifice, uncertain phase relationships between the electrical and acoustic signals, and artifacts caused by electrical and / or acoustic noise. For these reasons, it is desirable to have a conduit that includes at least 10 wavelengths of sound in the reference gas to minimize these artifacts. As mentioned in the previous paragraph, conventional phase detectors impose strict constraints on conduit length. In the case of a 360° phase-shift detector, the conduit path length, L, can be no greater than 5.452 wavelengths of the reference gas, while in the case of a 180° phase-shift detector, the path length can be no greater than 2.726 wavelengths of the reference gas.

[0064] However, by applying the previously described frequency division technique to a specific phase-shift detection method, the constraint on the catheter path length L can be removed and L can be extended. If the frequency is divided by n, for example, n = 8, then the maximum number of wavelengths and the maximum length L are each multiplied by a factor of n = 8. In other words, the upper limit of the catheter path length becomes:

[0065] L <nλ 最小 ·λ 最大 / (λ 最小 -λ 最大 )or

[0066] L<1 / 2nλ 最小 ·λ 最大 / (λ最小 -λ 最大 ).

[0067] This depends on whether the phase shift detector is 360 degrees or 180 degrees respectively.

[0068] On the other hand, even with frequency-splitting techniques, the upper limit on the catheter path length is not infinite. Long catheters also lead to problems including signal attenuation, large sample sizes, and bulky designs. It is desirable to limit the catheter length L to approximately 30 wavelengths, at which point the disadvantages of long path lengths become significant.

[0069] In the preferred embodiment, the wavelengths corresponding to approximately 23 wavelengths of sound in the reference gas (λ 参 This would result in a situation where the phase shift exceeds the limit of the chosen 180° phase detector, but the advantage of the relatively long signal path is maintained using the described frequency division technique.

[0070] The relatively simple and low-cost gas analyzer described herein measures ozone concentration in an ozone generation system with high resolution and accuracy, relatively independently of the oxygen concentration in the feed gas and independent of temperature. Furthermore, the oxygen concentration in the feed gas used for ozone generation is accurately measured. This provides an inexpensive and efficient way to generate ozone on-site and at the point of use.

[0071] The description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the above teachings. The embodiments have been chosen and described in order to best explain the principles of the invention and its practical application. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as suited to a particular application. The scope of the invention is defined by the appended claims.

Claims

1. An analyzer for deriving one or more gases from a first gas of known composition and velocity, each derived gas having a varying concentration of a component, the analyzer comprising: a first transducer driving a continuous acoustic wave in response to a fixed frequency signal source; a conduit acoustically coupled to the first transducer, the conduit selectively receiving and retaining a sample having the first gas and one or more derivative gases; a second transducer acoustically connected to the conduit opposite the first transducer, the second transducer receiving the acoustic wave from the first transducer through the conduit and generating a signal in response to the received acoustic wave; a processing unit that receives signals from the fixed frequency signal source and the second transducer, the processing unit determining a relative phase shift between the signals from the fixed frequency signal source and the second transducer for a gas sample in the conduit, the relative phase shift corresponding to a difference in acoustic velocity of one gas sample relative to another gas sample, the processing unit including circuitry for reducing the frequency of the signals received from the fixed frequency signal source and the second transducer to expand a measurement range of the relative phase shift; and A calculation unit determines the sonic velocity of the one or more gases derived from the first gas based on the first gas of known composition, and calculates the composition of samples of the one or more derived gases originating from the first gas as a reference.

2. The analyzer according to claim 1, wherein The circuits for reducing the frequency of signals received from the fixed frequency signal source and the second transducer each include a frequency divider circuit.

3. The analyzer according to claim 2, wherein The frequency divider circuit includes a digital counter circuit.

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

5. The analyzer according to claim 4, wherein The frequency divider circuit divides by 8.

6. The analyzer according to claim 1, wherein The first gas includes air.

7. The analyzer according to claim 6, wherein The first derived gas includes air having an increased oxygen concentration.

8. The analyzer according to claim 7, wherein The second derived gas includes air having an increased ozone concentration.

9. The analyzer according to claim 3, wherein The digital counter of the digital counter circuit has an adjusted baseline so that the measurement range of the relative phase shift is expanded and the calculation range of the gas composition is increased.

10. The analyzer according to claim 1, wherein The calculation unit calculates concentrations of components that vary in samples of the one or more derivative gases.

11. The analyzer according to claim 10, wherein The concentration of the component that varies in the sample of the one or more derivative gases is calculated based on the ratio of the components that vary in the sample of the one or more derivative gases.

12. The analyzer according to claim 11, wherein The proportion of the varying component in the sample of the one or more derivative gases is calculated based on a ratio, the numerator of which is the determined sound velocity of the sample of the one or more derivative gases and the denominator of which is 100% of the sound velocity of the varying component.

13. The analyzer according to claim 11, wherein Calculating the concentration of the varying component in the sample of the one or more derivative gases includes modifying an empirical correction factor.

14. The analyzer according to claim 1, wherein The conduit is maintained at a selected temperature.

15. The analyzer according to claim 14, wherein The conduit comprises an ozone resistant tube enclosed in a metal block to provide thermal mass to the conduit.

16. The analyzer according to claim 15, wherein The ozone-resistant tube includes polytetrafluoroethylene (PTFE), and the metal block includes aluminum.

17. The analyzer according to claim 2, wherein: The catheter has a path length L between the first and second transducers, wherein the divider circuit eliminates the constraint on L according to the following equation: L<λ 最小 ·l 最大 / (l 最小 -l 最大 ) or L<1 / 2λ 最小 ·l 最大 / (l 最小 -l 最大 ), These two formulas depend on whether the processing unit limits the relative phase shift to 360 degrees or 180 degrees, respectively, where λ 最小 = minimum wavelength and λ of the first gas and one or more gases 最大 = the maximum wavelength of the first gas and one or more gases.

18. The analyzer according to claim 17, wherein The conduit has a path length L: L <nλ 最小 ·l 最大 / (l 最小 -l 最大 ) or L<1 / 2nλ 最小 ·l 最大 / (l 最小 -l 最大 ), These two formulas depend on whether the processing unit limits the relative phase shift to 360 degrees or 180 degrees, respectively, and n comprises the divisor used for the divider circuit.

19. The analyzer according to claim 18, wherein The path length L of the conduit is greater than 10 wavelengths of the first gas.

20. The analyzer according to claim 19, wherein The path length L of the conduit is less than 30 wavelengths of the first gas.

21. A method of operating an analyzer for deriving one or more gases from a first gas of known composition and velocity, each derived gas having a varying concentration of a component, the method comprising: driving a continuous acoustic wave with a first transducer in response to a fixed frequency electrical signal through a conduit, the conduit holding a sample of the first gas or one or more derivative gases at a time; receiving, by a second transducer, the acoustic wave driven through the conduit and generating an electrical signal in response to the received acoustic wave, a phase shift between the fixed frequency electrical signal and the electrical signal generated in response to the received acoustic wave corresponding to a speed of sound in the gas sample; processing the fixed frequency electrical signal and the electrical signal generated in response to the received acoustic wave at a reduced frequency to expand a measurement range of the phase shift to thereby determine a relative phase shift between the fixed frequency electrical signal and the electrical signal generated in response to the received acoustic wave for a gas sample in the conduit, the relative phase shift corresponding to a difference in acoustic velocity of one gas sample relative to the other gas sample; determining a speed of sound of one or more gases derived from the first gas based on the known speed of sound of the first gas and based on the relative phase shift of gas samples of the first gas and one or more gases derived from the first gas in the conduit, and The composition of a gas sample of one or more gases derived from the first gas is calculated as a reference.

22. The method according to claim 21, 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 wave.

23. The method according to claim 22, wherein The frequency reduction step comprises dividing the frequency by a divisor having a value that is a power of two.

24. The method according to claim 23, wherein The frequency reduction step comprises dividing the frequency by 8.

25. The method according to claim 21, wherein The first gas comprises air, the first derived gas comprises air having an increased oxygen concentration, and the second derived gas comprises air having an increased ozone concentration.

26. The method according to claim 21, wherein The calculating step includes calculating the varying component concentrations in the sample of the one or more derivative gases.

27. The method according to claim 26, wherein The step of calculating the changed component includes calculating a concentration of the changed component in the sample of the one or more derivative gases based on a ratio of the changed component in the sample of the one or more derivative gases.

28. The method according to claim 27, wherein The step of calculating the changed component includes calculating the proportion of the changed component in the sample of the one or more derivative gases based on a ratio, the numerator of the ratio being the determined sound velocity of the sample of the one or more derivative gases and the denominator being 100% of the sound velocity of the changed component.

29. The method according to claim 27, wherein The step of calculating the varying composition includes modifying the proportion of the varying composition in the sample of the one or more derivative gases using an empirical correction factor.

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

31. The method of claim 21 further comprising providing a conduit having a path length greater than 10 wavelengths of sound in air.

32. A method of determining the composition of one or more gases derived from a first gas of known composition and velocity of sound, the method comprising: driving a continuous acoustic wave at a fixed frequency through the conduit, the phase shift between the acoustic wave entering the conduit and exiting the conduit corresponding to the speed of sound of the gas in the conduit; processing electronic signals corresponding to the continuous acoustic waves entering and exiting the conduit at a reduced frequency to extend the measurement range of the phase shift; changing the gas in the conduit between the first gas and one or more derivative gases; determining, within the expanded range, the sound velocities of the first gas of known composition and the one or more gases derived from the first gas based on a relative phase shift between the first gas of known composition and the one or more derived gases, the relative phase shift corresponding to a difference in sound velocity of one gas sample relative to the other gas sample; and The composition of the one or more derivative gases derived from the first gas is calculated as a reference.

33. The method according to claim 32, wherein Each derivative gas has a component concentration that initially varies from the first gas.

34. The method of claim 32, wherein: The processing step includes reducing the frequency of continuous acoustic waves entering and exiting the conduit at a reduced frequency to expand the measurement range of the phase shift.

35. The method according to claim 34, wherein The frequency reduction step comprises dividing the frequency by a divisor having a value that is a power of two.

36. The method according to claim 35, wherein The frequency reduction step comprises dividing the frequency by a divisor equal to eight.

37. The method of claim 32, wherein: The first gas comprises air, the first derived gas comprises air having an increased oxygen concentration, and the second derived gas comprises air having an increased ozone concentration.

38. The method of claim 32, wherein: The calculating step includes calculating the varying concentrations of components in the sample of the one or more derivative gases.

39. The method according to claim 38, wherein The step of calculating the changed component includes calculating a concentration of the changed component in the sample of the one or more derivative gases based on a ratio of the changed component in the sample of the one or more derivative gases.

40. The method of claim 39, wherein The step of calculating the changed component includes calculating the proportion of the changed component in the sample of the one or more derivative gases based on a ratio, the numerator of the ratio being the determined sound velocity of the sample of the one or more derivative gases and the denominator being 100% of the sound velocity of the changed component.

41. The method of claim 39, wherein: The step of calculating the varying composition includes modifying the proportion of the varying composition in the sample of the one or more derivative gases using an empirical correction factor.

42. The method of claim 32, further comprising maintaining the conduit at a selected temperature.

43. The method of claim 32, further comprising providing a conduit having a path length greater than 10 wavelengths of sound in air.

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