Acetone Detector

Abstract

To improve the detection accuracy of acetone and reduce costs.SOLUTION: An acetone detector 1 includes: a TVOC sensor 10 being a bridge circuit that includes a detection element 11 and a compensation element 12 and changes an output value Vm1 by combustion of acetone and ethanol; an ethanol sensor 20 being a bridge circuit that includes a detection element 21 and a compensation element 22 and changes an output value Vm2 by combustion of ethanol; and an MCU 30 that pulse-drives the TVOC sensor 10 and the ethanol sensor 20 and obtains a concentration of acetone based on the output value Vm1 of the TVOC sensor 10 and the output value Vm2 of the ethanol sensor 20.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an acetone detector. [Background technology]

[0002] A biological gas detection device that is intended to detect a desired gas with high accuracy even under the influence of an interference gas is known (see, for example, Patent Document 1). The biological gas detection device described in Patent Document 1 includes a semiconductor gas sensor that is sensitive to the desired gas and interference gas contained in the biological gas, an electrochemical gas sensor that is sensitive to the interference gas contained in the biological gas, and a desired gas concentration acquisition means that acquires the concentration of the desired gas based on the output values ​​of these gas sensors. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-1126 A Summary of the Invention [Problem to be solved by the invention]

[0004] The biogas detector described in Patent Document 1 uses two types of gas sensors, a semiconductor gas sensor and an electrochemical gas sensor, which have different principles for detecting gas, and therefore requires separate packages for each gas sensor, which increases costs. Also, since the semiconductor gas sensor does not have a temperature and humidity compensation element, there is a large deviation in characteristics with respect to temperature and humidity, and a large correction coefficient must be input, resulting in large errors.

[0005] In view of the above circumstances, an object of the present invention is to provide an acetone detector that can improve the accuracy of acetone detection while reducing costs. [Means for solving the problem]

[0006] The acetone detector of the present invention comprises: a first bridge circuit comprising a first heater and a first catalyst element having a first catalyst that adsorbs acetone and ethanol and burns them with the heat of the first heater; a first compensation element having a heater and a support heated by the heater, and an output value that changes due to combustion of acetone and ethanol; a second bridge circuit comprising a second heater and a second catalyst element having a second catalyst that adsorbs ethanol and burns it with the heat of the second heater; and a second compensation element having a heater and a catalyst heated by the heater, and an output value that changes due to combustion of ethanol; and a control unit that pulse-drives the first bridge circuit and the second bridge circuit by periodically applying two pulses having LOW / HIGH levels, and determines the concentration of acetone based on a first output value that is the difference between the output value of a first pulse and the output value of a second pulse of the first bridge circuit, and a second output value that is the difference between the output value of the first pulse and the output value of the second pulse of the second bridge circuit. Effect of the Invention

[0007] According to the present invention, an output value is obtained from the first bridge circuit for acetone and ethanol adsorbed on the first catalyst of the first bridge circuit, and an output value is obtained from the second bridge circuit for ethanol adsorbed on the second catalyst of the second bridge circuit, and the concentration of acetone is calculated based on these output values. This makes it possible to detect low concentrations of acetone. In addition, since the gas sensor is composed of the first bridge circuit and the second bridge circuit that are based on the same principle of adsorption and combustion, it is possible to mount a gas sensor for acetone and ethanol and a gas sensor for ethanol in a common package. Furthermore, the first bridge circuit and the second bridge circuit are pulse-driven by periodically applying two pulses having LOW / HIGH levels, and the concentration of acetone is calculated based on the first output value, which is the difference between the output value of the first pulse and the output value of the second pulse of the first bridge circuit, and the second output value, which is the difference between the output value of the first pulse and the output value of the second pulse of the second bridge circuit. That is, the difference between the state in which the gas molecules adsorbed on the catalyst are burned and the state in which the gas is not adsorbed on the catalyst is set as the first output value and the second output value. Therefore, the accuracy of acetone detection can be improved and costs can be reduced. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing a schematic configuration of an acetone detector according to one embodiment of the present invention. [Diagram 2] FIG. 2 is an exploded perspective view showing the acetone detector shown in FIG. [Diagram 3] FIG. 3 is a diagram showing a schematic configuration of the acetone detector shown in FIGS. 1 and 2, and a relationship between the outputs of the TVOC sensor and the ethanol sensor and the output of the acetone detector. [Figure 4] FIG. 4 is a diagram showing the output characteristics of the TVOC sensor and the ethanol sensor. [Diagram 5] FIG. 5 is a diagram illustrating the gas adsorption and combustion reaction that occurs in the detection element of the TVOC sensor. [Figure 6]FIG. 6 is a diagram for explaining the adsorption and combustion characteristics of the TVOC sensor and the ethanol sensor. [Figure 7] FIG. 7 is a linear graph showing the relationship between the output of a gas sensor exhibiting adsorption combustion characteristics and the gas concentration, and the relationship between the output of a gas sensor exhibiting catalytic combustion characteristics and the gas concentration. [Figure 8] FIG. 8 is a table showing the relationship between the ethanol concentration (ppb) and the output (mV) of the TVOC sensor and the ethanol sensor, and the ratio. [Figure 9] FIG. 9 is a graph showing the relationship between the ethanol concentration (ppb) and the output (mV) of the TVOC sensor and the ethanol sensor. [Figure 10] FIG. 10 is a diagram showing an outline of an evaluation device. [Figure 11] FIG. 11 is a timing chart showing a pulse drive pattern in an evaluation test of an acetone detector. [Figure 12] FIG. 12 is a graph showing the test results, which is a graph showing the relationship between the output (mV) of the TVOC sensor and the ethanol sensor and the elapsed time (min). [Figure 13] FIG. 13 is a double logarithmic graph showing the test results, and is a double logarithmic graph showing the relationship between the output (mV) of the TVOC sensor and the ethanol sensor and the acetone concentration (ppm) of the acetone gas. [Figure 14] FIG. 14 is a double logarithmic graph showing the relationship between the acetone concentration (ppm) and the output (mV) of the acetone detector when the pulse voltage is switched between a LOW voltage and a HIGH voltage and when the pulse voltage is switched between ON and OFF. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The present invention will be described below along with preferred embodiments. Note that the present invention is not limited to the embodiments shown below, and can be modified as appropriate within the scope of the present invention. In addition, in the embodiments shown below, some configurations are omitted from illustration and description, but for the details of the omitted technologies, publicly known or well-known technologies are applied as appropriate within the scope of no contradiction with the contents described below.

[0010] Fig. 1 is a diagram showing the schematic configuration of an acetone detector 1 according to one embodiment of the present invention. The acetone detector 1 shown in this figure is an adsorption and combustion type gas sensor that detects acetone in breath. Here, breath contains ethanol as an interference gas in addition to acetone to be detected. Therefore, the acetone detector 1 of this embodiment determines the concentration of acetone in breath using a calculation method that removes the influence of ethanol in breath.

[0011] The acetone detector 1 includes a TVOC sensor 10, an ethanol sensor 20, and an MCU (Micro Control Unit) 30. The TVOC sensor 10 outputs a TVOC value V m1 The TVOC sensor 10 constitutes a combustion-type gas sensor in which the temperature changes. The TVOC sensor 10 is a bridge circuit including a sensing element 11 and a compensating element 12. Acetone, which is the detection target, and ethanol, which is an interference gas, exhibit adsorption and combustion characteristics with respect to the sensing element 11. On the other hand, acetone and ethanol do not exhibit either adsorption or catalytic combustion characteristics with respect to the compensating element 12. The adsorption and catalytic combustion characteristics will be described later.

[0012] The TVOC sensor 10 includes a pair of parallel circuits 10A and 10B. The pair of parallel circuits 10A and 10B branch off from a conductor connected to a heater drive circuit 31. The pair of branched parallel circuits 10A and 10B are recombined with a conductor connected to a ground terminal Vg. In one parallel circuit 10A, a detection element 11 and a compensation element 12 are connected in series. In the other parallel circuit 10B, a pair of fixed resistors R1 and R2 and a variable resistor VR provided between the pair of fixed resistors R1 and R2 are connected in series. In this embodiment, the compensation element 12 is disposed on the power supply terminal Vd side, and the detection element 11 is disposed on the ground terminal Vg side.

[0013] The TVOC sensor 10 includes a terminal 13 connected between the detection element 11 and the compensation element 12, and a terminal 14 connected to a variable resistor VR. The TVOC sensor 10 outputs an output value V m1 to the MCU 30. In this TVOC sensor 10, the sensing element 11 exhibits sensitivity to acetone and ethanol, whereas the compensating element 12 exhibits no sensitivity to acetone, ethanol, and other gases.

[0014] Examples of the sensing element 11 include a bead type in which a coil is embedded in a bead-shaped carrier and a catalyst is supported on the carrier, and a microsensor type in which a heater is patterned on a substrate and a catalyst is supported, etc. Examples of the compensating element 12 include a bead type in which a coil is embedded in a bead-shaped carrier that does not support a catalyst, and a microsensor type in which a heater is patterned on a substrate and a catalyst is not supported, etc. Below, the bead-type sensing element 11 and compensating element 12 will be described as examples, but the present invention is not limited to these.

[0015] The detection element 11 includes a heater 111 and a combustion catalyst layer 112. The heater 111 generates heat when a voltage is applied. An example of the material of the heater 111 is platinum (Pt). The combustion catalyst layer 112 includes a carrier in which the heater 111 is embedded and a catalyst supported on the carrier, and is heated by the heater 111. The combustion catalyst layer 112 exhibits an adsorption combustion characteristic in which acetone, which is the detection target, and ethanol, which is an interference gas, are physically adsorbed and burned. Examples of the material of the combustion catalyst layer 112 include palladium / aluminum oxide (Pd / Al2O3), palladium / tin oxide (Pd / SnO2), palladium / cerium oxide (Pd / CeO2), and palladium / zirconium oxide (Pd / ZrO2). Examples of the material of the carrier of the combustion catalyst layer 112 include aluminum oxide, tin oxide, cerium oxide, and zirconium oxide. Palladium can be exemplified as the catalyst material of the combustion catalyst layer 112. Furthermore, palladium / aluminum oxide, which is one example of the material of the combustion catalyst layer 112, can be exemplified by aluminum oxide in which palladium is supported at a ratio of 15 wt %. Note that while platinum may be used as the catalyst material of the combustion catalyst layer 112, palladium, which does not adsorb carbon monoxide, is preferable as the catalyst material of the combustion catalyst layer 112.

[0016] It is polar molecular gases that are physically adsorbed to the combustion catalyst layer 112. Examples of polar molecular gases include alcohols such as ethanol, methanol, propanol, and butanol, and VOCs such as acetone, formaldehyde, toluene, xylene, and styrene. By using palladium as the catalyst material for the combustion catalyst layer 112, carbon monoxide (CO), methane (CH4), isobutane (C4H 10 ) or other non-polar or low polar molecular gases can be prevented from being physically adsorbed onto the combustion catalyst layer 112.

[0017] Compensating element 12 includes heater 121 and carrier 122. Heater 121 generates heat when a voltage is applied. An example of the material of heater 121 is platinum. Heater 121 is embedded in carrier 122, and is heated by heater 121. An example of the material of carrier 122 is aluminum oxide, tin oxide, cerium oxide, zirconium oxide, etc.

[0018] The ethanol sensor 20 outputs a value V m2 The ethanol sensor 20 is a bridge circuit including a detection element 21 and a compensation element 22. Ethanol exhibits different adsorption and combustion characteristics with respect to the detection element 21 and the compensation element 22.

[0019] The ethanol sensor 20 includes a pair of parallel circuits 20A and 20B. The pair of parallel circuits 20A and 20B branch off from a conductor connected to a heater drive circuit 31. The pair of branched parallel circuits 20A and 20B are recombined with a conductor connected to a ground terminal Vg. In one parallel circuit 20A, a detection element 21 and a compensation element 22 are connected in series. In the other parallel circuit 20B, a pair of fixed resistors R1 and R2 and a variable resistor VR provided between the pair of fixed resistors R1 and R2 are connected in series. In this embodiment, the compensation element 22 is disposed on the heater drive circuit 31 side, and the detection element 21 is disposed on the ground terminal Vg side.

[0020] The ethanol sensor 20 includes a terminal 23 connected between the detection element 21 and the compensation element 22, and a terminal 24 connected to a variable resistor VR. The ethanol sensor 20 outputs an output value V m2 to the MCU 30. In this ethanol sensor 20, the sensing element 21 and the compensation element 22 exhibit different response characteristics (adsorption and combustion characteristics) to ethanol, but exhibit similar response characteristics to gases other than ethanol.

[0021] Examples of the detection element 21 and the compensation element 22 include a bead type in which a coil is embedded in a bead-shaped carrier and a catalyst is supported on the carrier, and a microsensor type in which a heater is patterned on a substrate and a catalyst is supported, etc. Below, the detection element 21 and compensation element 22 of the bead sensor type will be described as an example, but the present invention is not limited to this.

[0022] The detector element 21 includes a heater 211 and a combustion catalyst layer 212. The heater 211 generates heat when a voltage is applied. An example of the material of the heater 211 is platinum. The combustion catalyst layer 212 includes a carrier in which the heater 211 is embedded and a catalyst supported on the carrier, and is heated by the heater 211. This combustion catalyst layer 212 exhibits a characteristic of physically adsorbing and burning ethanol. Examples of the material of the combustion catalyst layer 212 include platinum / aluminum oxide (Pt / Al2O3) containing lanthanum oxide (La2O3), platinum / aluminum oxide containing copper oxide (CuO), and the like. Examples of the catalyst material of the combustion catalyst layer 212 include platinum containing lanthanum oxide and platinum containing copper oxide. Also, an example of the material of the carrier of the combustion catalyst layer 212 is aluminum oxide. Furthermore, an example of platinum / aluminum oxide containing lanthanum oxide, which is an example of the combustion catalyst layer 212, is one in which platinum is supported at a ratio of 15 wt% to the aluminum oxide and lanthanum oxide is added at a ratio of 30 wt% to the aluminum oxide in the platinum / aluminum oxide catalyst. An example of platinum / aluminum oxide containing copper oxide, which is an example of the combustion catalyst layer 212, is one in which platinum is supported at a ratio of 15 wt% to the aluminum oxide and copper oxide is added at a ratio of 30 wt% to the aluminum oxide in the platinum / aluminum oxide catalyst.

[0023] The compensation element 22 includes a heater 221 and a combustion catalyst layer 222. The heater 221 generates heat when a voltage is applied. An example of the material of the heater 221 is platinum. The combustion catalyst layer 222 includes a carrier in which the heater 221 is embedded and a catalyst supported on the carrier, and is heated by the heater 221. This combustion catalyst layer 222 exhibits a characteristic of physically adsorbing and burning ethanol. Examples of the material of the combustion catalyst layer 222 include platinum / aluminum oxide containing chromium oxide (Cr2O3), platinum / aluminum oxide containing α-iron oxide (α-Fe2O3), and platinum / aluminum oxide containing tungsten oxide (WO3). Examples of the catalyst material of the combustion catalyst layer 222 include platinum containing chromium oxide, platinum containing α-iron oxide, and platinum containing tungsten oxide. An example of the material of the carrier of the combustion catalyst layer 222 is aluminum oxide. Furthermore, an example of platinum / aluminum oxide containing chromium oxide, which is an example of the combustion catalyst layer 222, is one in which platinum is supported at a ratio of 15 wt% to the aluminum oxide and chromium oxide is added at a ratio of 30 wt% to the aluminum oxide in the platinum / aluminum oxide catalyst.An example of platinum / aluminum oxide containing tungsten oxide, which is an example of the combustion catalyst layer 222, is one in which platinum is supported at a ratio of 15 wt% to the aluminum oxide and tungsten oxide is added at a ratio of 30 wt% to the aluminum oxide in the platinum / aluminum oxide catalyst.

[0024] Not only polar molecular gases such as ethanol, but also non-polar molecular gases or low polar molecular gases such as carbon monoxide are physically adsorbed to the combustion catalyst layers 212 and 222. However, while ethanol shows different response characteristics in the combustion catalyst layers 212 and 222, other gases show similar response characteristics in the combustion catalyst layers 212 and 222. Therefore, the output value V from the ethanol sensor 20 differs between ethanol and other gases. m2 This output value V m2 This difference makes it possible to distinguish ethanol from other gases.

[0025] The MCU 30 includes a heater drive circuit 31, an output detector 32, a memory 33, and an arithmetic circuit 34. The heater drive circuit 31 is connected to a power supply terminal Vd of the TVOC sensor 10 and a power supply terminal Vd of the ethanol sensor 20. The heater drive circuit 31 generates a pulsed drive voltage and applies it to the TVOC sensor 10 and the ethanol sensor 20.

[0026] The output detector 32 is connected to the terminals 13 and 14 of the TVOC sensor 10 and the terminals 23 and 24 of the ethanol sensor 20. The output detector 32 detects the above-mentioned output value V m1 ,V m2 is amplified and output to the arithmetic circuit 34.

[0027] The memory 33 stores a program for causing the MCU 30 to execute processing. The arithmetic circuit 34 calculates the output value V m1 ,V m2 The concentration of acetone is calculated based on this.

[0028] Fig. 2 is an exploded perspective view showing the acetone detector 1 shown in Fig. 1. As shown in this figure, the acetone detector 1 comprises a substrate 2, a TVOC sensor 10 and an ethanol sensor 20 mounted on the substrate 2, a cap 3 that covers the substrate 2, and leads 4 extending from the substrate 2. That is, in the acetone detector 1, the TVOC sensor 10 and the ethanol sensor 20, which have the same gas detection principle (both are adsorption and combustion type), are mounted in a common package 5.

[0029] 3 is a diagram showing a schematic configuration of the acetone detector 1 shown in FIGS. 1 and 2, and a diagram showing the relationship between the outputs of the TVOC sensor 10 and the ethanol sensor 20 and the output of the acetone detector 1. As shown in this diagram, an output value V m1 is output, and the output value V m2 will be output.

[0030] Fig. 4 is a diagram showing the output characteristics of the TVOC sensor 10 and the ethanol sensor 20. The lower part of Fig. 4 shows the output (mV) of the TVOC sensor 10 for air containing no ethanol or acetone, ethanol gas with a concentration of 3 ppm, and acetone gas with a concentration of 3 ppm. The upper part of Fig. 4 shows the output (mV) of the ethanol sensor 20 for air containing no ethanol or acetone, ethanol gas with a concentration of 3 ppm, and acetone gas with a concentration of 3 ppm. As shown in Fig. 4, the TVOC sensor 10 and the ethanol sensor 20 are sensitive to ethanol. On the other hand, while the TVOC sensor 10 is sensitive to acetone, the ethanol sensor 20 is not sensitive to acetone.

[0031] Here, when the acetone concentration in the breath is measured using the acetone detector 1, the TVOC sensor 10 is sensitive not only to acetone in the breath but also to ethanol, and the output value V corresponding to the combustion of acetone and ethanol in the breath in the combustion catalyst layer 112 is m1 Therefore, in order to detect the acetone concentration with the acetone detector 1, the output value V of the TVOC sensor 10 must be m1 Therefore, in the acetone detector 1 of this embodiment, synchronized pulse voltages are applied to the ethanol sensor 20 and the TVOC sensor 10, and the output value V of the ethanol sensor 20 is m2 Based on this, the output value V of the TVOC sensor 10 m1 The acetone output (V m1 -V m2 ) is required.

[0032] 5 is a diagram showing a schematic diagram of an adsorption and combustion reaction of gas G occurring in the detection element 11 of the TVOC sensor 10. This diagram shows a reaction in which polar molecular gases such as alcohols and VOCs as gas G are physically adsorbed onto the combustion catalyst layer 112 and combusted.

[0033] 5, gas G, which is a polar molecular gas such as alcohols or VOCs, is physically adsorbed to the combustion catalyst layer 112 while the heater 111 is OFF (or at a low voltage), and is combusted on the combustion catalyst layer 112 while the heater 111 is ON (or at a high voltage). Then, gas G is decomposed into water (H2O) and carbon dioxide (CO2) by combustion while the heater 111 is ON.

[0034] Here, since the gas G is physically adsorbed to the combustion catalyst layer 112 while the heater 111 is OFF (or at LOW voltage), the gas G is instantly combusted after the heater 111 is turned ON (or at HIGH voltage) and the temperature of the gas G is raised to or above the combustion temperature. In this manner, the combustion of the gas G that has been adsorbed to the combustion catalyst layer 112 is called adsorption combustion. In this adsorption combustion, the response from when the heater 111 is turned ON (or at HIGH voltage) until combustion occurs is relatively fast compared to catalytic combustion, in which a non-adsorbable gas collides with a catalyst and combusts.

[0035] Although not shown, the detection element 21 and compensation element 22 of the ethanol sensor 20 also adsorb and burn ethanol gas, similar to the detection element 11 of the TVOC sensor 10. The adsorption and combustion of ethanol gas in the ethanol sensor 20 has a faster reaction time from when the heaters 211, 221 are turned ON (or HIGH voltage) to when combustion occurs, compared to catalytic combustion.

[0036] FIG. 6 is a diagram for explaining the adsorption and combustion characteristics of the TVOC sensor 10 and the ethanol sensor 20. As shown in this figure, in the acetone detector 1 of this embodiment, the heater driving circuit 31 (see FIG. 1) applies a synchronized pulse voltage to the heaters 111, 121 of the TVOC sensor 10 and the heaters 211, 221 of the ethanol sensor 20 at a period of several seconds to several tens of seconds. Here, the heater driving circuit 31 pulse-drives the TVOC sensor 10 and the ethanol sensor 20 by periodically applying two pulses having LOW / HIGH levels. The arithmetic circuit 34 calculates the concentration of acetone based on an output value that is the difference between the output value of the first pulse and the output value of the second pulse of the TVOC sensor 10 and an output value that is the difference between the output value of the first pulse and the output value of the second pulse of the ethanol sensor 20 (see FIG. 11).

[0037] Here, the period of several seconds to several tens of seconds during which the pulse voltage applied to the heaters 111, 121, 211, 221 is OFF or LOW is the adsorption period during which acetone and ethanol are adsorbed to the combustion catalyst layer 112 of the TVOC sensor 10, and ethanol is adsorbed to the combustion catalyst layers 212, 222 of the ethanol sensor 20. When a LOW voltage is applied to the heaters 111, 121, 211, 221 during the adsorption period, it is preferable to set the LOW voltage so that the temperature of the combustion catalyst layers 112, 212, 222 is a temperature (e.g., 100 to 200°C) at which the moisture adsorbed to the catalyst can be removed.

[0038] The period of 50 to 100 ms during which the pulse voltage applied to the heaters 111, 121, 211, and 221 is ON or HIGH voltage is the combustion period during which acetone and ethanol adsorbed on the combustion catalyst layer 112 of the TVOC sensor 10 are burned, and ethanol adsorbed on the combustion catalyst layers 212 and 222 of the ethanol sensor 20 is burned. During this combustion period, the outputs (adsorption and combustion outputs) of the TVOC sensor 10 and the ethanol sensor 20 appear as peak outputs. That is, during the period of 50 to 100 ms during which the pulse voltage is ON or HIGH voltage, the TVOC sensor 10 shows high sensitivity to acetone and ethanol, and the ethanol sensor 20 shows high sensitivity to ethanol. In contrast, in catalytic combustion caused by collision of a non-adsorbing gas against a catalyst, the output does not appear as a peak output during the combustion period, but only appears as a steady output.

[0039] FIG. 7 is a linear graph showing the relationship between the output of a gas sensor exhibiting adsorption combustion characteristics (adsorption combustion output in the figure) and gas concentration, and the relationship between the output of a gas sensor exhibiting catalytic combustion characteristics (catalytic combustion output in the figure) and gas concentration. In this semi-logarithmic graph, the gas concentration on the X-axis is shown in logarithm. As shown in this semi-logarithmic graph, the ratio between the catalytic combustion output and the logarithm of the gas concentration is constant. In contrast, the ratio between the adsorption combustion output and the logarithm of the gas concentration is not constant, and is larger than the ratio between the catalytic combustion output and the logarithm of the gas concentration. In other words, it can be confirmed from this semi-logarithmic graph that the gas sensor exhibiting adsorption combustion characteristics exhibits higher sensitivity to low concentration gases than the gas sensor exhibiting catalytic combustion characteristics.

[0040] In the gas sensor showing the adsorption combustion characteristics, when the adsorbed gas is burned during the ON or HIGH voltage pulse voltage period, the peak output gradually decreases, and at the end of this period, a steady output due to catalytic combustion (combustion caused by gas molecules flying around colliding with a catalyst) remains. Here, the peak output is 5 to 10 times larger than the steady output, and is particularly large when the gas concentration is low. Therefore, in the gas sensor showing the adsorption combustion characteristics, the peak output is easier to distinguish from noise than in the gas sensor showing the catalytic combustion characteristics, so that the S / N ratio can be kept high and the detection accuracy can be improved.

[0041] In this manner, in the acetone detector 1 of this embodiment, the TVOC sensor 10 exhibits high sensitivity to acetone and ethanol contained in breath, and the ethanol sensor 20 exhibits high sensitivity to ethanol contained in breath. m1 and the output value V of the ethanol sensor 20 m2 In response to the above, the concentration of acetone is calculated using an arithmetic expression stored in the memory 33.

[0042] Fig. 8 is a table showing the relationship between the ethanol concentration (ppb), the output (mV) of the TVOC sensor 10 and the ethanol sensor 20, and the ratio α described below. Also, Fig. 9 is a graph showing the relationship between the ethanol concentration (ppb), and the output (mV) of the TVOC sensor 10 and the ethanol sensor 20. The outputs of the TVOC sensor 10 and the ethanol sensor 20 shown in these tables and graphs appear for ethanol gas that does not contain acetone and contains only ethanol.

[0043] As shown in these tables and graphs, the output of the TVOC sensor 10 for ethanol gas is a higher voltage than the output of the ethanol sensor 20 for ethanol gas. The ratio of the output of the TVOC sensor 10 for ethanol gas to the output of the ethanol sensor 20 for ethanol gas is as shown in the table of Fig. 8. Specifically, the ratio α is 3.95 for ethanol gas with a concentration of 1 to 100 ppb, the ratio α is 6.08 for ethanol gas with a concentration of 101 to 200 ppb, the ratio α is 7.79 for ethanol gas with a concentration of 201 to 400 ppb, the ratio α is 4.83 for ethanol gas with a concentration of 401 to 1000 ppb, and the ratio α is 2.44 for ethanol gas with a concentration of 1001 to 3000 ppb.

[0044] In the acetone detector 1 of this embodiment, a table showing the relationship between the output of the ethanol sensor 20 (hereinafter referred to as the ethanol sensor output) and the ethanol concentration, and a table showing the relationship between the ethanol concentration and the above-mentioned ratio α are stored in the memory 33 (see FIG. 1) of the MCU 30 shown in FIG. 1. The memory 33 also stores an arithmetic equation (the following equation (1)) for the acetone output of the TVOC sensor 10 (hereinafter referred to as the acetone output) defined according to the ethanol concentration.

[0045] Z = Y - αX … (1) where Z is the acetone output, Y is the output of the acetone sensor (hereinafter referred to as the acetone sensor output), and X is the ethanol sensor output. Also, α is the above ratio, which is 3.95 when the ethanol concentration is 1 to 100 ppb, 6.08 when the ethanol concentration is 101 to 200 ppb, 7.79 when the ethanol concentration is 201 to 400 ppb, 4.83 when the ethanol concentration is 401 to 1000 ppb, and 2.44 when the ethanol concentration is 1001 to 3000 ppb.

[0046] An evaluation test of the acetone detector 1 of this embodiment will be described below.

[0047] 10 is a diagram showing an outline of evaluation device 100. Evaluation device 100 shown in this figure is a device (gas chromatograph) used in evaluation tests of acetone detector 1. This evaluation device 100 includes cylinders 101 and 102, pressure reducing regulators 103 and 104, a thermostatic bath 100A, a mass flow controller (MFC) 105, a flowmeter 106, a mixer 107, a measurement chamber 108, a humidifier 109 equipped with a flowmeter, and an exhaust pump 110.

[0048] Cylinder 101 is filled with acetone gas with a concentration of 5 ppm, and cylinder 102 is filled with air. Pressure reducing regulator 103 is provided corresponding to cylinder 101, and pressure reducing regulator 104 is provided corresponding to cylinder 102. Mass flow controller 105, flow meter 106, mixer 107, measurement chamber 108, humidifier 109, and exhaust pump 110 are housed in thermostatic bath 100A. Thermostatic bath 100A is set to 24° C. and 30% RH. Humidifier 109 includes a dry flow meter 109D and a wet flow meter 109W.

[0049] Acetone gas with a concentration of 5 ppm is supplied to mixer 107 with its pressure adjusted by pressure reducing regulator 103 and its flow rate controlled by mass flow controller 105. On the other hand, air is pressure adjusted by pressure reducing regulator 104, humidified by humidifier 109, and supplied to mixer 107. In mixer 107, acetone gas with a concentration of 5 ppm and air are mixed. In evaluation device 100, the acetone concentration of acetone gas supplied to measurement chamber 108 can be adjusted within the range of 0.01 to 5 ppm by adjusting the flow rates of acetone gas and air supplied to mixer 107.

[0050] The measurement chamber 108 has a volume of 2.5 L, and an acetone detector 1 (see FIG. 1, etc.) is installed in this measurement chamber 108. This acetone detector 1 exhibits sensitivity to acetone in the gas supplied to the measurement chamber 108. The gas supplied to the measurement chamber 108 is exhausted from the measurement chamber 108 by an exhaust pump 110. The total flow rate of this evaluation device 100 is 500 cc / min.

[0051] In this evaluation test, air was first supplied from cylinder 102 to measurement chamber 108, and then acetone gas with acetone concentrations of 0.1 ppm, 0.2 ppm, 0.4 ppm, 1.0 ppm, and 3.0 ppm was supplied to measurement chamber 108 at 10-minute intervals, and the output of acetone detector 1 was monitored. The pulse drive pattern of acetone detector 1 is as shown in FIG.

[0052] 11 is a timing chart showing a pulse drive pattern in an evaluation test of the acetone detector 1. In this evaluation test, the acetone detector 1 was started every time acetone gas with an adjusted acetone concentration was supplied to the measurement chamber 108 (see FIG. 10) at 10-minute intervals. After starting the acetone detector 1, (1) application of a LOW voltage for a predetermined time L1 (e.g., 1 to 30 s), (2) application of a HIGH voltage for a predetermined time S1, (3) application of a LOW voltage for a predetermined time L2, and (4) application of a HIGH voltage for a predetermined time S2 were performed in the order of (1) to (4), and the pulse drive pattern of (1) to (4) was repeated.

[0053] The predetermined time L1 may be set to, for example, 1 to 30 s, which is longer than the predetermined times L2, S1, and S2, and is 10 s in this evaluation test. The period of the predetermined time L1 is an adsorption period during which acetone in the gas is adsorbed to the catalyst. The predetermined times S1 and S2 may be set to, for example, 50 to 100 ms, which is shorter than the predetermined time L1, and is 50 ms in this evaluation test. The period of the predetermined time S1 is a combustion period during which acetone adsorbed to the catalyst is burned. The predetermined time L2 may be set to, for example, 30 to 200 ms, which is shorter than the predetermined time L1, and is 30 ms in this evaluation test. The period of the predetermined time L2 is a non-adsorption period during which acetone in the gas is not adsorbed to the catalyst.

[0054] The LOW voltage was set to a voltage that could heat the catalyst to a temperature (e.g., 100-200°C) at which the moisture adsorbed on the catalyst could be removed, and the power was set to 16-17 mW. The HIGH voltage was set to a voltage that could heat the catalyst to a temperature (e.g., 460°C) at which the acetone adsorbed on the catalyst could be burned, and the power was set to 53 mW.

[0055] In this evaluation test, two pulses of HIGH voltage were applied every 10 seconds. At that time, the acetone adsorbed on the catalyst during the adsorption period of the predetermined time L1 was burned during the subsequent combustion period of the predetermined time S1, and the output value Vs1 of the TVOC sensor 10 and the ethanol sensor 20 was obtained 10 ms after the HIGH voltage was reached. In addition, the output value Vs2 of the TVOC sensor 10 and the ethanol sensor 20 was obtained 10 ms after the HIGH voltage was reached during the period of the predetermined time S2 following the non-adsorption period of the predetermined time L2. This output value Vs2 corresponds to the output in a state in which it can be said that acetone is not adsorbed on the catalyst (concentration is 0). The difference (Vs1-Vs2) between the output value Vs1 and the output value Vs2 was calculated to obtain the output value Vs1 of the TVOC sensor 10 and the ethanol sensor 20. m1 ,V m2 In other words, the difference between the state in which the gas molecules adsorbed on the catalyst are burned and the state in which the gas molecules are not adsorbed on the catalyst is the output value V m1 ,V m2 It was decided.

[0056] Fig. 12 is a graph showing the test results, which shows the relationship between the output (mV) of the TVOC sensor 10 and the ethanol sensor 20 and the elapsed time (min). Fig. 13 is a double logarithmic graph showing the test results, which shows the relationship between the output (mV) of the TVOC sensor 10 and the ethanol sensor 20 and the acetone concentration (ppm) of acetone gas. As shown in these graphs, it was confirmed that an output of the TVOC sensor 10 could be obtained for acetone gas of a low concentration (0.1 ppm).

[0057] 14 is a double logarithmic graph showing the relationship between the acetone concentration (ppm) and the output (mV) of the acetone detector 1 when the pulse voltage is switched between a LOW voltage and a HIGH voltage and when the pulse voltage is switched between ON and OFF. The LOW-HIGH results shown in this double logarithmic graph were obtained by applying a LOW voltage to the heaters 111, 121, 211, and 221 during the adsorption period in which acetone and ethanol are adsorbed into the combustion catalyst layers 112, 212, and 222, and raising the temperature of the combustion catalyst layers 112, 212, and 222 to 100 to 200°C.

[0058] As shown in this double logarithmic graph, the detection limits when the pulse voltage is switched between ON and OFF are 500 mV (output) and 20 ppm (acetone concentration), whereas the detection limits when the pulse voltage is switched between LOW and HIGH are 50 mV (output) and 0.05 ppm (acetone concentration), which are relatively low voltages and low concentrations. From the above, it was confirmed that significant effects can be obtained by switching the pulse voltage applied to the TVOC sensor 10 and the ethanol sensor 20 between LOW and HIGH voltages.

[0059] As described above, the acetone detector 1 of the present embodiment pulse-drives the adsorption and combustion type TVOC sensor 10 and the adsorption and combustion type ethanol sensor 20, and outputs an output value V from the TVOC sensor 10 for acetone and ethanol adsorbed on the catalyst of the TVOC sensor 10 during the adsorption period when the pulse voltage is OFF or LOW. m1 The output value V from the ethanol sensor 20 is obtained for the ethanol adsorbed on the catalyst of the ethanol sensor 20 during the adsorption period when the pulse voltage is OFF or LOW. m2 Then, the calculation circuit 34 of the MCU 30 calculates the output value V m1 ,V m2The concentration of acetone is calculated based on the above. Generally, it is believed that a semiconductor gas sensor is more suitable for detecting low concentrations of acetone than a catalytic combustion gas sensor. However, the acetone detector 1 of this embodiment, which is composed of the TVOC sensor 10 and the ethanol sensor 20, both of which are adsorption combustion gas sensors, is capable of exhibiting high sensitivity to low concentrations of acetone, equal to or higher than that of a semiconductor gas sensor.

[0060] Furthermore, the TVOC sensor 10 and the ethanol sensor 20 are pulse-driven by periodically applying two pulses having low and high levels, and the difference (Vs1-Vs2) between the output value Vs1 of the first pulse and the output value Vs2 of the second pulse of the TVOC sensor 10 is calculated as an output value V m1 The difference (Vs1-Vs2) between the output value Vs1 of the first pulse and the output value Vs2 of the second pulse of the ethanol sensor 20 is defined as the output value V m2 In other words, the difference between the state in which the gas molecules adsorbed on the catalyst are burned and the state in which the gas molecules are not adsorbed on the catalyst is the output value V of the TVOC sensor 10 and the ethanol sensor 20. m1 ,V m2 This makes it possible to improve the detection accuracy of the TVOC sensor 10 and the ethanol sensor 20.

[0061] Furthermore, since the acetone detector 1 of this embodiment is composed of the TVOC sensor 10 and the ethanol sensor 20, which are based on the same principle of adsorption and combustion, it is possible to mount the TVOC sensor 10 and the ethanol sensor 20 in a common package 5. This makes it possible to improve the accuracy of acetone detection and reduce costs.

[0062] Furthermore, because semiconductor gas sensors do not include a temperature and humidity compensating element, there is a large deviation in characteristics with respect to temperature and humidity, and a large correction coefficient must be input, resulting in large errors.In contrast, in the acetone detector 1 of this embodiment, the TVOC sensor 10 and the ethanol sensor 20 are composed of detection elements 11, 21 and compensation elements 12, 22, and the temperature and humidity characteristics of these elements are similar, so that the dependency on temperature and humidity is relatively small, and errors are small.

[0063] In the acetone detector 1 of this embodiment, the output value V of the ethanol sensor 20 is calculated based on the ratio α, which is a correction coefficient set according to the concentration of ethanol. m2 The corrected value (αX in the above formula (1)) and the output value V of the TVOC sensor 10 m1 The acetone concentration is calculated based on the difference (Z=Y-αX in equation (1) above) from (Y in equation (1) above). This makes it possible to remove the effects of ethanol as an interference gas, and to remove errors due to individual differences in the ethanol sensor 20, thereby enabling acetone to be detected with high accuracy.

[0064] Furthermore, in the acetone detector 1 of this embodiment, the TVOC sensor 10 and the ethanol sensor 20 are pulse-driven by switching between a LOW voltage, which is higher than the OFF voltage, and a HIGH voltage, which is higher than the LOW voltage. This makes it possible to evaporate moisture adsorbed to the catalysts of both sensors during the adsorption period when acetone and ethanol are adsorbed to the catalysts of the TVOC sensor 10 and the ethanol sensor 20. Therefore, it is possible to obtain outputs from both sensors by applying a HIGH voltage with the influence of moisture adsorbed to the catalysts removed, and it is also possible to obtain outputs for low concentrations of acetone (see FIG. 14).

[0065] Furthermore, in the acetone detector 1 of this embodiment, the catalyst included in the TVOC sensor 10 is palladium. This makes it possible to suppress adhesion of gas molecules with low adsorptivity, such as carbon monoxide, to the catalyst of the TVOC sensor 10, and to suppress the output of the TVOC sensor for non-target gases, such as carbon monoxide.

[0066] Here, it is conceivable to use mesoporous silica (m-SiO2) as the carrier, but mesoporous silica is a hydrophilic material and therefore easily absorbs water molecules. When water molecules are absorbed onto the carrier, the output of the sensor becomes unstable and the S / N ratio decreases. In contrast, in the acetone detector 1 of this embodiment, the carrier of the combustion catalyst layer 112 provided in the TVOC sensor 10 is aluminum oxide, which relatively suppresses the absorption of water molecules onto the carrier, thereby stabilizing the output of the TVOC sensor 10 and increasing the S / N ratio.

[0067] The present invention has been described above based on the above embodiment, but the present invention is not limited to the above embodiment, and modifications may be made without departing from the spirit of the present invention, and known or well-known technologies may be appropriately combined. [Explanation of symbols]

[0068] 1 Acetone detector 5 Package 10 TVOC sensor (first bridge circuit) 11 Detection element (first catalytic element) 111 Heater (1st heater) 112 Combustion catalyst layer (first catalyst) 12 Compensation element (first compensation element) 121 Heater 122 Carrier 20 Ethanol sensor (second bridge circuit) 21 Detector element (second catalytic element) 211 Heater (Second Heater) 212 Combustion catalyst layer (second catalyst) 221 Heater (Second Heater) 222 Combustion catalyst layer (second catalyst) 22 Compensation element (second compensation element) 221 Heater 222 Combustion catalyst layer (catalyst) 30 MCU (control unit) V m1 Output Value V m2 Output Value V s1 Output Value V s2 Output Value X Output Value Y Output Value α correction coefficient αX correction value

Claims

1. a first catalyst element including a first heater and a first catalyst that adsorbs acetone and ethanol and burns them with the heat of the first heater; and a first compensation element including a heater and a carrier heated by the heater, a first bridge circuit whose output value changes due to the combustion of acetone and ethanol; a second catalyst element including a second heater and a second catalyst that adsorbs ethanol and burns it with the heat of the second heater; and a second bridge circuit including a heater and a second compensation element including a catalyst heated by the heater, the second bridge circuit having an output value that changes with the combustion of ethanol; a control unit that drives the first bridge circuit and the second bridge circuit by periodically applying two pulses having LOW / HIGH levels, and determines the concentration of acetone based on a first output value that is a difference between an output value of a first pulse and an output value of a second pulse of the first bridge circuit, and a second output value that is a difference between an output value of the first pulse and an output value of a second pulse of the second bridge circuit; An acetone detector comprising:

2. The control unit is 2. The acetone detector according to claim 1, wherein the concentration of acetone is determined based on a difference between a correction value obtained by correcting the second output value of the second bridge circuit by a correction coefficient set in accordance with a concentration of ethanol and the first output value of the first bridge circuit.

3. 3. The acetone detector according to claim 1, wherein the first bridge circuit and the second bridge circuit are provided in a common package.

4. the first catalyst is palladium; 4. The acetone detector according to claim 1, wherein the support that supports the first catalyst is aluminum oxide.

Images