NDIR freon sensor

Abstract

CONSTITUTION: To provide an NDIR Freon sensor comprising a light source having an MEMS heater, a thermopile, a band-pass filter and an optical cell housing the light source, the thermopile and the band-pass filter, the light source having a black body film on a surface of the MEMS heater, and having no airtight windows between the black body film and the external part of the light source, the thermopile having a black body film covering a hot junction of the thermopile, the band-pass filter having a transmission center wavelength of about 9 μm.EFFECT: An NDIR Freon sensor can detect Freon with high sensitivity.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to an NDIR Freon sensor that uses the absorption of infrared radiation around 9 μm. [Background technology]

[0002] Freons such as R-32 (CH2F2) have a strong absorption peak near 9 μm due to C-F stretching vibration. Freons containing Cl atoms have an absorption peak near 10 μm due to C-Cl stretching vibration (Patent Document 1). Detecting Freons based on absorption near 9 μm can reduce interference with water vapor, CO2, and even organic solvents.

[0003] Many NDIR (non-dispersive infrared) gas sensors use a tungsten lamp with a tungsten coil sealed in a glass bulb. However, because glass does not transmit far infrared rays, tungsten lamps cannot be used around 9 μm. InSb, which is highly sensitive to infrared rays in the 3 to 5 μm range, is often used as the light receiving element in NDIR. However, compound semiconductors such as InSb require cooling for use around 9 μm, and this requires cooling the compound semiconductor with a Peltier element or liquid nitrogen. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2003-57178A Summary of the Invention [Problem to be solved by the invention]

[0005] The object of the present invention is to provide an NDIR gas sensor capable of detecting Freon with high sensitivity. The auxiliary objects of the present invention are as follows. 1) The light source does not require expensive window materials. 2) The photodetector does not require cooling. 3) Preventing interference from water vapor, CO2, and organic solvents in the detection of Freon. [Means for solving the problem]

[0006] The NDIR Freon sensor of the present invention comprises a light source having a MEMS heater, a thermopile, a bandpass filter, and an optical cell containing the light source, thermopile, and bandpass filter. The light source has a blackbody film on the surface of the MEMS heater, and there is no airtight window between the blackbody film and the outside of the light source. The thermopile includes a black film covering the hot junction of the thermopile. The bandpass filter has a central transmission wavelength of around 9 μm.

[0007] As shown in Figure 10, this invention can provide an NDIR Freon sensor with higher sensitivity than conventional Freon sensors. Furthermore, since the heater can be used exposed in a MEMS light source, expensive window materials such as Ge are not required for airtightness. Unlike semiconductor light-receiving elements such as InSb, thermopiles do not require cooling even when detecting wavelengths around 9 μm. As shown in Figure 11, there is little interference from water vapor in Freon detection. Furthermore, as shown in Figure 12, there is little interference from CO2 and organic solvents.

[0008] Preferably, the thickness of both the black body film of the light source and the black body film of the thermopile is 1 μm or more and 200 μm or less. Figure 8 shows the effect of a black body film made of a Pt black body film with a thickness of approximately 100 μm. By providing a black body film, the amount of far-infrared light around 9 μm from the MEMS heater increases several times. Similarly, by attaching a black body to the hot junction of the thermopile, the absorption efficiency of far-infrared light around 9 μm is improved and sensitivity is increased. Since a 100 μm black body film can produce significant effects, the thickness of both the black body film of the light source and the black body film of the thermopile is preferably 1 μm or more and 200 μm or less, more preferably 10 to 200 μm, and particularly preferably 50 to 200 μm. Note that a thickness of 1 μm corresponds to the case where the black body film is formed by a thin-film process such as sputtering.

[0009] Preferably, both the black body film of the light source and the black body film of the thermopile are at least one of a noble metal black film, a black noble metal plating film, a black Cr film, a black electroless nickel film, a black metal oxide powder film, a chromium nitride film, and a metal oxide laminate film. These materials are known as black body materials, and good black body films can be obtained with Pt black, so good black body films can also be obtained with these materials.

[0010] Preferably, the MEMS light source is turned on and off at a predetermined cycle, and the maximum temperature of the heater film when the MEMS light source is on is between 300°C and 500°C. Figure 8 shows the light intensity when the heater temperature is set to 400°C. Since the measurement is performed near 9 μm, Freon can be detected even with a light source whose maximum temperature is lower than 400°C. Furthermore, lowering the maximum temperature is effective in reducing power consumption and minimizing thermal degradation of the heater.

[0011] Preferably, the MEMS light source includes a Pt heater film, measures the resistance of the heater film, calculates a signal corresponding to the maximum temperature of the heater film based on the measured resistance, and controls the power supplied to the heater based on this signal. When a Freon sensor is operated over a wide temperature range, such as from -10°C to 50°C, fluctuations in the heater film temperature due to fluctuations in the ambient temperature become a problem. Another problem is fluctuations in the maximum temperature due to thermal degradation of the heater film. Therefore, by calculating a signal corresponding to the maximum temperature of the heater film based on the resistance of the heater film and controlling the power supplied to the heater, the operating temperature range of the Freon sensor can be expanded and the effects of thermal degradation of the heater can be compensated for. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a plan view of the main part of a MEMS light source according to an embodiment of the present invention; [Figure 2] Cross-sectional view of the MEMS light source of the embodiment [Figure 3] Cross-sectional view of a thermopile according to an embodiment [Figure 4] Block diagram of the NDIR Freon sensor of the embodiment [Figure 5]A diagram showing the infrared absorption spectrum of Freon (R-32) [Figure 6] A diagram showing the infrared absorption spectrum of water vapor and water [Figure 7] Infrared absorption spectrum of ethanol [Figure 8] Actual measurement of the emission spectrum of a MEMS light source with and without a blackbody [Figure 9] Measured emission spectrum of a tungsten lamp light source [Figure 10] 1 is a graph showing the freon concentration dependence of the freon sensor (9 μm) of the embodiment and the freon sensor (3.3 μm) of the comparative example. [Figure 11] Graph showing the relative humidity dependence of the Freon sensor (9 μm) of the embodiment and the Freon sensor (3.3 μm) of the comparative example. [Figure 12] A diagram showing the sensitivity of the Freon sensor (9 μm) of the embodiment and the Freon sensor (3.3 μm) of the comparative example to R-32 and other gases. DETAILED DESCRIPTION OF THE INVENTION

[0013] The best mode for carrying out the present invention is described below. When "to" is used to indicate a range, the range is inclusive of the upper and lower limits. [Example]

[0014] NDIR Freon sensor structure Figures 1 to 4 show an NDIR Freon sensor 40 according to an embodiment. Figures 1 and 2 show a MEMS light source 2, with 4 representing a Si substrate, 6 representing a cavity, and 8 representing a support film, which supports a Pt heater 11 and a blackbody film 10 laminated on the heater 11. 7 represents legs of the support film 8, and 12 represents wire bonding pads. 14 represents a ceramic or other housing that houses the Si substrate 4. An opening 15 in the housing 14 is either left empty or is provided with a cover that covers only part of the opening 15, such as a mesh to protect the Si substrate 4. If a glass or plastic window is provided in the opening 15, infrared rays around 9 μm will be absorbed. Ge windows transmit infrared rays around 9 μm, but are expensive and therefore not used.

[0015] The heater 11 only needs to be heat-resistant and oxidation-resistant. The black film 10 can be a film of a black precious metal such as Pt black or Pd black, or a black precious metal plating film, a black Cr film, a black electroless nickel film, a film of black metal oxide powder (e.g., black iron oxide powder), a chromium nitride film, or a metal oxide laminate film. The black precious metal plating film is, for example, a porous film made of dendritic precious metal particles. The black Cr film and black electroless nickel film can be formed, for example, by electroless plating. The chromium nitride film can be formed by vapor deposition, and the metal oxide laminate film can be formed by laminating metal oxide films with different transmission wavelengths.

[0016] The blackbody film 10 is formed by vapor deposition, sputtering, plating, or the like, and has a film thickness of, for example, 1 μm to 200 μm, preferably 10 μm to 200 μm, and particularly preferably 50 μm to 200 μm. The blackbody film 10 is a light source of infrared radiation in the vicinity of 9 μm, and is layered on the heater 11 and heated by the heat from the heater 10. The blackbody film 10 is heated, for example, at a frequency of 1 Hz to 25 Hz, preferably 1 Hz to 4 Hz, until the maximum temperature reaches, for example, 300°C to 500°C. Preferably, to prevent thermal degradation of the Pt heater 11 and reduce power consumption, the maximum temperature of the blackbody film 10 in each cycle is set to 300°C to 400°C.

[0017] 3 shows a thermopile 22, with a substrate 24 made of silicon or the like and having a cavity 25, a support film 26 on the cavity 25, and supporting the hot junction 27 of the thermopile. 28 is a cold junction, with a blackbody film 10 covering the hot junction 27. 30 is a housing, and a bandpass filter 32 is provided at an opening 31, with a wavelength around 9 μm, particularly a transmission center wavelength of 8.9 μm to 9.4 μm, preferably 9.0 μm to 9.4 μm. The material, film thickness, manufacturing method, etc. of the blackbody film 10 are the same for both the MEMS light source 2 and the thermopile 22.

[0018] 4 shows an example NDIR Freon sensor 40, in which the MEMS light source 2 and thermopile 22 are arranged in an optical cell 42, with 43 indicating an air vent. A power supply 44 supplies power to each component of the sensor 40, and a processing circuit 45 such as a microcomputer energizes the MEMS light source 2 for a fixed time (e.g., 25 ms to 100 ms, corresponding to a drive frequency of 10 to 4 Hz) via a drive circuit 47. As a result, the temperature of the heater 11 changes, for example, between room temperature and the maximum temperature.

[0019] The initial resistance value of the heater 11 is known, and the relationship between the resistance value and the temperature of the heater 11 is also known. The temperature coefficient of resistance of the heater 11 is also known. Lowering the maximum temperature of the heater 11 is advantageous in terms of long-term stability of the resistance value of the heater 11, but doing so makes the maximum temperature of the heater 11 more susceptible to fluctuations in room temperature. Therefore, preferably, the processing circuit 45 uses the resistance value of the heater 11 to feedback control the heater 11 so that the maximum temperature of the heater 11 is constant.

[0020] In this embodiment, the heater 11 is driven by a square wave, and the maximum temperature of the heater 11 is measured based on the resistance value immediately before the heater 11 is turned off. The heater 11 is turned on and off, for example, 1 to 25 times per second, preferably 1 to 4 times per second. A signal corresponding to the average maximum temperature of the heater is calculated, and the power supplied to the heater is adjusted based on this signal to maintain the maximum temperature of the heater 11. The maximum temperature of the heater 11 may be calculated using the ratio between the minimum resistance value of the heater 11 (corresponding to the resistance value while the heater 11 is off) and the maximum resistance value, in addition to the maximum resistance value of the heater 11. If the Freon sensor 40 includes a thermistor or the like for measuring room temperature, the power supplied to the heater 11 may be controlled according to the room temperature. Long-term changes in the resistance value of the heater 11 may also be detected based on the minimum resistance value of the heater 11 and the measured room temperature.

[0021] The light receiving circuit 48 extracts from the thermopile 22 a signal whose frequency coincides with the drive frequency of the MEMS light source, amplifies it, and inputs it to the processing circuit 45. The processing circuit 45 calculates the Freon concentration from the signal from the light receiving circuit 48, displays it on the display 50, and also outputs it to the outside from the output terminal 51. It is also possible to add a set of a bandpass filter with a transmission wavelength outside the vicinity of 9 μm and the thermopile 22 to detect the emission intensity of the MEMS light source 2 and correct the absorption signal due to Freon.

[0022] In this embodiment, the MEMS light source 2 is driven at a temperature of about 300 to 400°C, which is lower than that of the conventional example, and therefore the light source temperature is prone to fluctuate depending on the ambient temperature. For this reason, the temperature of the heater 11 is estimated using the resistance value or resistance temperature coefficient of the Pt heater 11, and the maximum temperature of the heater 11 is kept constant. This makes it possible to maintain a constant light output even if the ambient temperature or the resistance value of the Pt heater 11 fluctuates.

[0023] Absorption spectrum 5 shows the infrared absorption spectrum of R-32 gas. In the example, infrared absorption due to C—F stretching vibration with a central wavelength of around 9.1 μm was used. As a comparative example, an NDIR sensor was used that measures absorption due to C—H stretching vibration with a central wavelength of around 3.3 μm using a combination of a W lamp light source and an InSb infrared sensor.

[0024] Figure 6 shows the absorption by water vapor and liquid water. At high relative humidity, the amount of adsorbed water in the optical cell increases, and absorption by liquid water becomes a problem.

[0025] Figure 7 shows the absorption of ethanol, with absorption due to CH stretching vibrations at around 3.3 μm, and weak absorption above 7 μm. Note that while ethanol is shown as a representative organic solvent, LPG and acetone also have absorption near 3.3 μm.

[0026] Blackbody effect in MEMS light source 2 The MEMS light source 2 of Figures 1 and 2 was prototyped with and without a blackbody film 10 (Example) and without (Comparative Example). The blackbody film 10 was formed by dispersing Pt black (fine Pt powder) in an organic solvent, applying it to the heater 11, and baking it after drying. Because Pt black is highly reactive, it bonded to the heater 11 by heating. When observed with an optical microscope at 100x magnification, the blackbody film 11 appeared black and had a film thickness of approximately 100 μm. The Example and Comparative Example were identical except for the presence or absence of the blackbody film 10.

[0027] Electric power was applied to the MEMS light sources of the example and comparative example so that the temperature of the Pt heater 11 reached 400°C, and the emission spectra were measured. The results are shown in Figure 8. The absorption near 4 μm is thought to be due to CO2 in the air, and the absorption between 5 and 7 μm is thought to be due to water vapor, etc. The blackbody film 10 increased the light output near 9 μm by several times.

[0028] Figure 9 shows the emission spectrum of a tungsten lamp. Because of the glass bulb of the lamp, the output above 5 μm was small, and measurement at 9 μm was impossible.

[0029] FIG. 10 shows the sensitivity of the R-32 gas of the example (■) and the comparative example (●), with the example showing several times the sensitivity of the comparative example.

[0030] Figure 11 shows the effect of relative humidity on the output (indicated R-32 gas concentration) for the example (■) and the comparative example (◆). The actual R-32 concentration was 0. In the comparative example, an error of about 14,000 ppm occurred at a relative humidity of 80%.

[0031] 12 shows the sensitivity of the Freon sensor 40 of the embodiment to the R-32 gas to be detected and to hydrogen, propane, CO2, and ethanol, which are candidate interfering gases. The Freon sensor 40 has a small sensitivity to ethanol, but this is only a fraction of the sensitivity of R-32 gas, so it can be seen that the relative sensitivity to Freons is high.

[0032] Although the measurement of R-32 gas was shown in the example, other Freons can also be detected in the same way. The example has the following effects. 1) Freon can be detected with high sensitivity. 2) The MEMS light source 2 does not require expensive window materials. 3) The thermopile 22 does not require cooling. 4) There is little interference from water vapor, CO2, and organic solvents. 5) The use of far infrared rays allows the temperature of the heater 11 to be lowered, reducing fluctuations in heater resistance and reducing heater power. 6) The heater resistance is measured and feedback control is performed to keep the maximum heater temperature constant, so even if the light source temperature is low, the influence of the ambient temperature can be reduced. [Explanation of symbols]

[0033] 2 MEMS light source 4. Si substrate 6 cavities 7 legs 8 Support membrane 10 Black body membrane 11 Heater 12 pads 14 Housing 15 Aperture 22 Thermopile 24 Si substrate 25 cavities 26 Support membrane 27 Hot junction 28 cold junction 30 Housing 31 Aperture 32 Bandpass Filter 40 NDIR Freon sensor 42 Optical Cell 44 Power supply 45 Processing circuit 47 Drive Circuit 48 Photodetector circuit 50 Display 51 Output terminal

Claims

1. a light source having a MEMS heater, a thermopile, a bandpass filter, and an optical cell containing the light source, the thermopile, and the bandpass filter; the light source has a black body film on the surface of the MEMS heater, and there is no airtight window between the black body film and the outside of the light source; the thermopile comprises a blackbody film covering a hot junction of the thermopile; The bandpass filter has a transmission center wavelength of about 9 μm.

2. 2. The NDIR Freon sensor according to claim 1, wherein the thickness of each of the black body film of the light source and the black body film of the thermopile is 1 μm or more and 200 μm or less.

3. 3. The NDIR Freon sensor of claim 2, wherein the black body film of the light source and the black body film of the thermopile are both made of at least one of a noble metal black film, a black noble metal plating film, a black Cr film, a black electroless nickel film, a black metal oxide powder film, a chromium nitride film, and a metal oxide laminate film.

4. 2. The NDIR Freon sensor according to claim 1, wherein the MEMS light source is turned on / off at a predetermined cycle, and the maximum temperature of the heater film when the MEMS light source is turned on is 300° C. or more and 500° C. or less.

5. 5. The NDIR Freon sensor according to claim 4, wherein said MEMS light source comprises a Pt heater film, the resistance value of the heater film is measured, and the power input to the heater film is controlled based on the measured resistance value.

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