Method and apparatus for the detection and measurement of gases

The present invention relates to a gas detection device, specifically, to an instrument and method for detecting and measuring low-concentration gas.

There is a huge and growing demand for analytical equipment for analyzing various reaction products of industrial processes. When applied to the detection of toxic gases, it is necessary to determine the very low concentration of those substances of interest, generally as low as parts per million. The usual method of measuring gas concentration is to send a sample of the surrounding air into a light pipe and measure the absorption of a specific molecule of interest at a specific infrared wavelength. Since the concentration of this molecule may be quite low, the amount of infrared absorption to be measured is also very low. In order to increase the absorption, generally a very long long-distance light pipe is used, and the effective optical path length can be as long as 20 meters by adopting the method of multiple reflection between mirrors. The disadvantage of this method is that the multi-pass light pipe is expensive, bulky and heavy. Therefore, it is not suitable for the light and portability requirements of the application of such equipment, in order to meet the requirements of the United States Occupational Safety and Health Administration (OSHA) on the allowable concentration of gas in the workshop.

Only this kind of light pipe for gas measurement is available on the market. To use this kind of light pipe, it is necessary to fill the filtered air first to establish the "reference" absorption, then evacuate, and then inject the surrounding air, the absorption is measured twice, and the absorption values measured twice are compared to determine the sensation. The absorption of gas of interest. This kind of procedure is obviously very time-consuming, and it is easy to produce errors due to the drift of electrons between multiple measurements. If the gas concentration to be measured is very low, the theoretical difference between the two measurements is also very small. In order to generate a small difference signal, the two large optical radiation values must be subtracted, and any large drift in the infrared radiation value will mask the true absorption value.

Another shortcoming in the method of applying the currently available commercially available instruments is that an interference color filter is used to tune the infrared light source to the appropriate wavelength for absorption measurement, although the interference color filter can achieve the high infrared energy throughput required for this measurement. , But the resolution achieved at a specific infrared wavelength is extremely low, which leads to interference absorption of various gases, which in turn causes measurement errors.

The instrument for detecting a selected substance in a gas sample according to the present invention includes: a photo-acoustic detector, the detector includes a wall for defining a sealed chamber, the sealed chamber is provided with a valve, which is connected to the wall In order to allow gas samples to enter and exit the chamber, a device for detecting acoustic shock waves in the chamber, a window is opened on the opposite side of the sealed chamber to allow infrared rays to pass through; an infrared radiation connected to a photo-acoustic detector Source, in the infrared radiation source, a narrow-band pulsed infrared radiation directly passes through the chamber, interacts with the gas sample in the room, and generates a pulsed acoustic shock wave in the sealed chamber when the selected gas exists; an acoustic- The optical tunable filter is placed between the photo-acoustic detector and the infrared radiation source device. It includes a light-adjusted acousto-optic crystal through which infrared rays pass and is connected to the variable-frequency energy source and the acousto-optic crystal. The sensor emits sound waves in the crystal to interact with the selected narrow-band part of the infrared radiation to distinguish this part from the rest of the infrared radiation. The narrow-band part is the frequency of the radio frequency energy and the sound wave emitted to the crystal. function.

An object of the present invention is to provide an improved instrument for detecting and measuring extremely low concentration gaseous substances, and to provide a method for detecting and measuring gaseous substances.

Another object of the present invention is to provide a compact and portable gas detection and measurement device to comply with the regulations of the U.S. Occupational Safety and Health Administration on the measurement of toxic substances.

Conveniently, the wall of the sealed chamber includes a device for detecting acoustic shock waves in the chamber, and windows are opened on opposite sides of the sealed chamber through which infrared rays can pass. The infrared radiation source is connected to the photo-acoustic detector to provide infrared rays passing through the chamber. An acousto-optical tunable filter (AOTF) is placed between the photo-acoustic detector and the infrared radiation source. The acousto-optic tunable filter includes a light-adjustable acousto-optic crystal through which infrared rays pass at a predetermined angle with respect to the optical axis of the crystal. The acoustic sensor is connected with a variable frequency energy source and an acousto-optic crystal to emit sound waves into the crystal to interact with a selected infrared narrowband part. The interaction of sound waves and infrared rays can distinguish the selected narrow-band part from other infrared rays. The narrow frequency band is a function of the frequency of the radiometric energy and the sound wave. The selected narrow-band part of the infrared rays directly passes through the window of the sealed chamber and interacts with the gas sample in the chamber. When there is this predetermined gas in the sealed chamber, this interaction generates an acoustic shock wave.

The invention is described below with an example, and can refer to a brief schematic diagram of an embodiment of the gas detector.

The gas detection instrument of the present invention includes an acousto-optical tunable filter (AOTF) and a photo-acoustic (PA) detection tube. The acousto-optical tunable filter is made of appropriate materials, such as arsenic thallium selenide, and its function is a high-throughput source of high-resolution infrared selective filtering. The description of US Patent 4052121 discloses a non-collinear structure in the acousto-optical tunable filter. The description of U.S. Patent Application No. 345123 describes a system in which a narrow-band pass-tunable acousto-optic filter can be selectively tuned with a predetermined radio frequency signal to selectively send the correspondence we are interested in. A narrow frequency band for specific types of molecules for easy identification and analysis. The described system includes a microcomputer with a storage function to measure and compare the detection signals from an infrared detector, which has converted the filtered infrared signal into an electrical signal. The memory provides control signals for the computer and for controlling the sequence and frequency of radio frequency energy applied to the tuning filter. In this way, infrared rays near the middle range can be used to analyze the absorption band corresponding to a predetermined kind of molecule, and the generated feedback signal can control the combustion process and so on. In the present invention, the acousto-optical tunable filter used enables the filter to be quickly electronically tuned, and is tuned to a selected infrared passband by means of acousto-optic interaction with infrared rays passing through the crystal.

The photo-acoustic (PA) detector is a relatively new device for measuring the absorption of trace gases. They are applied to the fact that light energy in the infrared region can be absorbed by the excitation of molecular oscillations, and the light energy is quickly converted into heat, which is later converted into pressure waves in the medium. Therefore, if the infrared source is modulated at a predetermined frequency, the pressure wave will form a sound wave of that frequency, which can be detected by an acoustic sensor such as a microphone. The signal generated is proportional to the energy absorbed. If the absorbing medium is a gas and the concentration of the absorbed substance is very low, the sound energy generated will be proportional to the concentration of the absorbed substance and the intensity of the incident infrared light. The applicant stated that when the light source is tuned by the acousto-optical tunable filter, the type of substance can be identified by its characteristic infrared absorption wavelength. Therefore, photo-acoustic detectors and infrared adjustable light sources can be used to identify and measure the concentration of any number of different gases in a background gas (such as air).

There are many types of photo-acoustic detectors. For gas detection, the medium can be installed in a sealed room with a window. The window is opened for infrared rays. There are some acoustic detectors on the wall to detect the absorbed energy.

In order to increase the sensitivity, the sealed room should be able to produce sound resonance at the modulation frequency of the infrared source. Photo-acoustic spectroscopy is well discussed in the article entitled "Photo-acoustic spectroscopy" by West et al., contained in "Scientific Testing Equipment", July 1983, Volume 54 (No. 7) . Its content is comprehensively elaborated here as a reference. Compared with many commonly used absorption methods, photo-acoustic detection has some obvious advantages. The detection component is a microphone, so expensive infrared detectors are not required. The signal generated is proportional to the concentration of the substance. Therefore, there is no need to detect small changes in large signals, like the usual light absorption techniques described in the background section of this article. The photo-acoustic detector device is compact and light in weight, so it is suitable for portable instruments.

Acousto-optical tunable filter and photo-acoustic detector are used in combination with broadband infrared radiation source. The best source of infrared radiation is Nernst (Nernst) luminous body, which provides the broadband infrared radiation used in the system, or silicon carbide hot rod. The high energy flux and wavelength resolution of the acousto-optical tunable filter in the infrared region have been described in the literature, and can be found in the "Automatic Acousto-Optic Tunable Filter Infrared Analyzer" by Steinbuegge et al. As seen in the text, it is contained in SPIE, Vol. 268, page 160, 1981. The content is quoted here as a reference.

Turning now to this figure, the reference symbol 1 is used to denote the acousto-optical tunable filter combined with the photo-acoustic detector according to the present invention. The main components of the system include infrared radiation source 3, acousto-optical tunable filter 5, photo-acoustic detection sampler 7 and data sorting and control device 9. As mentioned above, the infrared radiation source 3 is preferably a broadband infrared radiation source, such as a Nernst luminous body. The acousto-optical tunable filter is optically collimated between the infrared radiation source 3 and the photo-acoustic detector 7. It includes an optical input screen 11, an optical output screen 13 and a sensor 15. The sensor is placed on one side of the acousto-optic filter to form a non-linear filter. The sensor 15 is connected to a variable frequency radiation source, which is included in the data processing and control device 9.

The photo-acoustic detector 7 includes a wall 17 that defines a sealed chamber 19 with valves 21 and 23. These two valves are combined with the wall 17 so that the gas sample can be introduced into the chamber 19 from the valve and can be removed from the chamber 19 discharge. The sealed chamber wall 17 includes a device 25 for detecting acoustic shock waves in the sealed chamber 19, and window devices 27 and 29 placed on opposite sides of the chamber 19, through which infrared rays can pass. The detection device 25 maintains telecommunications contact with the data sorting and control device 9 for data processing.

As mentioned earlier, the acousto-optical tunable filter is combined with the photo-acoustic detector, and it is set up and working as a trace gas detection device. The infrared light source is a thin wound nickel-chromium heating wire, working temperature is 1200°C, with a parabolic mirror, which can focus infrared rays into the acousto-optical tunable filter, the useful wavelength of the acousto-optical tunable filter The tuning range is 1.5-1.6 microns. The narrow-band light from the filter enters the photo-acoustic tube through a window through which infrared rays can pass. Install a microphone to eliminate external vibration and reduce background noise. The photo-acoustic chamber in this embodiment produces resonance at 4KHZ, and the acousto-optical tunable filter produces pulses at this same frequency. The control system can be a device that scans the wavelength range in order to obtain a complete absorption, or it can be a device that jumps to a selected wavelength to measure a predetermined gas concentration. Performance tests have been performed on this system with methane and benzene, and the results are consistent with the theoretical performance. The device briefly described and illustrated here can measure the gas concentration in the parts per million range specified by the U.S. Occupational Safety and Health Administration. These measurements can be completed in a few seconds, compared with 5 to 10 minutes using a traditional absorption trace gas analyzer.

The above-mentioned instrument is a combination of the following devices: a broadband infrared light source, an acousto-optic filter and a photo-acoustic detector that can selectively pass infrared rays of a certain wavelength, which combine to form a novel detection and measurement low Instruments for concentration of gases.