Infrared sensing principle
People have always known:
1. Many materials can absorb infrared radiation (due to intramolecular vibration)
2. For any material, its absorption capacity changes with the change of wavelength (its absorption spectrum)
3. Different materials have different absorption spectra
The basic principle of infrared gas sensor operation is to rely on the discovery of the above facts. Table 1 shows typical infrared spectra, including carbon monoxide, propane, hexane, and carbon dioxide.
Table 1: Absorption spectrum
Design principles
All infrared gas sensors have basic components: an infrared source (that is, an incandescent lamp), a probe (such as a thermal battery, a pyrotechnic probe), a method of selecting an appropriate wavelength (such as a light band passing through an interference filter), and a sample element. The radiation passes from the radiation source through the sample element and the wavelength selector. The choice of wavelength has a considerable influence on the relative selectivity of the sensor. The radiation that is not absorbed by the sample is measured by the probe and provides the measurement result of the concentration value of the target gas in the sample. The other probe (or channel) in the sample is set to another wavelength and will not be diluted by any possible wavelength in the sample. This is usually used to provide reference measurements.
Another element that enhances the performance of infrared sensors is the temperature sensor. All these components must have temperature accessories to compensate to provide accurate gas concentration values. The temperature sensor (usually a thermistor) should be placed in the probe or very close to the probe.
The infrared sensor can provide effective measurement values for the measurement of target gas molecules between the infrared source and the probe. Therefore, the output signal changes not only with the gas concentration, but also with the influence of air pressure, that is, they are part of the pressure equipment. In order to ensure the high accuracy of the measurement, air pressure compensation must be provided. This shows that sensors with a longer optical path (radiation distance from the radiation source to the probe) have higher sensitivity and require a lower mechanical range but increased resolution.
If the target gas is a gas, and the fixed optical device is at a constant pressure, the output signal (and signal / sound ratio) will increase with the gas concentration to a trend similar to exponential decay, that is, the infrared sensor is fixed nonlinear sensor. The accuracy of the measurement decreases with increasing gas concentration.
The above description of each component is a very typical infrared sensor, but in any practical system, supporting electronics are required. The more commonly used detection technique is to use an amplification device to amplify the extremely small analog signal output by the probe. The amplified output signal can be used to improve the accuracy of the measurement after being filtered by the simulation.
The infrared source also needs to have a circuit, which usually adjusts the output of the infrared source through fluctuations (maybe the previous design used fixed lighting and mechanical hammers). This makes the radiation emission intensity change periodically, and makes the use of simultaneous monitoring technology possible.
For temperature and air pressure compensation, a computer system is usually used in a microprocessor. This first requires converting the analog signal into a digital signal, and then the compensated data will be transmitted to the user in some form.
Figure 2 is a schematic diagram of a typical dual-channel infrared sensor and its independent supporting electronic system.
Figure 2: Block schematic of high accuracy 2 channel infrared gas sensor
For more detailed information, please contact us or visit our website:
Beijing Leshilianchuang Technology Co., Ltd.
Address: Room C918, Jiahao International Center, 116 Zizhuyuan Road, Haidian District, Beijing 100097
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