Carbon Dioxide Gas Detectors – CO2 detectors
The infrared spectroscopy is actually based on the concept of gas molecules that absorb infrared light and other gases that occur at a specific wavelength. Usually, a thermopile which has a built-in filter is useful to detect the specific amount of gas. For example, carbon dioxide is considered to be a strong gas which has a high absorption rate at a wavelength of 4.26 µm. A band filter is put to use so that it can remove the existing light out from the wavelength. Gas molecules also have the ability to absorb radiation energy emitted from the lamps. The absorption rate follows the Lamber-Beer law which is I = Io*e –kcl.
A little elaboration of this law always helps readers to get better clarity. I is the transmitted infrared (IR) from the thermopile detector side. The Io is initial intensity at the IR source. k is the gas absorption coefficient. c is the gas concentration. I is the length of the absorption path from the light source to the thermopile detector.
The thermopile is effective in finding the light intensity change. The output voltage will be –
V = n * Δα * (Tbody – Tamb)
Over here Δα is the difference in the Seebeck coefficients in the thermopile materials. n is the number thermocouples in the thermopile detector. Tbody is the blackbody temperature which emits thermal radiation and Tamb is the temperature of the surrounding ambient.
Within the chamber, the infrared (IR) lamp radiation is considered as the ideal black body radiation. The radiation a blackbody emits is a result of the difference in temperature between the ambient and the blackbody, this entire process is known as thermal radiation. As per the Stefan-Boltzmann law, the thermal radiation per unit area is –
RT = σ * (Tbody4 – Tamb4)
Over here σ = 5.67 * 10-8 W/(m2 *K4) is the Stefan-Boltzmann constant. For better understanding, let us assume that there is no loss in light intensity while it travels through the chamber, in such a situation RT = I. So, let us rearrange the entire equation and the thermopile output voltage becomes:
V = n * Δα * [I0 * e -kcl ] / [σ * (Tbody2 + Tamb2 ) * (Tbody + Tamb)]
Examining this equation will help us understand that thermopile output voltage will get affected because of the ambient temperature along with the complex relationship because of the uncertainty in the lamp intensity. In order to get better accuracy, special attention needs to be given to the design implementation. The temperature compensation is the best way to maintain accuracy in the system. In order to accomplish this, the thermistors are integrated into the thermopile sensor and there are changes in the resistance which depends on the ambient temperature of the surrounding. To get accuracy in measurement it is important to have a constant and steady voltage that can successfully excite the thermistor.
The carbon dioxide gas detection system is an important feature and it is important to have an in-depth understanding of the same.
Traditional Discrete Op Amp Signal Conditioning
The traditionally discrete amps are used for the NDIR systems and the AC coupling helps in removing the signal chain offset. In order to successfully handle a two-channel, it is essential to use a quad op amp that can be configured in a dual change on the front end. Active filtering also needs to be built in the signal path.
Carbon Dioxide Detector
NDIR is an industry term known as non-dispersive infrared and is considered to be the most common type of sensor that is used to effectively measure carbon dioxide. Infrared lamp directs the waves of light into a tube filled with air towards the IR light detector. This light detector measures the right amount of infrared light that hits it successfully. When the light passes through the tube, the gas molecules of the same size as that of the wavelength of the infrared light absorb the infrared light. However, the other wavelengths of light are passed through. The residual light hits the optical filter which absorbs another wavelength of light except for the exact wavelength that is absorbed by carbon dioxide. The infrared detector comprehends an exact amount of light which is not absorbed by the optical filter or the carbon dioxide molecules. This difference is proportional with respect to the carbon dioxide molecules present in the air inside the tube.
Gas Detection System
NDIR sensor is used extensively in the system and it comprises the infrared lamp, a thermistor and two thermopile channels. In order to save more power, it is best to prevent overheating the device. The lamp source is modulated with 50% duty cycle along with the frequency of 1 to 3Hz. The Reference channel and the Detector are directly connected to the inputs of the LMP91051. The filter capacitors are well connected with the common mode reference to each of the input in order to ensure low pass filtering. The external filtering option of LMP91051 is disabled and the pins A0 and A1 are made short internally within the chip. There is no need for high pass filtering because the internal offset DAC is taken to cancel all the offset errors in the signal chain. This makes the measurements faster compared to the traditional AC couple system. NDIR sensor has an internal thermistor connected to the resistor bridge and this is buffered with the amplifier. The entire system is powered off with the help of a single supply of the 3V.
Carbon Dioxide Detector Placement, Method and Settings
The NDIR system is well integrated into the infrared lamp and it is pulsed with 50% duty cycle which results in small 100’s uV RC waveforms. To bring about an overall improvement in measurement accuracy, the signals need to be amplified. There is the need for peak to peak waveform voltage for both the reference channel and the active channel when the comparisons are brought into the limelight. There is a need for active DC offset adjustment which is essential to make sure that the output does not saturate and the signal chain offset errors are skilfully removed. It is necessary to achieve accurate sampling and for that multiple samples need to be used on each of the channels before you make the move of switching channels. The sampling needs to be synced perfectly to the lamp pulses so that the data is captured within the expected time which is relative to lamp switching. The same sample can be looked at over several lamp cycles to determine the noise performance.
Carbon Dioxide Gas Detector Performance
The functional features of LMP91051 were clearly demonstrated over the carbon dioxide concentration, the signal path gains, offsets and the frequency. Thermopiles used mainly in NDIR sensing have high internal resistance (~100 Kohm). Along with this, the signal path can get coupled with the 50/60Hz power main noise and then this will become noticeable in the entire setup of high gain application. Noise is always present in the signal path in the DC coupled system which without any filtering. In order to reduce the noise, a low-pass filtering needs to be added or the average filtering needs performance in the digital domain. In such a situation, the trade off is usually in case the cut-off frequency and that is too low, then the sensor signal might not have ample time for complete settlement.
The best and the most effective way to limit the wide band noise is by creating a low pass filter that has inherent source impedance. Thermopile sensors must have a source impedance which is up to 100 Kohms. The thermal noise has the ability to dominate the system noise. The right placement of capacitors from sensor outputs to the system common mode forms RC low pass filter and reduces the noise effects. In case of the verified 2Hz lamp frequency along with the thermopile source impedance which is of 85 Kohm, 6Hz low pass filter gets formed
FC = 1/(2π*R*C) = 1/(2π*85kohm*0.33uF) ~ 6Hz
An important point to be noted here is the lamp bias potential effect on the accuracy of the system. The light bulb glows because the current flows through the filament. The intensity of light emitted mostly depends on the lamp bias voltage and the light intensity, in turn, intensifies the effect of the thermopile output voltage. This is the primary reason why having a steady and constant light intensity is essential that can be over the entire sensor lifetime. Having a well regulated and clean power supply is instrumental in maintaining continuous emission of light. In order to understand the resolution, noise performance and span of the system, take note of the following measurement and calculation. PGA output captured with 12 bit A/D along with the voltage reference of 3V. A 6Hz low pass filter.
It is always essential to understand the mathematics and the mechanics of the carbon dioxide gas monitor so that the results you receive are high on accuracy.
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