0~2000ppm Range and 232ppm Resolution

CO₂ Thermal Conductivity Sensors

Xiang Zheng Tu

 

Figure 1 shows a thermopile thermal conductivity sensor provided by POSIFA Microsystems Company. The sensor mainly comprises a silicon chip, a hot plate suspending over a cavity recessed into the silicon chip, a resistor and a thermopile both disposed on the hot plate. The resistor is heated by applying a square pulse voltage and the thermopile is used to measure the temperature difference between the hot plate and the silicon chip. The temperature difference depends upon the thermal conductivity of the gas or gas mixture filled in the cavity. Since the sensor is ultra miniature its thermal time constant is short enough to allow for heating the sensor to work with very narrow pulses of electricity. So the power consumption throughout sensor's operation is quite low.

Figure 2 shows measurement data for indoor air. The heating square pulse voltage is supplied by Agilent 8110A 150 MHz Pulse Generator. The square pulse voltage is chosen to have: period = 1s, width = 20ms and amplitude = 8.96V. The out voltage of the sensor is measured by a TDS Digitizing Oscilloscope, which is shown as 1.86mV.

Figure 3 shows measurement data for 40% carbon dioxide and 60% nitrogen.

The heating pulse voltage is maintained as the same. The out voltage of the sensor is shown as 1.924V which is higher than the out voltage measured for indoor air. 

Using the above measurement data the sensitivity of the thermopile thermal conductivity sensor for carbon dioxide in nitrogen or in indoor air can be calculated as 1.65mV/1%.

In order to determine the resolution of a practical sensor measurement system in terms of voltage, we have to make a few calculations.

·      Assume the system capable of making measurements across 0 to 5V range,

·      Using a18-bits A/D converter, and

·      Using an averaging technique for reducing the noise contribution from four counts to one count.

Therefore, the smallest theoretical change we can detect is 153μV or 232ppm carbon dioxide in nitrogen.