Air Quality Sensors: New CO2 Sensors for Air Quality Monitoring （2）

Published:2011/8/3 22:01:00 Author:Amy From:SeekIC

Christian Voit---Unitronic AG

The TGS4160 gas sensor

The TGS4160 consists of a CO2-sensitive solid electrolyte cell with an internal thermistor for temperature compensa­tion. Figure 2 shows the construction of the sensor and a cross-section through the sensor element. A NASICON disc with a diameter of 4 mm and a thickness of 0.7 mm is printed with gold electrodes on each side. A layer of lithium carbonate (U2CO3) is added on the cathode side and then calcined at 600 =C. A thin disc of aluminium oxide ceramic, carrying a printed platinum healing ele­ment, is laminated onto the anode side. The edges of the sensor are sealed with glass. Platinum wires are bonded to the sensor element and spot-welded to the connection pins on the sensor base. The sensor base is made from glass fibre reinforced polybutylene terephthalate, and over it a mounted a two-layer stainless steel mesh as a flame retardant. The external enclosure is made from rein­forced polyamide and includes a zeolite adsorption filter to prevent ingress of interference gases such as ethanol.



Signal conditioning

For good sensor performance and long life it is important to measure the cell voltage using as high an impedance as possible. The cell has a high internal resistance and so a low impedance measurement would give a false read­ing, and furthermore a significant external current would flow, causing a continual flow of ions in one direction. Over a long period this would damage the sensor. The input impedance of the measurement circuit should there­fore be at least 100 MQ and the bias current should be less than 1 pA. A typical basic circuit is shown in Figure 3.



The output EMF is relatively stable as long as the sensor is heated. Long unhealed storage, however, especially at high humidity, can lead to a significant drop in EMF. Nevertheless, the difference EMF (JkEMF) between the value at 350 ppm CO2 and the value at higher atmos­pheric CO2 concentrations remains very stable. The AEMF of the TGS4160 shows a linear dependence on the logarithm of the CO2 concentration (Figure 4) and only at higher humidities (over 60 %) does it start to show a slight humidity dependence. Because of these properties, the sensor is suitable for use in rapidly-react­ing CO2 air monitoring systems.



Because of the drift that can occur, the sensor is not suit­able for measuring absolute CO2 concentrations, but only the CO2 concentration relative to the ’natural’ con­centration in fresh air. Special signal conditioning is thus required on the cell output voltage, which is carried out in software using a dedicated microcontroller running an application-specific algorithm (Figure 5). To evaluate the AEMF the reference level corresponding to 350 ppm CO2 must first be determined. To do this we assume that the natural concentration of CO2 in fresh air never falls below 350 ppm and that there will be fresh air in the measured environment at least from lime to time. Given these assumptions we can take the highest EMF meas­ured over a specified period (of say 24 hours) to be the reference EMF for 350 ppm. If during a 24 hour period the current reference level is not reached, the algorithm automatically reduces the stored reference value by a set amount, until the measured EMF once again exceeds the stored reference value. In this v/ay a drift in either direc­tion is automatically compensated for: the measuring sys­tem is self-calibrating. TGS4160s have been in continuous testing since 1996. So far no noticeable changes in performance or sensor failures have been observed.

Example circuit

The algorithm described above has been optimized in the light of practical experience and programmed into a microcontroller, which is available as a standard compo­nent. Figure 6 shows the TGS4160 along with the ready-made AM4 module, which includes all the electron­ics required for a ventilation control system apart from the power driver stage. The sensor has already been suc­cessfully used in ventilation control applications, and its suitability for mass production has been demonstrated.

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