Published:2011/8/3 4:19:00 Author:Amy From:SeekIC
Christian Voit---Unitronic AG
A CO2 sensor has been developed for specialist applications in air quality monitoring and control, offering good long-term stability, high selectivity, and a low humidity dependence.
The concentration of CO2 can be used as a good indicator of air quality in the office or in the home. Until now, an obstacle to the widespread adoption of control systems based on CO2 concentration measurement has been the lack of stable, accurate and yet economical sensors for this unreactive gas. Now, however, with the introduction of new solid electrolyte sensors, rapidly-reacting measurement systems for air quality control can be produced.
The concentration of CO2 in fresh air is generally around 350 ppm (parts per million), although this is of course subject to variations due to natural as well as artificial causes. These variations are nevertheless of low amplitude and take place over long periods. In inhabited living and working spaces the CO2 concentration can quickly rise to many thousands of parts per million as a result of respiration, smoking for other reasons. Figure 1 shows the evolution over time of CO2 concentration in a test room holding between 10 and 80 people. Of course many other substances have an influence on air quality (and in particular on the perception of odours). Research has shown, however, that CO2 is the key indicator for determining air quality. Subjective perceptions of air quality certainly correlate with a gradual rise in the level of CO2, especially bearing in mind that CO2 is an odourless gas. If one goes into a room with a high CO2 concentration, one immediately feels the need for fresh air (the so-called ’meeting room effect’). Table 1 shows the effect of various levels of CO2 on humans.
A range of sensor technologies
The most widely-used technique for CO2 measurement is by NDIR (non-dispersive infra-red) absorption. This exploits the physical property of CO2 that it absorbs infra-red light at a wavelength of 4.27 u.m. The CO2 partial pressure can be measured very selectively and accurately using this method (especially in the case of high absolute concentrations of CO2, even up to 100 %). Unfortunately, for measuring lower concentrations a long optical path through the gas is required in order to obtain a sufficiently significant absorption effect. The infra-red method is stable over long periods of time and has no moving parts. It is, however, bulky and requires precision optics and is therefore correspondingly expensive.
There are a few CO2 sensors employing liquid electrolytes on the market. A decisive disadvantage of these cells is their limited life, the poor stability of their outputs and hence the lack of reproducibility of readings. There is also a general risk of leakage with cells that contain a liquid: this can lead to damage to the sensor or to nearby components. These sensors have therefore not been used in ventilation control systems.
Certain solids possess the property that ions can move within them. This makes them ideal to function as the electrolyte in a gas sensor. Particularly well suited to detecting gaseous oxides such as CO2 or NOx are materials based on a natrium (i.e., sodium) super-ionic conductor, or NASICON, structure. NASICON materials, which have been known for some time, are ceramics capable of conducting sodium ions. The chemical formula is Na]+xZr2SixP3.xOi2, with 0<x<3. The highest ion conductivity is exhibited when x=2. Many studies have sought to find an arrangement whereby an electrochemical cell is constructed from NASICON material together with chemically active layers, giving an electrical voltage which depends on the ambient gas concentration. The principal obstacle to commercial mass production was the lack of stability and reproducibility of the solid electrolyte CO2 sensors, and their high humidity dependence. The Japanese semiconductor gas sensor manufacturer Figaro has now managed to develop a process for making long-term stable CO2 sensors as standard components with reproducible properties and low humidity dependence.
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