The MQ135 data sheet does not specify response curves for any reducing gases, so while the sensor should be sensitive to such gases, the user would have to experimentally determine the response curves themselves. Here, the sensing mechanism is the opposite the reducing gas adsorbs onto the sensor surface, consuming additional electrons from the conduction band, and thereby lowers the conductivity further. SnO2 gas sensors can also detect the presence of reducing gases such as NOx. This process ultimately also returns electrons to the SnO2 conduction band, and therefore results in increased conductivity.
CO 2 then in turn reacts with the adsorbed hydroxide, forming carbonate (CO 3 2-). Here, the sensing mechanism is different instead of reacting with adsorbed oxygen directly, CO 2 sensing relies on water vapor first reacting with adsorbed oxygen to form adsorbed hydroxide (OH –). By quantifying the conductivity response to the presence of various gases, a thin SnO 2 surface can be used to determine the concentration of the gas. This reaction releases the donor electrons back into the conduction band, thereby raising the conductivity of the sensor. In the presence of a flammable gas, the gas will also adsorb onto the sensor surface, where it consumes the adsorbed oxygen species to form CO 2 and H 2O. Therefore, SnO2 has low conductivity in clean air. The adsorption reaction consumes electrons from the conduction band to form negatively charged oxygen species. However, SnO 2 when exposed to air readily adsorbs oxygen onto its surface. SnO 2 is an n-type semiconductor, in which donor electrons are excited to the conduction band at elevated temperatures. The MQ135 consists of a surface covered in a thin layer of SnO 2, and a heater resistor which serves to raise the temperature of the SnO 2 surface to several hundred degrees Celsius.
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This article will, firstly, discuss the working principles of the MQ135 sensor and how to read the data sheet, and secondly present how to use it with the PSLab. Much has been written about using the MQ135 as a CO 2 sensor, mostly focusing on using it together with Arduino. This makes it an attractive low-cost alternative to more specialized (and more expensive) CO 2-specific sensors. Nevertheless, according to the data sheet the MQ135 is capable of measuring the concentrations of several gases, one of which is CO 2.
It is marketed as a generalized “air quality” sensor, rather than precision device for measuring the concentration of any specific gas. The MQ135 is a cheap gas sensor that is primarily intended for detecting the presence of flammable gases.