Electrochemical Gas Measurement
Electrochemical sensors work according to the battery principle: measuring, counter and reference electrodes are located in an electrolyte. The gas diffuses into the sensor through a membrane and reacts at the measuring electrode. Measuring and counter electrodes generate a current in proportion to the gas concentration. The reference electrode generates a steady potential, thus minimizing drift of the measured values. The manufacturers of electrochemical sensors aim to reduce the reaction to a specific gas. This works in known measuring environments, such as ambient air, with sufficient accuracy. However, multigas applications can lead to significant cross-sensitivities. As is the case with a battery, the operating lifetime is relatively short, and aging begins immediately and continuously after initial use.
Semiconductor Detectors
Semiconductor detectors are typical mass products. Usually, the gas flows through a semipermeable membrane. The concentration is measured via a variable resistance. For example, this can be a semiconductor with a metal oxide coated surface on one side and a reference semiconductor on the other side. The measuring effect is based on the accumulation of certain gas molecules on the coated surface of the detector. These molecules modify the resistance of the semiconductor, while the uncoated reference section shows no effect. The resistance difference between the reference section and the active section indicates the concentration of the desired gas, which can be determined with this method. The semiconductor's sensitivity to a specific gas can be modified by the temperature of the semiconductor.
Thermal Detectors
Thermal detectors are based on a heating wire, which is kept at a constant temperature by a controlled electric current. The energy output of the heating wire to the atmosphere depends on the thermal conductivity of the gases. Since the gases have different thermal conductivity, the energy output is an indicator for the gas. On the other hand, the energy release is compensated by increasing the heat flow. Finally, the electrical signal of the current control becomes an indicator for the gas concentration. Similar to the semiconductor detector, a semi-permeable membrane can be used to improve the gas selectivity.
Photoionization Detectors
A photoionization detector (PID) measures aromatic hydrocarbons, solvents or a variety of organic and inorganic substances, among others. Prerequisite for the measurement of these substances is their ionizability. The substances are exposed to the UV light of a gas discharge lamp. The radiation ionizes the substances and is then exposed to the electric field between the electrodes of the measuring chamber. The strength of the resulting current is directly proportional to the concentration of ionized molecules in the detector chamber. This allows conclusions about the concentration of the substances in the air. A PID is therefore suitable for the detection of entire groups of pollutants, but can, with appropriate adjustment, also be used for the measurement of individual substances.
Catalytic Sensors
This sensor is based on the heat tone principle. Combustible gases and vapors can burn with atmospheric oxygen, releasing reaction heat. For this purpose, a pellistor (a porous ceramic ball <1 mm with an embedded platinum spiral coated with a catalyst) is heated to several hundred degrees celsius. The temperature of the pellistor increases due to the combustion of the gases. The combustion leads to an increase in temperature and thus to an increase in resistance of the platinum spiral. A bridge circuit with a second pellistor without a catalytically coated platinum spiral eliminates the effect of the ambient temperature. The heating by the combustion at the first pellistor leads to a small temperature increase, which is used as a parameter for the gas concentration. Oxygen for combustion is taken from the ambient air.
