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In direct measurements the temperature values and also the concentration of those gas elements which are detected by independent electrochemical sensors are obtained. The electrochemical cell indications are proportional to the volume concentration of the detected elements expressed in ppm (parts per million).
Ambient temperature is mostly measured using a thermistor. This is a material that changes its resistance proportional to the temperature. The commonest type is probably the platinum thermistor. Flue gas temperature is generally measured with a thermocouple. A thermocouple produces a tiny electrical potential proportional to a tmeperature difference between two points. There are different types of thermocouple available for different temperature ranges, but most of them cover the combustion gas range.
The following quantities are obtained by means of direct measurement:
flue gas temperature Tgas and ambient temperature Ta, expressed in [°C ]
volume concentration of CO [ ppm ]
volume concentration of NO [ ppm ]
volume concentration of SO2 [ ppm ]
volume concentration of O2 [%]
volume concentration of optional cells NO2, H2S, H2, Cl2, HCl (ppm)
In addition to these standard results it is possible that carbon dioxide, CO2 or methane CH4 may also be measured. With these main factors all the other parameters for flue gas are calculated. There are certain sensors that have a different method of measuring to the standard volume concnetration indication generally used. This is particularly common with oxygen. Here, the sensor shows the partial pressure of the gas instead of a volume concentration. Naturally it is then necessary to know the ambient pressure in order to calcualte the volume concentration if needed. Generally it is enough to know the height above sea level and to calculate the ambient pressure on this basis. These sensors are used in medical equipment where the partial pressure reading is most important, hence the difference in technology. More accurate readings will require exact knowledge of the ambient pressure, however. This is also important when using infrared or similar technology. Here, the sensor basically "counts" the number of molecules in the light path, which will also be a function of pressure. More sophisticated infrared analysers are therefore also fitted with a sensor for ambient pressure to correct for these effects.
Some instruments have connections for external sensors or current/voltage analogue inputs. these results can then be shown on the display or stored with the concnetrations and tmeperatures obtained from the standard sensors. These may show the fuel flow rate or gas flow velocity measured with an anemometer.