Voltage measurements, thermometer

Fig. 9.12. Capacitance of a capacitive glass thermometer as a function of temperature at the same frequency (4.7 kHz) for three measuring voltages. The measurements were carried out in magnetic field of 0.0 T (o),...

During the reaction of the hot catalyst surface with a flammable gas the temperature of the device increases. The Platinum coil itself serves at the same time as a resistance thermometer. The resistance increase of the coil then is a direct measure for the amount of combusted gas. Usually the amount of heat that develops during combustion is small and amounts to 800 kj/mol for methane, for example [8], Therefore the sensor is connected in a bridge circuit to a second resistor which shows the same setup as the pellistor but is catalytically inactive. The bridge voltage is then controlled by the temperature difference of the two sensors (see Fig. 5.34). 

A thermocouple is a junction of two metals that produces a voltage proportional to temperature and is therefore a device for measuring temperature. A thermocouple can be calibrated with the use of known temperatures, such as the freezing point of water (slushy ice-water mixture) or temperatures known because they are measured with a calibrated thermometer. 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

Thermocouples tend to be relatively inexpensive, bnt the instruments to measure voltages and to record the temperatnres are generally more expensive. To obtain reliable temperature measurements, the following factors are important relative to the installation of the thermocouple plus other types of thermometers ... 

A mercury thermometer is not a good measuring device to be used for control since its measurement cannot be readily transmitted. On the other hand, a thermocouple is acceptable because it develops an electric voltage which can be readily transmitted. Thus transmission is a very crucial factor in selecting the measuring devices. 

Temperature measurements below 100°K are generally accomplished using one of four basic sensing elements—thermocouples, resistance thermometers, semiconductors, and gas thermometers. The first three devices depend on the conversion of an output voltage to the desired temperature information. They are, therefore, basically electrical in nature. The latter, depending only on the unique temperature-pressure relationship of a pure gas, is inherently thermodynamic in character. 

The measuring equipment consists of a voltage source (1), an ampere-meter (2), a volt-meter (3), two thermometers (4 f5), a thermostatic unit (6) and a recorder (7), They are connected to the cell as presented in fig, 4, 21,... 

A resistance thermometer uses the temperature dependency of the electric resistance of the used material. Therefore, a well-defined and very low current is applied onto the sensor by measuring the voltage simultaneously. For precise measurements, pure platinum is often used since the temperature coefficient is almost constant and the variations of the electric resistance are higher than for alloys. When semiconductors are used, much higher temperature coefficients and therewith higher sensitivity can be obtained these thermometers are called thermistors. 

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