Thermistor output voltage

Flow-through conductivity sensors suitable for insertion in pipelines (see Fig. 6.47a) are now available for use at temperatures up to 480 K. and pressures up to 1700 kN/m2(64). As conductivity is temperature sensitive, a thermistor is usually included in the detector circuit as part of a temperature compensator. Screw-in cells (Fig. 6.476) will withstand higher pressures. More recently, electrodeless methods of measuring conductivity have become available. In this case the solution is placed between two energised toroids. The output voltage of the instrument (from the output toroid circuit) is proportional to the conductivity of the solution provided that the input voltage remains constant. This type of conductivity meter can be used under much more severe conditions, e.g. with highly corrosive or dirty systems 43 . 

The two thermistors on which the solution droplet and the solvent droplet are placed are arranged in an Wheatstone bridge circuit in such a way that the temperature rise can be measured very accurately as a function of the bridge imbalance output voltage, AV. The operating equation is... 

Contact temperature sensors represent a class of temperature probes that are used to determine the temperature of a medium through the thermal equilibrium attained between the sensor and the medium when in contact. The change in the medium temperature is inferred from a corresponding variation in the sensor resistance or output voltage, which can be used to obtain the temperature difference via an a priori calibration of the sensor output with temperature. The commonly implemented contact temperature sensors include thermocouples, resistance temperature detectors (RTDs), and thermistors. 

The temperature rise is usually measured with calibrated thermistors or thermocouples. Thermistors are more sensitive than thermocouples, but the advantages of using thermocouples (consisting of a junction of two metals) are their small size and mass, and the known dependence of the output voltage on temperature. 

Another version utilizes a thermistor that consists of a material with a large temperature coefficient a — AR/dT)/R of the resistance R. If a constant current i is fed through R (Fig. 4.75b), the incident power P that causes a temperature increase AT produces the voltage output signal... 

Calibration of a DTA involves adjustment of instrumental electronics, handling and manipulation of the data in order to ensure the accuracy of the measured quantities temperature, heat capacity and enthalpy [614,615,621]. Temperature sensors such as thermocouples, resistivity thermometers or thermistors may experience drifts that affect the mathematical relationship between the voltage or resistance and the absolute temperature. Also, significant differences between the true internal temperature of a sample with poor thermal conductivity and the temperature recorded by a probe in contact with the sample cup can develop when the sample is subjected to faster temperature scans. The important quantity measured in DTA experiments is the AT output from which enthalpy or heat capacity information is extracted. The proportionality constant must thus be determined using a known enthalpy or heat capacity - the power-compensated DSC requires lower attentiveness as it works already in units of power. The factors such as mass of the specimen, its form and placement, interfaces and surface within the sample and at its contact to holder, atmosphere... 

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