Resistor values, NTCs

The pH (or pI) term of the Nemst equation contains the electrode slope factor as a linear temperature relationship. This means that a pH determination requires the instantaneous input, either manual or automatic, of the prevailing temperature value into the potentiometer. In the manual procedure the temperature compensation knob is previously set on the actual value. In the automatic procedure the adjustment is permanently achieved in direct connection with a temperature probe immersed in the solution close to the indicator electrode the probe usually consists of a Pt or Ni resistance thermometer or a thermistor normally based on an NTC resistor. An interesting development in 1980 was the Orion Model 611 pH meter, in which the pH electrode itself is used to sense the solution temperature (see below). 

There are numerous uses for resistors with high values of the temperature coefficient of resistance (TCR) and they may be negative (NTC) or positive (PTC). An obvious application is in temperature indicators that use negligible power to monitor resistance changes. Compensation for the variation of the properties of other components with temperature may sometimes be possible in this case the applied power may be appreciable and the resulting effect on the temperature-sensitive resistor (TSR) must be taken into account. 

The preceding two equations imply that, for high accuracy, high n values must be considered. In practice n = 3 gives, for the known thermistors, a good fit and an accuracy of about 10-3°C. For a suitable narrow range of temperatures the following simplified equation may be conveniently used to represent the pT characteristics of an NTC resistor ... 

The value of c is often negligible for NTC resistors, thus the variation of resistance as a function of temperature may be written as ... 

Thermistors are resistors with a temperature-dependent value of their resistance. There are three types of thermistors CTT (critical temperature thermistors), NTC (negative temperature coefficient), and PTC (positive temperature coefficient) thermistors. Their thermal behavior is shown in Figure 9.7. 

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