Contact - electrical thermometers

Contact - electrical thermometers

1. Resistance thermometers

Principle is that the electrical resistance of the sensors is strongly temperature dependent, and changes with temperature in a predictable way.

Platinum Resistance

Glass coated 100 ohm Pt100 platinum resistance sensor

Standard Platinum Resistance Thermometers (SPRTs) are the most accurate.
However, they are only suitable for laboratory use, and in industry more rugged industrial platinum resistance thermometers are used, variously known as IPRTs, Pt100s, RTDs (resistance temperature detectors).
In a thermometer, the high purity platinum wire sensor is located near the tip of a closed protective tube, to make a probe which can be inserted into the measurement environment.
Most sensors are made with two wires emerging from the instrument, the resistance of these wires is included in the measurement and errors of a few °C may result.
Some compensation for the lead resistances can be achieved by connecting a third wire to one side of the sensor (3-wire connection), but best accuracy requires four wires, two for passing the current and two for sensing the voltage across the Pt100 resistance.
Good sensitivity can be achieved: measurements routinely made with a precision of better than a thousandth part of 1°C.

Some Pt100/RTD sensors suitable for insertion in steel protective tubes or on flat surfaces. The smallest are about 1 mm in diameter.

Thermistors

Thermistors for use in current limiting circuits 

Semiconducting materials such as thermistors are very temperature sensitive and resistance increases very strongly as the temperature falls.Well suited for use in small probes with fast response, e.g. as current limiters in electronic circuits and in medical thermometry, where good sensitivity is achieved over limited temperature ranges.
Most common are negative temperature coefficient (NTC) types.
Since resistances are large, generally several kilohms, 2-wire connections can usually be used without significant error. Thermistors are not standardised, and the manufacturer’s specification must be referred to.