Castle Group Ltd

Temperature can be measured in two ways:
By either contact measurement or non contact measurement. The method you need to use will depend on the application you are using it for, you may need either or a combination of both.

Classic contact applications

Objects with high heat capacity

Metals

Large metal masses

Objects with smooth surfaces

Polished Steel Plate

Polished Heating Pipes

Probes

When measuring temperature there are several different probes you can choose from. The choice of probe will be determined by the measuring task. Probes are selected by using the following criteria.

• Range
• Accuracy
• Location Design
• Response time & Resistance

A Thermometer is a device that measures temperature or a temperature gradient using several different principles.

Thermometers are available in various types.

A common method includes what is known as a bi metallic strip. This is a small strip of metal made up of 2 metals which will bend with the changes in temperature.

A mercury glass thermometer consists of mercury in a glass tube that has calibrated marks which allow the temperature to be read by looking at the length of mercury in the tube. This length will vary according to the temperature. Thermometers usually have a bulb of mercury at the end that contains the majority of the mercury. With a change in temperature the volume of mercury will expand or contract and this is amplified in a much narrower bore of tube. The empty space may be a vacuum or filled with nitrogen.

A thermistor is a type of resistor that is used to measure temperature changes by relying on a change in resistance. They have a negative temperature co efficient thus as the temperature increases resistance decreases.

Thermo Couples – Thermocouple Temperature measurement is based on thermoelectric effect. These consist of two wires welded together. These wires are made of different metals or metal alloys. Voltages and maximum tolerances are defined in the IEC 584 Standards. An NIcr-Ni is the most popular thermocouple.

Rule Of Thumb

Thermocouple probes are quick and have a wide measuring range where as NTC & resistance probes are slower but more accurate.

The wider the measuring range of a probe the more universal the applications.

You should first choose the probe that is suitable for your measuring range.

Which Probe to use?

Immersion Probe

Liquids, Powders & Air

Penetration Probe

Plastic, Semi-Solid Environments

Air Probe

For rapid measurements

Surface Probe

Smooth flat surfaces (e.g. Irons)