Design, Manufacture and Supply of Process Instrumentation

Temperature Measurement Principles


Temperature Measurement With Status Instruments

 

Temperature Sensors

 

Temperature is the most widely measured parameter, this is intended as a practical guide to temperature measurement.

To measure temperature, you need a temperature sensor. Most industrial applications are covered by an RTD or Thermocouple sensor.
 

RTD Sensors

The resistance to the flow of electricity in metallic materials varies with temperature. This can be used to good effect in platinum resistance detectors. Platinum is particularly stable both electrically and mechanically and is also stable with respect to time, producing a relatively linear change in resistance versus temperature.

Because the output resistance change to temperature is relatively small, it follows that lead lengths and resistances are therefore important features. In general when lead lengths are short, or can be considered as an acceptable additive content, two wire configuration is sufficient.

Three wire is the most commonly used and unless otherwise specified is supplied as standard, the third wire is the compensator for lead length and providing that all three wire have equal resistance, compensates for any ZERO or SPAN errors. (Not true for all bridges).

In theory an RTD can be used up to 850 Deg C, but in reality, we tend to switch to Thermocouples over 600 Deg C. This is because when an RTD is encased in a metal sheath (the temperature probe) it can get contaminated above this temperature.
 
The advantages of RTD sensors is high accuracy, stability and interchangeability.
 

Thermocouple Sensors

 

A thermocouple is a junction of two dissimilar materials which when joined at both ends and with an indicating meter connected in one wire, will cause an electromotive force (emf) to register if the temperature of the two ends are different.      

By placing one end at the measuring point called the hot junction and the other at a different point normally a colder temperature and called the cold junction the difference emf generated can be measured. The emf is proportional to the difference in temperature between the two junctions. Traditionally the cold junction was maintained at 0°C by placing it in crushed ice.

Nowadays the cold junction is measured electronically and then automatically compensated for and the emf can then be related to the temperature of the hot or measuring junction. Various combinations of material can be used with predictable output signals depending upon the application, these materials are documented under published standards for example:-
British-BS.EN, German-DIN, American-ANSI, European-IEC.

Thermocouples are generally less accurate than Platinum Resistance but are more robust, lower cost and can operate over a wider temperature range. They are available to suit a wide range of process temperatures and environments as shown below and can have an isolated or grounded junction i.e. the thermocouple is separate from, or joined to, the outer sheath. Grounded thermocouples offer faster response but can cause problems in the process due to earth loops etc.

For this reason all thermocouples sold by Status Instruments Ltd are isolated junction as standard.

Grounded junctions must be specifically requested.


 
 
 
 
Accepted Payment Methods
10000
8000