Thermocouples are the most common sensors for measuring temperature in industrial use. It consists of two dissimilar metals such as iron and constantan electrically, they joined together at one end. When the junction of the two metals is heated or cooled a voltage is produced that can be correlated back to the temperature.
The voltage across the thermocouple increases as the temperature rises. A suitably calibrated instrument, capable of measuring small voltages can be used to measure the change. The process temperature is obtained from the voltage, either from a graph or by using thermocouple tables. Thermocouple tables list the voltages corresponding to each temperature.
A table is required for each thermocouple type. The relationship between millivolts and temperature is not linear. In microprocessor-based equipment, the conversion is done based on the data stored in the device.
A thermocouple requires a reference junction which is placed in series with the sensing junction. The reference junction is used to correct the sensing device. The technique of cold junction compensation is where the the actual ambient temperature is measured and a correction made to the thermocouple reading. The correction is made by adjusting the voltage by an amount equal to the difference between the actual temperature and 0 ºC.
For accurate temperature measurement, the reference junction temperature must remain constant or suitable compensation provided if it should change. To reduce inaccuracies, most thermocouples are now installed with instruments which provide automatic reference compensation. Modern microprocessor instruments also have all the main thermocouple (and RTD) characteristics available in the memory of the instrument.
- Fusion welded to form pure joint
- Maintains integrity of circuit
- Grounded junctions mean good thermal contact
- Ungrounded junctions provide electrical isolation
- Sheath material often stainless steel
- Mineral insulated thermocouple has become the standard
- The materials of construction consist of a variety of temperature and corrosion resistant sheaths and have a high purity in the order of 99.4% of magnesium oxide insulation.
Thermocouples Installation Techniques
- Use largest size thermocouple wire possible
- Avoid stress and vibration
- Use transmitters if possible
Thermocouples Application Details
- Thermopaste to minimise problems with air gaps
- Clean bore of thermowell when changing
- New thermocouple of different mass to old one…means longer response time
- Grounding requires care
- Extension cable may also need to be replaced
- Care with swap between bonded and insulated junction