Temperature Sensors can be broken down into 4 basic types: thermocouple, RTD (Resistance Temperature Detector), thermistor, and infrared.
Thermocouples are two different metals joined at a junction point. The combination of metals generates a millivolt signal that is proportional to the temperature of the junction. Different metals are combined to measure different temperature ranges, and these metal combinations are known as “types”. For most industrial processes up to 2000 Degree F, the most common thermocouple types are J and K followed by T.
Thermocouple junctions are of three types: grounded, ungrounded and exposed tip. A grounded thermocouple has the junction touching the sheath material – there is continuity between leads and thermocouple sheath. Ungrounded junction is electrically and physically isolated from the sheath material. The junction of an exposed tip thermocouple is exposed to the surrounding environment.
RTDs are less common in general industrial applications than thermocouples. RTDs measure change in resistance of the sensor tip. The resistance of a RTD changes in proportion to temperature. Platinum is commonly used as the resistor. RTDs are more accurate than thermocouples, and also more expensive. There are two common types of RTD, a two wire and a three wire. Three wire sensors are more accurate than two wire sensors, and both are more accurate than a thermocouple. The three wire sensor is preferred when there is longer distance from sensor to the controller or when precise accuracy is important.
It should be pointed out that, for most industrial processes, the difference in accuracy is not a concern. The difference in accuracy between a thermocouple and a RTD may be a degree or two.
Because the RTD changes resistance, a current must be applied to the sensor, usually originating at the control instrument. Unlike a thermocouple, which is self-powered, a RTD has to have an outside power source
Thermistors are not generally seen in industrial settings to limited temperature range, difficultly in switching replacing probes, and expense. Upper range for a thermistor is approximately 150° C. Thermistors must be calibrated to the controlling instrument for accuracy. Interchangeable thermistors are expensive relative to other temperature sensing alternatives
Things to Consider When Ordering Thermocouples/RTDs:
1) Temperature range: The type of thermocouple used partially depends on the temperature range to be measured. The most common types in industry are J and K. Type J has a range of 0° – 1500° F. Type K has a range of 0°-2500° F. K is normally used in the 750° – 2000° F range, J used in the 200° – 800° F range. Type T has an approximate range of -250° F to 400° F. There are other thermocouple types with different ranges for specialized applications. RDTs are effective -200° C to 700° C
2) Operating Environment: Once the temperature range has been established, the sensor must be deployed in the most effective way to measure the material. Since the measuring junction may need mechanical protection, there is a tradeoff between temperature measurement and durability of the sensor. The more insulation between the sensing junction and the target, the slower the response of the sensor.
3) Type of Controller Used: Some controllers require ungrounded thermocouples and some controllers can use either grounded or ungrounded ones.
4) Process Response Time: Most industrial processes are constrained by the speed of a temperature sensor. One notable exception is measuring hot air. Some air heaters respond faster than a grounded or ungrounded thermocouple can sense, potentially resulting in erratic processes or heater failure.
Thermocouple (and RTD) design Checklist:
Check back soon for a printable checklist to use to determine your thermocouple needs!
When necessary to measure a target which cannot be touched (e.g. parts moving down a conveyor) infrared sensors are a good solution. Care must be taken when choosing these sensors since selection criteria involve more than temperature range.