Thermocouples and RTDs are the two most common type of industrial temperature sensors. A thermocouple is a temperature sensor that produces a micro-voltage relative to a temperature change which can be measured, conditioned, and amplified into a standard process signal. RTD’s are temperature sensors that measure a change in electrical resistance relative to the temperature surrounding the RTD element. Each have their advantages and disadvantages, and should be applied understanding their respective strengths and weaknesses.
RTD STRENGTHS:
RTD’s are commonly used in applications where repeatability and accuracy are important considerations. Properly constructed Platinum RTD’s have very repeatable resistance vs. temperature characteristics over time. If a process will be run at a specific temperature, the specific resistance of the RTD at that temperature can be determined in the laboratory and it will not vary significantly over time. RTD’s also allow for easier interchangeability since their original variation is much lower than that of thermocouples. For example, a Type K thermocouple used at 400°F has a standard limit of error of – 4°F. A 100 Ohm DIN, Grade B platinum RTD has an interchageability of – 2.2°F at this same temperature. RTD’s also can be used with standard instrumentation cable for connection to display or control equipment where thermocouples must have matching thermocouple wire to obtain an accurate measurement.
RTD WEAKNESSES:
In the same configuration, you can expect to pay from 4 to 10 times more for an RTD than for a base metal thermocouple. RTD’s are more expensive than thermocouples because there is more construction required to make the RTD including manufacture of the sensing element, the hooking up of extension wires and assembly of the sensor. RTD’s do not do as well as thermocouples in high vibration and mechanical shock environments due to the construction of the sensing element. RTD’s are also limited in temperature to approximately 1200°F where thermocouples can be used as high as 3100°F
THERMOCOUPLE STRENGTHS:
Thermocouples can be used to temperatures as high as 3100°F, generally cost less than RTD’s and they can be made smaller in size (down to approximately .020” dia) to allow for faster response to temperature. Thermocouples are also more durable than RTD’s and can therefore be used in high vibration and shock applications.
THERMOCOUPLE WEAKNESSES:
Thermocouples are less stable than RTD’s when exposed to moderate or high temperature conditions. In critical applications, thermocouples should be removed and tested under controlled conditions in order to verify performance. Thermocouple extension wire must be used in hooking up thermocouple sensors to thermocouple instrument or control equipment. Use of instrumentation wire (plated copper) will result in errors when ambient temperatures change.
SUMMARY:
Both thermocouples and RTD’s are useful instruments for determining process temperature. RTD’s provide higher accuracy than thermocouples in their temperature range because platinum is a more stable material than are most thermocouple materials. RTD’s also use standard instrumentation wire to connect to the measurement or control equipment.
Thermocouples are generally less expensive than RTD’s, they are more durable in high vibration or mechanical shock applications and are usable to higher temperatures. Thermocouples can be made smaller in size than most RTD’s so they can be formed to fit a particular application.
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Abstracted from Pyromation's White Paper "How to Select and Use the Right Temperature Sensor".
