Resistance Thermometers vs Thermocouples: Key Differences

Thermocouples (TCs) and Resistance Thermometers (RTs) are both contact temperature sensors. They need to be in the environment they are measuring for them to work. Both devices are commonly used in the process industry and have significant differences which we will touch upon in this article.

So Many Names

Resistance Thermometers can be referred to by many names, some of these include Resistance Thermometer Detector, RTs, RTDs, Pt100s, Pt1000s, Platinum Resistance Thermometers (PRTs), Pt500 or Pt200 sensor. A thermocouple is also called a TC, thermal junction, thermoelectric thermometer, Pyro or Pyrometer. Occasionally you may see a reference to the specific type of thermocouple such as T1 T2, K, R or other less common alloy pairs.

What is a Resistance Thermometer?

A Resistance Thermometer Detector (RTD) is a variable resistor which changes its resistance depending on its surrounding temperature. It creates an internal resistance that can read with a simple circuit.

What is a Thermocouple?

A Thermocouple is essentially a simple assembly based on two different alloyed wires. It produces a reliable millivolt signal when it is in a thermal gradient. The signal is not linear, but instrumentation usually deals with that. The highest temperature thermocouples use Platinum Alloys.

These rare metals are very expensive so very thin sections are used. Thermocouples therefore usually rely on an external protection sheath to protect the thin alloy wires from the process conditions.

Temperature Sensor Differences

Resistance Thermometer Detectors (RTDs) are generally used to monitor temperatures between -70oC and 500oC. RTDs provide a practically linear output for the vast majority of applications. RTD’s can be designed into equipment without a specific protection sheath or enclosure.

When some stand-alone protection is needed, the smallest practical sheath size is approximately diameter 3mm. Because of lead wire resistance, Resistance Thermometer Detectors usually have 3 or 4 wires and these wires are normally red and white.

Thermocouples (TCs) only have two wires and can be very small sensors indeed, much smaller than RTD’S. Thermocouples can be made in sheath diameters as small as 0.25mm. TCs are more robust due to their simplicity and can withstand much higher temperatures.

Type B will survive around 1600oC with the appropriate protection sheath material. Thermocouples can be more expensive due to the rare metals used within the construction of the sensor.

RTDs are manufactured as a discreet chip. The chip is made from a very thin platinum wire or layer (hence the Pt in some of the names for these sensors) which is on a ceramic backing or in a ceramic tube.

A Resistance Thermometer Detector is more accurate than a Thermocouple due to smaller tolerances. RTDs are usually cheaper to manufacture than TCs as the RTD lead wires are usually copper or nickel, depending on the application temperature.

A Thermocouple requires controlled conductor alloys throughout its length, including specialist extension or compensation cables. Thermocouple circuits need to deal with cold junction compensation although the vast majority of modern instruments handle cold junction compensation automatically.

An RTD is a discrete device that can easily be put onto a circuit board. If an RTD has significant lead lengths then, 3 or even 4 wires are needed to maintain accuracy. These RTD extension wires must be identical to each other in terms of resistance per meter.

Application of Devices

Resistance Thermometer Detectors are typically found in medical devices, for example an anesthetic dispenser, used within research, are part of HVAC (Heating, Ventilation and Air Conditioning) devices as well as within OEM (Original Equipment Manufacturing) like electric motors.

In high temperature environments, for example within molten metal applications, cement manufacturing, furnaces and ovens, thermocouples are typically used. A high temperature thermocouple must be robust and tends to end up quite large. The diameter increases to give thicker protection sheath sections.

Probes end up quite long as connection to plant cables or instrumentation must occur in relatively cool areas. After an extended lifetime, thermocouples usually fail because of contamination that has entered the assembly.

It is very common in industrial thermocouples or resistance thermometers that the output in converted to 4-20mA loop powered signal for transmission around a plant.

Sensor positioning within a process can be very important. Inadequate immersion will give a low reading temperature error, while a direct line of sight between heaters and sensors may cause a high reading because of radiation error.

There are a lot of points to consider that are not always obvious when choosing between temperature sensors. A few considerations have been highlighted above. Further consideration can include pressure containment, vibration, ingress protection and of course cost.

There is no set answer to what sensor is best as it is dependent on the application and process environment. Our advice as temperature sensor specialists is simple, please just ask us at Peak Sensors and we will be happy to help.