Identifying Temperature Sensor Type

Identifying Temperature Sensors

We often receive phone calls from potential customers who have a temperature sensor in their hands, but little other information about it. This presents us with a number of problems. So if you want to know what is the difference between a thermocouple and pt100, please continue reading. The first issue is identifying exactly what type of temperature sensor it is.

Sometimes people ask for a Pt100 thermocouple. This is an incorrect name for a temperature sensor design as Pt100s and Thermocouples are two very different technologies.

Most industrial plant contact temperature sensors are thermocouples, resistance thermometers, or thermistors. To identify the type of sensor, our advice goes along the lines of:

  1. Is there any visible information on the instrument that displays the temperature?
  2. What temperatures are usually recorded?
  3. How many and what colour are any wires on the sensor or subsequent cabling?
  4. Disconnect the sensor from the system and take a resistance measurement across any terminals on the sensor with a multimeter.

How To Identify A Temperature Sensor Type (Step-By-Step)?

Step 1: Instrumentation Information

This may not be straightforward due to Jargon. Possibilities are:

There is also a range of other less common sensor assemblies which exact but are extremely rare.

Step 2: Temperature Sensor Range

The temperature sensor range that each sensor type can work at can hold vital information as to which sensor the customer is currently using. The usual maximum temperatures of sensors can be summarised as below, however it is not an absolute limit and individual designs can have other limitations.

MI thermocouple with a head
Sensor Type Common limit
K 1100°C
J 800°C
N 1200°C
R 1500°C
S 1500°C
B 1600°C
RTD 500°C

Step 3: Quantity and Colour of Wires

Colour codes can identify a sensor. If there are 2 wires, it could be any type of sensor. The options on Thermocouple colours are summarised here.

On top of that, a red and white wire will usually signify a resistance thermometer. American (ANSI) colour codes is another possibility.

If there are 3 wires, two red and one white, it is almost certainly a 3 wire Resistance thermometer. A third wire could also be a sheath grounding wire, while the other two are a thermocouple.

If there are 4 wires, two red and two white, it is probably a 4-wire resistance thermometer. Any other colours can be again cross-checked with the above tables as you are probably looking at a duplex thermocouple with two measuring points.

Step 4: Physical Readings

This requires some disassembly and should only be done by experienced, authorised personnel. A resistance reading across the sensor terminals can be decisive. Assuming the temperature sensor is quite small (less than say 1 metre long) a very low resistance reading at any temperature (less than 10 Ώ) will indicate that it is a thermocouple. Around 110Ώ at room temperature will almost certainly be a Pt100, or 1100Ώ at room temp a Pt 1000. A much higher resistance at room temperature is probably a thermistor. Open circuit readings simply indicate that the sensor is broken, and you probably knew that anyway.

We hope that by following this step by step guide, you should find it easy to identify your temperature sensor or assist others to do so.

View our temperature sensors

Rare Metal Thermocouples

Base Metal Thermocouples

Base Metal Cable Thermocouples

Cable Resistance Thermometers

If you want to order a temperature sensor or you are unsure exactly what you need, get in touch and we can help you.

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