Thermocouple temperature sensor

How to choose a thermocouple?

Thermocouples are very popular temperature sensors. They are inexpensive, interchangeable, robust and can measure a wide temperature range. 

Thermocouple measurement probes essentially consist of two dissimilar metals/alloys. When joining, they will generate a weak signal if there is a temperature difference between the hot junction (measuring junction) and the cold junction or reference junction. 

The signal depends only on the temperature difference. And, as such, a thermocouple probe cannot be tested properly if both junctions are at the same temperature. 

Although almost any type of metal can be used to make a temperature sensor, a number of standard types are used because they have predictable output voltages and large temperature gradients.

It is essential that the wiring (cold junction) to the meter be of compensating or extending material of the same rating as the thermocouple. The use of copper wires or other materials will cause loss of EMC and therefore an error. 

The law of intermediate metals states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided both junctions are at the same temperature. 

This law is also important in the construction of junctions. It is acceptable to make a junction by soldering the two metals, as the soldering will not affect the temperature reading. In practice, thermocouple junctions are made by soldering together the two metals. This ensures that temperature measurement performance is not limited by the melting point of the solder.

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What is a thermocouple?

Sensors are available as "bare" thermocouples that are inexpensive and offer fast response. They are integrated in temperature probes or mineral insulated metal tubes. 

A wide variety of sensors are available, suitable for different measurement applications (industrial, scientific, food temperature, medical research, etc.). When choosing a thermocouple, consideration must be given to the type of sensor, the insulation and the construction of the temperature probe. All of this will have an effect on the measurable temperature range, measurement accuracy and reliability of the temperature reading.

Thermocouple curve


Thermocouple type K (Chromel / Alumel)

Type K is the “general purpose” thermocouple. It is inexpensive and, due to its popularity, it is available in a wide variety of probes. 

Thermocouples are available with a temperature range of -200 to +1200 degrees celsius. 

The sensitivity is around 41uV/°C. 

Use Type K unless you have a good reason not to.

Material + Chromel / - Alumel

Color + Yellow / - Red

Thermocouple type J (Iron / Constantan)

The limited measuring range (-40 to +750°C) makes this type of probe less popular than type K. 

The primary application for this temperature probe is with older equipment that cannot accept "modern" thermocouples. J types should not be used above 760°C, as an abrupt magnetic transformation will cause permanent decalibration.

Material + Iron / - Constantan

Color White / Red

Thermocouple type N (Nicrosil / Nisil)

High stability and resistance to oxidation make type N suitable for high temperature measurements without the cost of platinum types (B, R, S). Designed to be an improved "K" thermocouple measurement probe, it is becoming more and more popular.

Material + Nicrosil /

Nile Color + Orange / - Red

Types B, R and S are all "noble" metal thermocouples and have similar characteristics. 

These measuring instruments are the most stable of all thermocouples. But due to their low sensitivity (around 10uV/0C), they are usually only used for high temperature measurements (>600°C). These noble metal temperature sensors all require high purity ceramic protective sheaths for use in industrial applications.

Thermocouple type B (Platinum / Rhodium)

Suitable for high temperature measurements up to 1800 ° C.

Type R thermocouple (platinum / rhodium)

This thermocouple temperature sensor is suitable for high temperature measurements up to 1600°C. However, it has low sensitivity (10uV/°C) and high cost.

Thermocouple type S (platinum / rhodium)

This temperature probe is suitable for high temperature measurements up to 1600°C. Due to its high stability, type S is used as a standard for the melting point of gold (1064,43°C).

Instructions

Precautions and considerations when using thermocouples

Most temperature measurement problems and errors are due to a lack of understanding of how thermocouples work. 

These temperature indicators can suffer from aging and the accuracy can vary accordingly, especially after prolonged exposure to temperatures at the extremes of their useful operating range. Here are some of the most common issues to consider.

Problems connecting thermocouple measuring devices

Many measurement errors are caused by unintentional junctions. Any joining of two dissimilar metals will result in a joining. If you need to increase the lead length of your measuring device, you must use the extension cable type (e.g. Type K for Type K thermocouples). 

Using another type of wire will present a thermocouple sensor junction. The connectors used must be made of the correct material and correct polarity must be observed. Any shorting of the wires in the terminal header or connector will create another junction and the instrument will read that temperature and not the hot junction temperature.

Lead resistance

To improve response times, thermocouples are made of thin wire (in the case of platinum types, cost is also a consideration). This can lead to high resistance of the measurement system which can make it susceptible to noise and can also cause errors due to the input impedance of the measurement instrument. 

A typical exposed junction temperature sensor with 32AWG (0,25mm diameter) wire will have a resistance of approximately 15 ohms/meter. If thermocouples with thin wires or long cables are needed, it's worth keeping the wires short and then using an extension wire (which is much thicker, so lower resistance) to run between the thermocouple and the measuring instrument.

interference

The output of a thermocouple is a weak signal, so it is prone to electrical interference. If your sensor is in a noisy environment (for example, near an electric motor), it is recommended to use a shielded extension cable. If the noise sensor is suspected, turn off all suspect equipment and check if the reading changes.

There is no possible maintenance function on a thermocouple, but programmed calibration checks are recommended.

  • Thermocouples drift in calibration, but the rate of drift depends on time and temperature.
  • In a known temperature source, check the thermocouple output with the Thermocouple C degrees from the table.
  • Thermocouples or their wiring can short or open, causing error signals. Another fault condition for data loggers or transmitters is low insulation resistance between the conductors and ground, which causes the thermocouple loop to ground.

If the thermocouple shows one of the 3 fault conditions, it must be replaced.

More information on thermocouples below!

Temperature range Conversion table (T ° / mV)