BIGpedia - Thermocouple - Encyclopedia and Dictionary Online

In electronics, thermocouples are a widely used kind of temperature sensor. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1 °C can be difficult to achieve.

A thermopile is a group of thermocouples connected in series.

Contents

1 How they work

2 Linearisation

3 Thermocouples

4 Precautions and considerations for using thermocouples

4.1 Connection problems
4.2 Lead resistance
4.3 Decalibration
4.4 Noise
4.5 Common mode voltage
4.6 Thermal shunting

5 Applications

5.1 Thermocuples in heating appliances

6 References

7 See also

How they work

In 1822, an Estonian physicist named Thomas Seebeck discovered (accidentally) that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a small voltage. Thermocouples rely on this discovery, the so-called Seebeck effect.

Thermocouples produce a voltage output dependent on the temperature difference between the junctions of two dissimilar metal wires. Note that thermocouples do not measure temperature, they measure temperature difference.

To measure temperature, one of the junctions must be maintained at a known (reference) temperature. This junction is known as the “cold junction”. It may be held at 0 °C in an ice flask, or the cold junction may be simulated electronically by adding an appropriate EMF.

Thermocouples always measure difference, even if you can’t see a cold junction. In the example here, the thermocouple wires will join copper PCB tracks. There is no physical ice point, the cold junction is simulated. A sensor measures the temperature at the connector to compensate electrically.

Linearisation

As well as dealing with cold-junction compensation, the measuring instrument must also allow for the fact that the thermocouple output is nonlinear. The relationship between temperature and output voltage is a complex polynomial equation (fifth to ninth order depending on thermocouple type). Analogue methods of linearisation are used in low-cost themocouple meters. Microprocessor-based instruments store thermocouple tables in computer memory to eliminate this source of error.

Thermocouples

Thermocouples are available either as bare wire "bead" thermocouples which offer low cost and fast response times, or built into probes. A wide variety of probes are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research, etc.). One word of warning: when selecting probes, take care to ensure they have the correct type of connector. The two common types of connector are "standard" with round pins and "miniature" with flat pins. This causes some confusion as "miniature" connectors are more popular than "standard" types.

When choosing a thermocouple, consideration should be given to both the thermocouple type, insulation, and probe construction. All of these will have an effect on the measurable temperature range, accuracy, and reliability of the readings. Listed below is a (somewhat subjective) guide to thermocouple types.

Type K (Chromel (Ni-Cr alloy) / Alumel (Ni-Al alloy))

Type K is the "general purpose" thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the −200 °C to +1200 °C range. Sensitivity is approximately 41 µV/°C. Use type K unless you have a good reason not to.

Type E (Chromel / Constantan (Cu-Ni alloy))

Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.

Type J (Iron / Constantan)

Limited range (−40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that can not accept "modern" thermocouples. J types should not be used above 760 °C as an abrupt magnetic transformation will cause permanent decalibration.

Type N (Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy))

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

Thermocouple types B, R, and S are all noble metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approximately 10 µV/°C) they are usually only used for high temperature measurement (>300 °C).

Type B (Platinum-Rhodium/Pt-Rh)

Suited for high temperature measurements up to 1800 °C. Unusually type B thermocouples (due to the shape of their temperature-voltage curve) give the same output at 0 °C and 42 °C. This makes them useless below 50 °C.

Type R (Platinum / Rhodium)

Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Suited for high temperature measurements up to 1600 °C. Low sensitivity (10 µV/°C) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

Type T (Copper / Constantan)

Suited for measurements in the −200 to 0 °C range. The positive conductor is made of copper, and the negative conductor is made of constantan.

Thermocouples are usually selected to ensure that the measuring equipment does not limit the range of temperatures that can be measured. Note that thermocouples with low sensitivity (B, R, and S) have a correspondingly lower resolution.

Precautions and considerations for using thermocouples

Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Listed below are some of the more common problems and pitfalls to be aware of.

Connection problems

Many measurement errors are caused by unintentional thermocouple junctions. Remember that any junction of two different metals will cause a junction. If you need to increase the length of the leads from your thermocouple, you must use the correct type of thermocouple extension wire (e.g., type K for type K thermocouples). Using any other type of wire will introduce a thermocouple junction. Any connectors used must be made of the correct thermocouple material and correct polarity must be observed.

Lead resistance

To minimise thermal shunting and improve response times, thermocouples are made of thin wire (in the case of platinum types, cost is also a consideration). This can cause the thermocouple to have a high resistance which can make it sensitive to noise and can also cause errors due to the input impedance of the measuring instrument. A typical exposed junction thermocouple with 32-AWG wire (0.25 mm diameter) will have a resistance of about 15 ohms per meter. If thermocouples with thin leads or long cables are needed, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so it has a lower resistance) to run between the thermocouple and measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.

Decalibration

Decalibration is the process of unintentionally altering the makeup of thermocouple wire. The usual cause is the diffusion of atmospheric particles into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specifications of the probe insulation.

Noise

The output from a thermocouple is a small signal, so it is prone to electrical noise pick up. Most measuring instruments reject any common mode noise (signals that are the same on both wires) so noise can be minimised by twisting the cable together to help ensure both wires pick up the same noise signal. Additionally, the TC-08 uses an integrating analog-to-digital converter which helps average out any remaining noise. If operating in an extremely noisy environment, (such as near a large motor) it is worthwhile considering using a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see if the reading changes.

Common mode voltage

Although thermocouple signal are very small, much larger voltages often exist at the input to the measuring instrument. These voltages can be caused either by inductive pick up (a problem when testing the temperature of motor windings and transformers) or by "earthed" junctions. A typical example of an "earthed" junction would be measuring the temperature of a hot water pipe with a non insulated thermocouple. If there are any poor earth connections a few volts may exist between the pipe and the earth of the measuring instrument. These signals are again common mode (the same in both thermocouple wires) so will not cause a problem with most instruments provided they are not too large. Common mode voltages can be minimised using the same cabling precautions outlined for noise, and also by using insulated thermocouples.

Thermal shunting

All thermocouples have some mass. Heating this mass takes energy so will affect the temperature you are trying to measure. Consider for example measuring the temperature of liquid in a test tube: there are two potential problems. The first is that heat energy will travel up the thermocouple wire and dissipate to the atmosphere so reducing the temperature of the liquid around the wires. A similar problem can occur if the thermocouple is not sufficiently immersed in the liquid, due to the cooler ambient air temperature on the wires, thermal conduction may cause the thermocouple junction to be a different temperature to the liquid itself. In the above example a thermocouple with thinner wires may help, as it will cause a steeper gradient of temperature along the thermocouple wire at the junction between the liquid and ambient air. If thermocouples with thin wires are used, consideration must be paid to lead resistance. The use of a thermocouple with thin wires connected to much thicker thermocouple extension wire often offers the best compromise.

Applications

Thermocuples in heating appliances

Many gas-fed heating appliances such as ovens, water heaters, space heaters, and other appliances that use a pilot light, use automatic control valves operated by thermocouples for safety. These automatic shut-off valves prevent the accumulation of dangerous gas in case the pilot light should be accidentally extinguished.

Such heating appliances typically employ a main burner which is cycled on and off under the control of a thermostatic valve or thermostat (TS). The main burner is lit by the pilot light on each cycle.

Should the pilot light ever be blown out or lose its steady supply of gas, the gas flowing out of the burner will not be ignited and will flow, uncombusted, into the surrounding environment.

To prevent such a hazard, the TS needs to "know" if and when the pilot light is not working, and will then withhold the supply of gas to the burner.

A thermocouple "senses" when the pilot light is not burning, and the TS "senses" the voltage from the TC. The tip of the TC extends into the envelope of flame that serves as the pilot. No flame, no heat in the tip; no heat in the tip, no voltage imposed on the wire that goes from the TC back to the TS.

References

Original text used by permission of Pico Technology:

Thermocouple application note

Manufacturer of Thermocouples