Thermocouples are the most common temperature sensors. They are cheap, interchangeable, have standard connectors and may measure an array of temperatures. The principle limitation is accuracy, system errors of lower than 1°C can be tough to obtain.

The Direction They Work

In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that this junction between two metals generates a voltage that is a function of temperature. Thermocouples rely on this Seebeck effect. Although nearly every two kinds of metal enables you to make a thermocouple, numerous standard types are being used simply because they possess predictable output voltages and huge temperature gradients.

A K type thermocouple is regarded as the popular and uses nickel-chromium and nickel-aluminium alloys to generate voltage.Standard tables show the voltage produced by thermocouples at any given temperature, so the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is really not easy to simply connect up a voltmeter towards the cartridge heater with thermocouple to measure this voltage, since the connection of your voltmeter leads can certainly make another, undesired thermocouple junction.

Cold Junction Compensation (CJC)

To create accurate measurements, this should be compensated for by using a technique generally known as cold junction compensation (CJC). Should you be wondering why connecting a voltmeter into a thermocouple is not going to make several additional thermocouple junctions (leads connecting for the thermocouple, contributes to the meter, inside the meter etc), what the law states of intermediate metals states that the third metal, inserted involving the two dissimilar metals of the thermocouple junction could have no effect provided that the two junctions are at the identical temperature. This law is likewise important in the making of thermocouple junctions. It is acceptable to create a thermocouple junction by soldering the two metals together since the solder will never impact the reading. In reality, thermocouple junctions are produced by welding both metals together (usually by capacitive discharge). This makes certain that the performance is just not limited with the melting reason for solder.

All standard thermocouple tables provide for this second thermocouple junction by assuming that it must be kept at exactly zero degrees centigrade. Traditionally it was completed with a carefully constructed ice bath (hence the word ‘cold’ junction compensation). Maintaining a ice bath is not really practical for almost all measurement applications, so instead the exact temperature at the point of connection from the thermocouple wires towards the measuring instrument is recorded.

Typically cold junction temperature is sensed with a precision thermistor in good thermal exposure to the input connectors in the measuring instrument. This second temperature reading, in addition to the reading from your thermocouple is employed by the measuring instrument to calculate the actual temperature at the thermocouple tip. Cheaper critical applications, the CJC is performed from a semiconductor temperature sensor. By combining the signal from this semiconductor together with the signal through the thermocouple, the appropriate reading can be found minus the need or expense to record two temperatures. Comprehension of cold junction compensation is essential; any error within the measurement of cold junction temperature will result in the same error from the measured temperature in the thermocouple tip.

And also working with CJC, the measuring instrument also must permit the reality that the thermocouple output is non linear. The relationship between temperature and output voltage is a complex polynomial equation (5th to 9th order dependant upon thermocouple type). Analogue strategies for linearisation are being used in affordable themocouple meters. High accuracy instruments store thermocouple tables in computer memory to eliminate this way to obtain error.

Thermocouple Selection

Thermocouples can be found either as bare wire ‘bead’ thermocouples that provide low priced and fast response times, or built into probes. Numerous types of probes are offered, ideal for different measuring applications (industrial, scientific, food temperature, scientific research etc). One word of warning: when choosing probes take care to ensure they may have the appropriate sort of connector. Both the common kinds of connector are ‘standard’ with round pins and ‘miniature’ with flat pins, this causes some confusion as ‘miniature’ connectors are definitely more popular than ‘standard’ types.

Types

When choosing a thermocouple consideration should be given to the two thermocouple type, insulation and probe construction. Every one of these could have an impact on the measurable temperature range, accuracy and reliability of the readings. Shown below is really a subjective help guide thermocouple types.

When selecting thermocouple types, make sure that your measuring equipment fails to limit the plethora of temperatures that could be measured. Remember that thermocouples with low sensitivity (B, R and S) use a correspondingly lower resolution. The table below summarises the useful operating limits to the various thermocouple types which are described in depth inside the following paragraphs.

Type K is definitely the ‘general purpose’ thermocouple. It is low priced and, owing to its popularity, it is available in numerous probes. Thermocouples can be purchased in the -200°C to 1200°C range. Sensitivity is approx 41uV/°C. Use type K unless there is a good reason never to.

Type E (Chromel / Constantan)

Type E includes a high output (68uV/°C) that makes it well suitable for 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 cannot accept ‘modern’ thermocouples. J types really should not be used above 760°C as being an abrupt magnetic transformation can cause permanent decalibration.

Type N (Nicrosil / Nisil)

High stability and potential to deal with high temperature oxidation makes type N appropriate for high temperature measurements without the fee for platinum (B,R,S) types. Designed to be an ‘improved’ type K, it is gaining popularity.

Thermocouple types B, R and S are ‘noble’ metal thermocouples and exhibit similar characteristics. Those are the most stable of most thermocouples, but because of their low sensitivity (approx 10uV/0C) these are usually only utilized for high temperature measurement (>300°C).

Type B (Platinum / Rhodium)

Suitable for high temperature measurements around 1800°C. Unusually type B thermocouples (because of the shape of their temperature / voltage curve) supply the same output at 0°C and 42°C. This makes them useless below 50°C.

Type R (Platinum / Rhodium)

Suitable for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost means they are unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Suitable for high temperature measurements around 1600°C. Low sensitivity (10uV/vC) and cost makes them unsuitable for general purpose use. For its high stability type S is used as being the standard of calibration for that melting reason for gold (1064.43°C).

Precautions and Considerations for Using Thermocouples

Most measurement problems and errors with thermocouples are due to too little knowledge of how thermocouples work. Thermocouples can are afflicted by ageing and accuracy can vary greatly consequently especially after prolonged being exposed to temperatures with the extremities of the useful operating range. Listed below are the more usual problems and pitfalls to be aware of.

Connection problems

Many measurement errors are generated by unintentional thermocouple junctions. Remember that any junction of two different metals may cause a junction. If you want to increase the length of the leads from your thermocouple, you have to utilize the correct form of thermocouple extension wire (eg type K for type K thermocouples). Using any other type of wire will introduce a thermocouple junction. Any connectors used has to be manufactured from the proper thermocouple material and correct polarity has to be observed.

Lead Resistance

To minimise thermal shunting and improve response times, thermocouples are constructed with thin wire (with regards to platinum types cost is also a consideration). This may increase the risk for thermocouple to get a high resistance that can make it responsive to noise and will also cause errors due to the input impedance of the measuring instrument. A normal exposed junction thermocouple with 32AWG wire (.25mm diameter) may have a resistance of approximately 15 ohms / meter. If thermocouples with thin leads or long cables are needed, it really is worth keeping the thermocouple leads short then using thermocouple extension wire (which can be much thicker, so includes a lower resistance) to work between the thermocouple and measuring instrument. It usually is a good precaution to study the resistance of your own thermocouple before use.

Decalibration

Decalibration is the procedure of unintentionally altering the makeup of thermocouple wire. The usual cause is the diffusion of atmospheric particles to the metal with the extremes of operating temperature. Another cause is impurities and chemicals through the insulation diffusing in to the thermocouple wire. If operating at high temperatures, look into the specifications of your probe insulation.

Noise

The output from a thermocouple is really a small signal, so it is vulnerable to electrical noise grab. Most measuring instruments reject any common mode noise (signals that are identical for both wires) so noise might be minimised by twisting the cable together to help ensure both wires grab exactly the same noise signal. Additionally, an integrating analog to digital converter may be used to helps average out any remaining noise. If operating in an extremely noisy environment, (for example near dexmpky44 large motor) it can be worthwhile considering by using a screened extension cable. If noise pickup is suspected first shut down all suspect equipment and see in the event the reading changes.

Common Mode Voltage

Although thermocouple signal are really small, bigger voltages often exist at the input for the measuring instrument. These voltages can be caused either by inductive pick up (an issue when testing the temperature of motor windings and transformers) or by ‘earthed’ junctions. A standard instance of an ‘earthed’ junction could be measuring the temperature of the very hot water pipe by using a non insulated thermocouple. If there are actually any poor earth connections several volts may exist involving the pipe and also the earth from the measuring instrument. These signals are again common mode (the identical within both thermocouple wires) so is not going to cause an issue with most instruments provided they are certainly not too big.

Thermal Shunting

All thermocouples incorporate some mass. Heating this mass takes energy so will change the temperature you are hoping to measure. Consider as an example measuring the temperature of liquid inside a test tube: there are 2 potential problems. The very first is that heat energy will travel in the thermocouple wire and dissipate towards the atmosphere so reducing the temperature from the liquid across the wires. An identical problem can happen in the event the thermocouple is just not sufficiently immersed from the liquid, because of the cooler ambient air temperature about the wires, thermal conduction might cause the thermocouple junction to be a different temperature to the liquid itself. Within the above example a thermocouple with thinner wires can help, mainly because it will result in a steeper gradient of temperature down the thermocouple wire in the junction in between the liquid and ambient air. If thermocouples with thin wires are employed, consideration should be paid to lead resistance. The use of a thermocouple with thin wires associated with much thicker thermocouple extension wire often gives the best compromise.