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Thermocouples are the most popular temperature sensors. These are cheap, interchangeable, have standard connectors and may measure a wide range of temperatures. The primary limitation is accuracy, system errors of under 1°C can be tough to achieve.

How They Work

In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is actually a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any 2 types of metal enables you to create a thermocouple, several standard types are being used simply because they possess predictable output voltages and big temperature gradients.

A K type thermocouple is easily the most popular and uses nickel-chromium and nickel-aluminium alloys to produce voltage.Standard tables show the voltage manufactured by thermocouples at virtually any temperature, hence the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is far from possible to simply connect up a voltmeter on the thermocouple temperature sensor to measure this voltage, because the connection in the voltmeter leads can certainly make a 2nd, undesired thermocouple junction.

Cold Junction Compensation (CJC)

To produce accurate measurements, this must be compensated for by using a technique known as cold junction compensation (CJC). Should you be wondering why connecting a voltmeter to some thermocouple will not make several additional thermocouple junctions (leads connecting on the thermocouple, brings about the meter, in the meter etc), legislation of intermediate metals states a third metal, inserted involving the two dissimilar metals of any thermocouple junction may have no effect given that the 2 junctions tend to be at a similar temperature. This law is likewise essential in the making of thermocouple junctions. It is actually acceptable to create a thermocouple junction by soldering the 2 metals together as being the solder is not going to change the reading. In practice, thermocouple junctions are created by welding the 2 metals together (usually by capacitive discharge). This ensures that the performance will not be limited through the melting point of solder.

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

Typically cold junction temperature is sensed by way of a precision thermistor in good thermal experience of the input connectors of your measuring instrument. This second temperature reading, together with the reading in the thermocouple itself is used by the measuring instrument to calculate the real temperature with the thermocouple tip. At a discount critical applications, the CJC is performed with a semiconductor temperature sensor. By combining the signal from this semiconductor with all the signal through the thermocouple, the right reading can be had without the need or expense to record two temperatures. Understanding of cold junction compensation is important; any error within the measurement of cold junction temperature will cause the same error from the measured temperature from the thermocouple tip.

And also dealing with CJC, the measuring instrument also needs to permit the point that the thermocouple output is non linear. The connection between temperature and output voltage can be a complex polynomial equation (5th to 9th order based on thermocouple type). Analogue ways of linearisation are employed in low priced themocouple meters. High accuracy instruments store thermocouple tables in computer memory to eliminate this source of error.

Thermocouple Selection

Thermocouples are available either as bare wire ‘bead’ thermocouples that offer low cost and fast response times, or that are part of probes. Numerous probes can be found, suited to different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes make sure to ensure they have the correct kind of connector. The 2 common kinds of connector are ‘standard’ with round pins and ‘miniature’ with flat pins, this leads to some confusion as ‘miniature’ connectors tend to be more popular than ‘standard’ types.

Types

In choosing a thermocouple consideration should be provided to the thermocouple type, insulation and probe construction. Every one of these may have an impact on the measurable temperature range, accuracy and longevity of the readings. Further down is a subjective help guide to thermocouple types.

When deciding on thermocouple types, ensure that your measuring equipment will not limit all the different temperatures that may be measured. Be aware that thermocouples with low sensitivity (B, R and S) use a correspondingly lower resolution. The table below summarises the useful operating limits for your various thermocouple types that happen to be described in greater detail from the following paragraphs.

Type K is the ‘general purpose’ thermocouple. It can be low priced and, owing to its popularity, it comes in a multitude of probes. Thermocouples can be purchased in the -200°C to 1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you do have a valid reason to not.

Type E (Chromel / Constantan)

Type E carries a high output (68uV/°C) which makes it well designed for low temperature (cryogenic) use. Another property is it is non-magnetic.

Type J (Iron / Constantan)

Limited range (-40 to 750°C) makes type J less popular than type K. The primary application is to use old equipment that cannot accept ‘modern’ thermocouples. J types must not be used above 760°C as an abrupt magnetic transformation will result in permanent decalibration.

Type N (Nicrosil / Nisil)

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

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

Type B (Platinum / Rhodium)

Best for high temperature measurements around 1800°C. Unusually type B thermocouples (due to model of their temperature / voltage curve) provide the same output at 0°C and 42°C. This makes them useless below 50°C.

Type R (Platinum / Rhodium)

Best for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and cost causes them to be unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Suitable for high temperature measurements around 1600°C. Low sensitivity (10uV/vC) and cost means they are unsuitable for general purpose use. Because of its high stability type S is utilized as the standard of calibration for that melting point of gold (1064.43°C).

Precautions and Things to consider for Using Thermocouples

Most measurement problems and errors with thermocouples result from an absence of comprehension of how thermocouples work. Thermocouples can are afflicted by ageing and accuracy may vary consequently especially after prolonged contact with temperatures with the extremities of their useful operating range. Further down are one of the more common problems and pitfalls to be familiar with.

Connection problems

Many measurement errors develop from unintentional thermocouple junctions. Remember that any junction of two different metals may cause a junction. If you need to increase the length of the leads from your thermocouple, you need to use the correct sort 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 needs to be made from the appropriate thermocouple material and correct polarity must be observed.

Lead Resistance

To minimise thermal shunting and improve response times, thermocouples are constructed with thin wire (in the case of platinum types cost is yet another consideration). This will make the thermocouple to have a high resistance which can make it responsive to noise and will also cause errors because of the input impedance of your measuring instrument. An average exposed junction thermocouple with 32AWG wire (.25mm diameter) may have a resistance of around 15 ohms / meter. If thermocouples with thin leads or long cables are essential, it is actually worth keeping the thermocouple leads short and after that using thermocouple extension wire (that is much thicker, so carries a lower resistance) to work involving the thermocouple and measuring instrument. It is always an effective precaution to study the resistance of your 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 into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from your insulation diffusing in the thermocouple wire. If operating at high temperatures, check the specifications of your probe insulation.

Noise

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

Common Mode Voltage

Although thermocouple signal are incredibly small, much larger voltages often exist on the input towards the measuring instrument. These voltages can be caused either by inductive pick-up (a difficulty when testing the temperature of motor windings and transformers) or by ‘earthed’ junctions. A typical instance of an ‘earthed’ junction could be measuring the temperature of a boiling water pipe using a non insulated thermocouple. If there are actually any poor earth connections a few volts may exist in between the pipe along with the earth of your measuring instrument. These signals are again common mode (the same within both thermocouple wires) so will not cause a problem with most instruments provided they are not too big.

Thermal Shunting

All thermocouples get some mass. Heating this mass takes energy so will affect the temperature you are hoping to measure. Consider for example measuring the temperature of liquid in a test tube: the two main potential issues. First is that heat energy will travel up the thermocouple wire and dissipate to the atmosphere so reducing the temperature of the liquid throughout the wires. A comparable problem can occur in case the thermocouple will not be sufficiently immersed within the liquid, as a result of cooler ambient air temperature about the wires, thermal conduction may cause the thermocouple junction to become different temperature for the liquid itself. In the above example a thermocouple with thinner wires could help, since it will result in a steeper gradient of temperature along the thermocouple wire on the junction between the liquid and ambient air. If thermocouples with thin wires are being used, consideration should be paid to lead resistance. Using a thermocouple with thin wires attached to much thicker thermocouple extension wire often provides the best compromise.