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Thermocouples are the most common temperature sensors. These 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 obtain.

The Way That They Work

In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage and that is a function of temperature. Thermocouples rely on this Seebeck effect. Although just about any 2 types of metal may be used to make a thermocouple, a number of standard types are utilized because they possess predictable output voltages and huge 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 made by thermocouples at any given temperature, hence the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is really not possible to simply connect up a voltmeter for the thermocouple to measure this voltage, for the reason that connection of the voltmeter leads will make an additional, undesired thermocouple junction.

Cold Junction Compensation (CJC)

To create accurate measurements, this must be compensated for through a technique called cold junction compensation (CJC). In case you are wondering why connecting a voltmeter to your thermocouple will not make several additional thermocouple junctions (leads connecting for the thermocouple, contributes to the meter, within the meter etc), what the law states of intermediate metals states a third metal, inserted involving the two dissimilar metals of a thermocouple junction will have no effect so long as the 2 junctions are at a similar temperature. This law is likewise crucial in the building of thermocouple junctions. It is acceptable to generate a thermocouple junction by soldering both the metals together as the solder is not going to modify the reading. In practice, thermocouple junctions are produced by welding both metals together (usually by capacitive discharge). This makes certain that the performance is just not limited from the melting point of solder.

All standard thermocouple tables enable this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this is carried out with a carefully constructed ice bath (hence the phrase ‘cold’ junction compensation). Maintaining a ice bath is just not practical for many measurement applications, so instead the particular temperature at the point of connection of your thermocouple wires on the measuring instrument is recorded.

Typically cold junction temperature is sensed by way of a precision thermistor in good thermal connection with the input connectors of your measuring instrument. This second temperature reading, together with the reading from the thermocouple is made use of by the measuring instrument to calculate the true temperature on the thermocouple tip. At a discount critical applications, the CJC is carried out by a semiconductor temperature sensor. By combining the signal using this semiconductor together with the signal in the thermocouple, the correct reading can be found minus the need or expense to record two temperatures. Comprehension of cold junction compensation is important; any error from the measurement of cold junction temperature will result in the same error inside the measured temperature in the thermocouple tip.

In addition to coping with CJC, the measuring instrument should also provide for the point that the thermocouple output is non linear. The relationship between temperature and output voltage is actually 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 remove this way to obtain error.

Thermocouple Selection

Thermocouples can be purchased either as bare wire ‘bead’ thermocouples that offers inexpensive and fast response times, or included in probes. A wide variety of probes can be found, suited to different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes take care to ensure they already have the appropriate form of connector. Both the common forms of connector are ‘standard’ with round pins and ‘miniature’ with flat pins, this causes some confusion as ‘miniature’ connectors tend to be more popular than ‘standard’ types.

Types

In choosing a thermocouple consideration needs to be given to both the thermocouple type, insulation and probe construction. Many of these can have an effect on the measurable temperature range, accuracy and reliability of the readings. Shown below is actually a subjective help guide thermocouple types.

When choosing thermocouple types, make sure that your measuring equipment will not limit all the different temperatures that can be measured. Be aware that thermocouples with low sensitivity (B, R and S) possess a correspondingly lower resolution. The table below summarises the useful operating limits for the various thermocouple types which are described in more detail within the following paragraphs.

Type K is definitely the ‘general purpose’ thermocouple. It really is low cost and, owing to its popularity, it is available in numerous types of probes. Thermocouples are available 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 carries a high output (68uV/°C) that makes it well fitted 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 primary application is using old equipment that cannot accept ‘modern’ thermocouples. J types really should not be used above 760°C for 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 suitable for high temperature measurements without the fee for platinum (B,R,S) types. Built to be an ‘improved’ type K, it is becoming more popular.

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

Type B (Platinum / Rhodium)

Designed for high temperature measurements up to 1800°C. Unusually type B thermocouples (as a result of form of their temperature / voltage curve) give the same output at 0°C and 42°C. As a result them useless below 50°C.

Type R (Platinum / Rhodium)

Best for high temperature measurements approximately 1600°C. Low sensitivity (10uV/°C) and cost ensures they are unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Designed for high temperature measurements as much as 1600°C. Low sensitivity (10uV/vC) and high cost causes them to be unsuitable for general purpose use. Due to the high stability type S is used because the standard of calibration for that melting reason for gold (1064.43°C).

Precautions and Things to consider for Using Thermocouples

Most measurement problems and errors with thermocouples are caused by not enough idea of how thermocouples work. Thermocouples can experience ageing and accuracy could differ consequently especially after prolonged contact with temperatures at the extremities of their useful operating range. Shown below are one of the more widespread problems and pitfalls to be familiar with.

Connection problems

Many measurement errors are caused by unintentional thermocouple junctions. Keep in mind that any junction of two different metals will cause a junction. If you want to increase the size of the leads through your thermocouple, you should take advantage of the correct type 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 must be made from the correct thermocouple material and correct polarity needs to be observed.

Lead Resistance

To minimise thermal shunting and improve response times, thermocouples are made from thin wire (with regards to platinum types cost is another consideration). This can result in the thermocouple to have a high resistance that make it responsive to noise and might also cause errors due to input impedance of your measuring instrument. A typical exposed junction thermocouple with 32AWG wire (.25mm diameter) can have a resistance of approximately 15 ohms / meter. If thermocouples with thin leads or long cables are needed, it is actually worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so carries a lower resistance) to operate involving the thermocouple and measuring instrument. It usually is a good precaution to look at the resistance of your own thermocouple before use.

Decalibration

Decalibration is the method of unintentionally altering the makeup of thermocouple wire. The standard cause will be the diffusion of atmospheric particles in the metal on the extremes of operating temperature. Another cause is impurities and chemicals through the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specifications of your probe insulation.

Noise

The output from the thermocouple is actually a small signal, so it is vulnerable to electrical noise grab. Most measuring instruments reject any common mode noise (signals that are exactly the same on both wires) so noise might be minimised by twisting the cable together to help ensure both wires pick up exactly the same noise signal. Additionally, an integrating analog to digital converter may be used to helps average out any remaining noise. If operating inside an extremely noisy environment, (like near dexmpky44 large motor) it really is worthwhile considering employing a screened extension cable. If noise pickup is suspected first turn off all suspect equipment to see when the reading changes.

Common Mode Voltage

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

Thermal Shunting

All thermocouples incorporate some mass. Heating this mass takes energy so will impact the temperature you try to measure. Consider for instance measuring the temperature of liquid in a test tube: the two main potential issues. The initial one is that heat energy will travel within the thermocouple wire and dissipate for the atmosphere so reducing the temperature in the liquid round the wires. A similar problem may appear when the thermocouple will not be sufficiently immersed in the liquid, as a result of cooler ambient air temperature about the wires, thermal conduction may cause the thermocouple junction as a different temperature on the liquid itself. From the above example a thermocouple with thinner wires might help, as it will result in a steeper gradient of temperature over the thermocouple wire in the junction between your liquid and ambient air. If thermocouples with thin wires are used, consideration must be paid to lead resistance. Utilizing a thermocouple with thin wires linked to much thicker thermocouple extension wire often offers the best compromise.