Thermocouples are the most popular temperature sensors. They can be cheap, interchangeable, have standard connectors and can measure an array of temperatures. The principle limitation is accuracy, system errors of less than 1°C can be difficult to accomplish.
The Way That They Work
In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) the junction between two metals generates a voltage that is a function of temperature. Thermocouples rely on this Seebeck effect. Although just about any 2 kinds of metal enables you to make a thermocouple, numerous standard types are used because they possess predictable output voltages and large temperature gradients.
A K type thermocouple is the most popular and uses nickel-chromium and nickel-aluminium alloys to create voltage.Standard tables show the voltage manufactured by thermocouples at any temperature, therefore the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is far from possible to simply connect up a voltmeter to the thermocouple to measure this voltage, as the connection of the voltmeter leads will make an additional, undesired thermocouple junction.
Cold Junction Compensation (CJC)
To create accurate measurements, this has to be compensated for using a technique referred to as cold junction compensation (CJC). Should you be wondering why connecting a voltmeter into a thermocouple does not make several additional thermocouple junctions (leads connecting towards the thermocouple, contributes to the meter, within the meter etc), what the law states of intermediate metals states a third metal, inserted between your two dissimilar metals of any thermocouple junction will have no effect provided both junctions are at a similar temperature. This law is additionally essential in the building of thermocouple junctions. It is acceptable to produce a thermocouple junction by soldering the two metals together since the solder will never modify the reading. In reality, thermocouple junctions are manufactured by welding both metals together (usually by capacitive discharge). This makes certain that the performance is not really limited by 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 it was carried out with a carefully constructed ice bath (hence the expression ‘cold’ junction compensation). Maintaining a ice bath is just not practical for the majority of measurement applications, so instead the exact temperature at the aim of connection of your thermocouple wires on the measuring instrument is recorded.
Typically cold junction temperature is sensed with a precision thermistor in good thermal exposure to the input connectors of your measuring instrument. This second temperature reading, together with the reading from the thermocouple itself is made use of by the measuring instrument to calculate the real temperature with the thermocouple tip. For less critical applications, the CJC is conducted by way of a semiconductor temperature sensor. By combining the signal from this semiconductor using the signal from your thermocouple, the proper reading can be acquired without having the need or expense to record two temperatures. Idea of cold junction compensation is very important; any error within the measurement of cold junction temperature will cause the same error in the measured temperature from the thermocouple tip.
And also handling CJC, the measuring instrument also must permit the reality that the thermocouple output is non linear. Your relationship between temperature and output voltage is really a complex polynomial equation (5th to 9th order depending on thermocouple type). Analogue methods of linearisation are utilized in affordable themocouple meters. High accuracy instruments store thermocouple tables in computer memory to eliminate this way to obtain error.
Thermocouples can be purchased either as bare wire ‘bead’ thermocouples that provide inexpensive and fast response times, or that are part of probes. A multitude of probes are offered, appropriate 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 right type of connector. The two common varieties 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 picking a thermocouple consideration must be given to the thermocouple type, insulation and probe construction. Many of these can have an effect on the measurable temperature range, accuracy and longevity of the readings. Listed below is a subjective guide to thermocouple types.
When picking thermocouple types, make sure that your measuring equipment is not going to limit the range of temperatures that can be measured. Remember that thermocouples with low sensitivity (B, R and S) have got a correspondingly lower resolution. The table below summarises the useful operating limits for that various thermocouple types that are described in greater detail within the following paragraphs.
Type K will be the ‘general purpose’ thermocouple. It is actually affordable and, because of its popularity, it can be purchased in numerous probes. Thermocouples can be purchased in the -200°C to 1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you will 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 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 must not be used above 760°C being an abrupt magnetic transformation may 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. Made to be an ‘improved’ type K, it really is gaining popularity.
Thermocouple types B, R and S are common ‘noble’ metal thermocouples and exhibit similar characteristics. They are the most stable of thermocouples, but because of the low sensitivity (approx 10uV/0C) these are usually only useful for high temperature measurement (>300°C).
Type B (Platinum / Rhodium)
Best for high temperature measurements as much as 1800°C. Unusually type B thermocouples (because of the form 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)
Suitable for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost ensures they are unsuitable for general purpose use.
Type S (Platinum / Rhodium)
Suitable for high temperature measurements as much as 1600°C. Low sensitivity (10uV/vC) and high cost ensures they are unsuitable for general purpose use. Because of its high stability type S is utilized since 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 caused by an absence of knowledge of how thermocouples work. Thermocouples can experience ageing and accuracy can vary consequently especially after prolonged exposure to temperatures at the extremities with their useful operating range. Further down are the more prevalent problems and pitfalls to understand.
Many measurement errors are caused by unintentional thermocouple junctions. Do not forget that any junction of two different metals will result in a junction. If you need to increase the length of the leads through your thermocouple, you must make use 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 has to be manufactured from the right thermocouple material and correct polarity needs to be observed.
To minimise thermal shunting and improve response times, thermocouples are constructed with thin wire (in the matter of platinum types cost is yet another consideration). This may increase the risk for thermocouple to get a high resistance which can make it responsive to noise and may also cause errors because of the input impedance from the measuring instrument. An average exposed junction thermocouple with 32AWG wire (.25mm diameter) will 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 after which using thermocouple extension wire (which is much thicker, so features a lower resistance) to perform involving the thermocouple and measuring instrument. It will always be an excellent precaution to appraise the resistance of your respective thermocouple before use.
Decalibration is the procedure of unintentionally altering the makeup of thermocouple wire. The standard cause is definitely the diffusion of atmospheric particles into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from your insulation diffusing into the thermocouple wire. If operating at high temperatures, examine the specifications of the probe insulation.
The output from your thermocouple can be a small signal, so it will be at risk of electrical noise get. 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 assist ensure both wires get the same noise signal. Additionally, an integrating analog to digital converter could be used to helps average out any remaining noise. If operating in an extremely noisy environment, (such as near dexmpky44 large motor) it is actually worthwhile considering utilizing a screened extension cable. If noise pickup is suspected first shut down all suspect equipment and see if the reading changes.
Common Mode Voltage
Although thermocouple signal are very small, much bigger voltages often exist at the input on the measuring instrument. These voltages could 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 will be measuring the temperature of the hot water pipe with a non insulated thermocouple. If you will find any poor earth connections several volts may exist between the pipe and also the earth in the measuring instrument. These signals are again common mode (exactly the same both in thermocouple wires) so will not likely cause an issue with most instruments provided they are certainly not too big.
All thermocouples incorporate some mass. Heating this mass takes energy so will change the temperature you are trying to measure. Consider for instance measuring the temperature of liquid inside a test tube: there are two potential issues. First is that heat energy will travel within the thermocouple wire and dissipate for the atmosphere so lowering the temperature of the liquid around the wires. A comparable problem can happen if the thermocouple is not sufficiently immersed inside the liquid, because of the cooler ambient air temperature around the wires, thermal conduction can cause the thermocouple junction to become different temperature towards the liquid itself. Inside the above example a thermocouple with thinner wires might help, mainly because it will cause a steeper gradient of temperature over the thermocouple wire with the junction in between the liquid and ambient air. If thermocouples with thin wires are used, consideration needs to be paid to lead resistance. The use of a thermocouple with thin wires connected to much thicker thermocouple extension wire often gives the best compromise.