Thermocouples are the most popular temperature sensors. They can be cheap, interchangeable, have standard connectors and can measure a variety of temperatures. The key limitation is accuracy, system errors of less than 1°C can be challenging to accomplish.
The Way They Work
In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage that is a purpose of temperature. Thermocouples depend on this Seebeck effect. Although virtually any 2 kinds of metal can be used to make a thermocouple, several standard types are utilized simply because they possess predictable output voltages and large temperature gradients.
A K type thermocouple is considered the most popular and uses nickel-chromium and nickel-aluminium alloys to build voltage.Standard tables show the voltage produced by thermocouples at any temperature, and so the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is not easy to simply connect up a voltmeter for the thermocouple temperature sensor to measure this voltage, since the connection in the voltmeter leads is likely to make a second, undesired thermocouple junction.
Cold Junction Compensation (CJC)
To help make accurate measurements, this should be compensated for using a technique called cold junction compensation (CJC). Should you be wondering why connecting a voltmeter to a thermocouple does not make several additional thermocouple junctions (leads connecting to the thermocouple, contributes to the meter, inside of the meter etc), what the law states of intermediate metals states which a third metal, inserted involving the two dissimilar metals of any thermocouple junction could have no effect provided the two junctions tend to be at a similar temperature. This law is likewise crucial in the making of thermocouple junctions. It really is acceptable to create a thermocouple junction by soldering both the metals together as the solder will not likely impact the reading. In reality, thermocouple junctions are produced by welding both metals together (usually by capacitive discharge). This makes sure that the performance is not limited from the melting point of solder.
All standard thermocouple tables provide for this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this was done with a carefully constructed ice bath (hence the term ‘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 the thermocouple wires towards the measuring instrument is recorded.
Typically cold junction temperature is sensed from a precision thermistor in good thermal connection with the input connectors of the measuring instrument. This second temperature reading, combined with the reading from the thermocouple itself is utilized by the measuring instrument to calculate the real temperature on the thermocouple tip. At a lower price critical applications, the CJC is carried out with a semiconductor temperature sensor. By combining the signal using this semiconductor with the signal from the thermocouple, the right reading can be acquired without the need or expense to record two temperatures. Comprehension of cold junction compensation is very important; any error in the measurement of cold junction temperature will cause the same error inside the measured temperature from the thermocouple tip.
And also coping with CJC, the measuring instrument also needs to permit the point that the thermocouple output is non linear. The relationship between temperature and output voltage can be a complex polynomial equation (5th to 9th order dependant upon thermocouple type). Analogue ways of linearisation are used in inexpensive themocouple meters. High accuracy instruments store thermocouple tables in computer memory to get rid of this source of error.
Thermocouples are offered either as bare wire ‘bead’ thermocouples that offers affordable and fast response times, or included in probes. A multitude of probes are available, suited to different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when choosing probes take care to ensure they already have the proper form of connector. The 2 common varieties of connector are ‘standard’ with round pins and ‘miniature’ with flat pins, this leads to some confusion as ‘miniature’ connectors are more popular than ‘standard’ types.
When picking a thermocouple consideration ought to be made available to the thermocouple type, insulation and probe construction. All of these can have an impact on the measurable temperature range, accuracy and longevity of the readings. Listed here is a subjective help guide thermocouple types.
When selecting thermocouple types, make sure that your measuring equipment does not limit all the different temperatures which can be measured. Keep in mind that thermocouples with low sensitivity (B, R and S) have a correspondingly lower resolution. The table below summarises the useful operating limits for that various thermocouple types that happen to be described in more detail in the following paragraphs.
Type K is the ‘general purpose’ thermocouple. It really is low cost and, because of its popularity, it is available in numerous probes. Thermocouples can be found 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 has a high output (68uV/°C) making 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 key application is to use old equipment that cannot accept ‘modern’ thermocouples. J types should not be used above 760°C as an abrupt magnetic transformation can cause permanent decalibration.
Type N (Nicrosil / Nisil)
High stability and resistance to high temperature oxidation makes type N ideal for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an ‘improved’ type K, it really is becoming more popular.
Thermocouple types B, R and S are typical ‘noble’ metal thermocouples and exhibit similar characteristics. Those are the most stable of all thermocouples, but because of the low sensitivity (approx 10uV/0C) these are usually only utilized for high temperature measurement (>300°C).
Type B (Platinum / Rhodium)
Suited for high temperature measurements up to 1800°C. Unusually type B thermocouples (because of the 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)
Suited for high temperature measurements around 1600°C. Low sensitivity (10uV/°C) and cost ensures they are unsuitable for general purpose use.
Type S (Platinum / Rhodium)
Suited for high temperature measurements approximately 1600°C. Low sensitivity (10uV/vC) and high cost ensures they are unsuitable for general purpose use. Due to its high stability type S is used because the standard of calibration for your melting reason for gold (1064.43°C).
Precautions and Considerations for Using Thermocouples
Most measurement problems and errors with thermocouples are caused by not enough knowledge of how thermocouples work. Thermocouples can suffer from ageing and accuracy may vary consequently especially after prolonged contact with temperatures on the extremities of the useful operating range. Listed below are among the more widespread problems and pitfalls to be aware of.
Many measurement errors are due to unintentional thermocouple junctions. Remember that any junction of two different metals will cause a junction. If you wish to increase the length of the leads from your thermocouple, you have to use the correct form of thermocouple extension wire (eg type K for type K thermocouples). Using any other kind of wire will introduce a thermocouple junction. Any connectors used has to be made of the appropriate thermocouple material and correct polarity needs to be observed.
To minimise thermal shunting and improve response times, thermocouples are made of thin wire (when it comes to platinum types cost is also a consideration). This will make the thermocouple to experience a high resistance that will make it sensitive to noise and might also cause errors due to input impedance from the measuring instrument. An average exposed junction thermocouple with 32AWG wire (.25mm diameter) will have a resistance of around 15 ohms / meter. If thermocouples with thin leads or long cables are important, it can be worth keeping the thermocouple leads short after which using thermocouple extension wire (which is much thicker, so carries a lower resistance) to work between your thermocouple and measuring instrument. It is always a good precaution to appraise the resistance of your thermocouple before use.
Decalibration is the procedure of unintentionally altering the makeup of thermocouple wire. The typical cause may be the diffusion of atmospheric particles in the metal in the extremes of operating temperature. Another cause is impurities and chemicals in the insulation diffusing in the thermocouple wire. If operating at high temperatures, look at the specifications of your probe insulation.
The output coming from a thermocouple is a small signal, it is therefore vulnerable to electrical noise get. Most measuring instruments reject any common mode noise (signals that are exactly the same on both wires) so noise may be minimised by twisting the cable together to help ensure both wires get exactly the same noise signal. Additionally, an integrating analog to digital converter enables you to helps average out any remaining noise. If operating within an extremely noisy environment, (such as near dexmpky44 large motor) it is worthwhile considering utilizing a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see when the reading changes.
Common Mode Voltage
Although thermocouple signal are extremely small, much bigger voltages often exist on the input to the measuring instrument. These voltages might be caused either by inductive pick-up (a challenge when testing the temperature of motor windings and transformers) or by ‘earthed’ junctions. A typical illustration of an ‘earthed’ junction can be measuring the temperature of any warm water pipe using a non insulated thermocouple. If there are actually any poor earth connections a few volts may exist between your pipe as well as the earth of your measuring instrument. These signals are again common mode (the same in thermocouple wires) so will not cause an issue with most instruments provided they are not too large.
All thermocouples incorporate some mass. Heating this mass takes energy so will change the temperature you are attempting to measure. Consider for instance measuring the temperature of liquid in the test tube: there are 2 potential issues. The first is that heat energy will travel up the thermocouple wire and dissipate towards the atmosphere so reducing the temperature of your liquid around the wires. The same problem can occur when the thermocouple is just not sufficiently immersed from the liquid, due to cooler ambient air temperature around the wires, thermal conduction can cause the thermocouple junction to be a different temperature to the liquid itself. In the above example a thermocouple with thinner wires may help, because it will cause a steeper gradient of temperature across the thermocouple wire with the junction involving the liquid and ambient air. If thermocouples with thin wires are utilized, consideration needs to be paid to lead resistance. Utilizing a thermocouple with thin wires connected to much thicker thermocouple extension wire often gives the best compromise.