Thermocouples are the most common temperature sensors. They are cheap, interchangeable, have standard connectors and will measure a wide range of temperatures. The primary limitation is accuracy, system errors of lower than 1°C can be hard to accomplish.
The Direction 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 count on this Seebeck effect. Although nearly every two kinds of metal can be used to produce a thermocouple, several standard types are employed simply because they possess predictable output voltages and large temperature gradients.
A K type thermocouple is easily the most popular and uses nickel-chromium and nickel-aluminium alloys to build voltage.Standard tables show the voltage manufactured by thermocouples at any temperature, and so the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is far from easy to simply connect up a voltmeter on the thermocouplers to measure this voltage, as the connection in the voltmeter leads is likely to make an additional, undesired thermocouple junction.
Cold Junction Compensation (CJC)
To make accurate measurements, this needs to be compensated for through a technique called 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), what the law states of intermediate metals states which a third metal, inserted between your two dissimilar metals of your thermocouple junction may have no effect provided that the 2 junctions have reached exactly the same temperature. This law is additionally essential in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by soldering both the metals together as being the solder will not change the reading. In practice, thermocouple junctions are produced by welding the two metals together (usually by capacitive discharge). This makes sure 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 must be kept at exactly zero degrees centigrade. Traditionally this is done with a carefully constructed ice bath (hence the word ‘cold’ junction compensation). Maintaining a ice bath is not practical for almost all measurement applications, so instead the actual temperature at the aim of connection in the thermocouple wires towards the measuring instrument is recorded.
Typically cold junction temperature is sensed by a precision thermistor in good thermal experience of the input connectors of your measuring instrument. This second temperature reading, together with the reading through the thermocouple itself is employed by the measuring instrument to calculate the actual temperature in the thermocouple tip. At a discount critical applications, the CJC is carried out with a semiconductor temperature sensor. By combining the signal from this semiconductor together with the signal from the thermocouple, the right reading can be acquired minus the need or expense to record two temperatures. Comprehension of cold junction compensation is essential; any error from the measurement of cold junction temperature will lead to the same error from the measured temperature from the thermocouple tip.
Along with handling 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 depending on thermocouple type). Analogue ways of linearisation are being used in inexpensive themocouple meters. High accuracy instruments store thermocouple tables in computer memory to reduce this source of error.
Thermocouples are available either as bare wire ‘bead’ thermocouples that provide affordable and fast response times, or built in probes. A wide variety of probes are offered, suited to different measuring applications (industrial, scientific, food temperature, scientific research etc). One word of warning: when deciding on probes make sure to ensure they have the proper sort of connector. Both common varieties 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.
In choosing a thermocouple consideration needs to be provided to both thermocouple type, insulation and probe construction. Most of these will have an impact on the measurable temperature range, accuracy and reliability of the readings. Listed below is really a subjective help guide to thermocouple types.
When picking thermocouple types, ensure that your measuring equipment will not limit all the different temperatures that may be measured. Keep in mind that thermocouples with low sensitivity (B, R and S) have got a correspondingly lower resolution. The table below summarises the useful operating limits to the various thermocouple types that happen to be described in depth inside the following paragraphs.
Type K may be the ‘general purpose’ thermocouple. It is low priced and, due to its popularity, it is available in a multitude of probes. Thermocouples can be bought in the -200°C to 1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you have a valid reason never to.
Type E (Chromel / Constantan)
Type E carries a high output (68uV/°C) rendering 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 using old equipment that cannot accept ‘modern’ thermocouples. J types really should not be used above 760°C being an abrupt magnetic transformation may cause permanent decalibration.
Type N (Nicrosil / Nisil)
High stability and effectiveness against high temperature oxidation makes type N suitable for high temperature measurements without the price of platinum (B,R,S) types. Designed to be an ‘improved’ type K, it is becoming more popular.
Thermocouple types B, R and S are all ‘noble’ metal thermocouples and exhibit similar characteristics. These are most stable of most 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)
Best for high temperature measurements approximately 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. As a result them useless below 50°C.
Type R (Platinum / Rhodium)
Suitable for high temperature measurements around 1600°C. Low sensitivity (10uV/°C) and high cost ensures they are unsuitable for general purpose use.
Type S (Platinum / Rhodium)
Best for high temperature measurements up to 1600°C. Low sensitivity (10uV/vC) and high cost means they are unsuitable for general purpose use. Because of its high stability type S is utilized because the standard of calibration to the melting reason for gold (1064.43°C).
Precautions and Considerations for Using Thermocouples
Most measurement problems and errors with thermocouples are caused by a lack of understanding of how thermocouples work. Thermocouples can suffer from ageing and accuracy can vary consequently especially after prolonged exposure to temperatures with the extremities of their useful operating range. Listed below are the more widespread problems and pitfalls to pay attention to.
Many measurement errors are due to unintentional thermocouple junctions. Remember that any junction of two different metals will result in a junction. If you need to increase the duration of the leads through your thermocouple, you should take advantage of the correct kind 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 created from the correct thermocouple material and correct polarity should be observed.
To minimise thermal shunting and improve response times, thermocouples are made of thin wire (when it comes to platinum types cost is another consideration). This will result in the thermocouple to get a high resistance that make it understanding of noise and may also cause errors due to the input impedance of the measuring instrument. An average 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 important, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so has a lower resistance) to run between the thermocouple and measuring instrument. It usually is a good precaution to study the resistance of your own thermocouple before use.
Decalibration is the method of unintentionally altering the makeup of thermocouple wire. The typical cause will be the diffusion of atmospheric particles into the metal on the extremes of operating temperature. Another cause is impurities and chemicals from the insulation diffusing in to the thermocouple wire. If operating at high temperatures, check the specifications in the probe insulation.
The output from a thermocouple is actually a small signal, it is therefore prone to electrical noise get. Most measuring instruments reject any common mode noise (signals that are the same on both wires) so noise could be minimised by twisting the cable together to assist ensure both wires pick-up the identical noise signal. Additionally, an integrating analog to digital converter can be used to helps average out any remaining noise. If operating in a extremely noisy environment, (for example near dexmpky44 large motor) it really is worthwhile considering using a screened extension cable. If noise pickup is suspected first shut down all suspect equipment and find out in case the reading changes.
Common Mode Voltage
Although thermocouple signal are really small, larger voltages often exist on the input towards the measuring instrument. These voltages might be caused either by inductive get (an issue when testing the temperature of motor windings and transformers) or by ‘earthed’ junctions. An average demonstration of an ‘earthed’ junction could be measuring the temperature of any hot water pipe by using a non insulated thermocouple. If there are any poor earth connections a number of volts may exist between your pipe as well as the earth of the measuring instrument. These signals are again common mode (the same within both thermocouple wires) so is not going to cause an issue with most instruments provided they are not too big.
All thermocouples incorporate some mass. Heating this mass takes energy so will affect the temperature you try to measure. Consider by way of example measuring the temperature of liquid in a test tube: there are 2 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 throughout the wires. A comparable problem can occur when the thermocouple will not be sufficiently immersed inside the liquid, because of the 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 result in a steeper gradient of temperature over the thermocouple wire in the junction between the liquid and ambient air. If thermocouples with thin wires are utilized, consideration must be paid to lead resistance. The use of a thermocouple with thin wires connected to much thicker thermocouple extension wire often provides the best compromise.