Thermocouples are typically the most popular temperature sensors. These are cheap, interchangeable, have standard connectors and might measure a wide array of temperatures. The principle limitation is accuracy, system errors of lower than 1°C can be difficult to accomplish.
The Direction They Work
In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that this junction between two metals generates a voltage which is a function of temperature. Thermocouples count on this Seebeck effect. Although nearly every 2 kinds of metal may be used to create a thermocouple, several standard types are used since they possess predictable output voltages and big temperature gradients.
A K type thermocouple is the most popular and uses nickel-chromium and nickel-aluminium alloys to produce voltage.Standard tables show the voltage produced by thermocouples at virtually any temperature, therefore the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is not necessarily possible to simply connect up a voltmeter on the cartridge heater with thermocouple to measure this voltage, since the connection of your voltmeter leads can certainly make a second, undesired thermocouple junction.
Cold Junction Compensation (CJC)
To produce accurate measurements, this has to be compensated for by using a technique called cold junction compensation (CJC). Should you be wondering why connecting a voltmeter to your thermocouple will not make several additional thermocouple junctions (leads connecting towards the thermocouple, contributes to the meter, in the meter etc), legislation of intermediate metals states that the third metal, inserted involving the two dissimilar metals of a thermocouple junction can have no effect provided the two junctions have reached the identical temperature. This law can also be important in the making of thermocouple junctions. It is acceptable to produce a thermocouple junction by soldering both metals together as the solder will never affect the reading. In reality, thermocouple junctions are created by welding the 2 metals together (usually by capacitive discharge). This makes certain 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 must be kept at exactly zero degrees centigrade. Traditionally this became done with a carefully constructed ice bath (hence the phrase ‘cold’ junction compensation). Maintaining a ice bath is not really practical for almost all measurement applications, so instead the actual temperature at the aim of connection of the 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 from the measuring instrument. This second temperature reading, along with the reading in the thermocouple is made use of by the measuring instrument to calculate the genuine temperature at the thermocouple tip. At a discount critical applications, the CJC is conducted by way of a semiconductor temperature sensor. By combining the signal with this semiconductor together with the signal from the thermocouple, the correct reading can be found without 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 from the measured temperature from the thermocouple tip.
As well as working with CJC, the measuring instrument must also allow for the truth that the thermocouple output is non linear. The relationship between temperature and output voltage is a complex polynomial equation (5th to 9th order dependant upon thermocouple type). Analogue strategies for linearisation are utilized in low cost themocouple meters. High accuracy instruments store thermocouple tables in computer memory to remove this method to obtain error.
Thermocouples are offered either as bare wire ‘bead’ thermocouples that offer low cost and fast response times, or that are part of probes. A multitude of probes can be purchased, ideal for different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when deciding on probes make sure to ensure they already have the appropriate type of connector. Both the common forms of connector are ‘standard’ with round pins and ‘miniature’ with flat pins, this leads to some confusion as ‘miniature’ connectors will be more popular than ‘standard’ types.
When picking a thermocouple consideration needs to be provided to the two thermocouple type, insulation and probe construction. Every one of these could have an effect on the measurable temperature range, accuracy and longevity of the readings. Listed here can be a subjective guide to thermocouple types.
When picking thermocouple types, make sure that your measuring equipment does not limit the plethora of temperatures that may be measured. Note that thermocouples with low sensitivity (B, R and S) have a correspondingly lower resolution. The table below summarises the useful operating limits to the various thermocouple types that happen to be described in greater detail in the following paragraphs.
Type K will be the ‘general purpose’ thermocouple. It can be low cost and, due to its popularity, it can be purchased in numerous types of probes. Thermocouples can be found in the -200°C to 1200°C range. Sensitivity is approx 41uV/°C. Use type K unless there is a valid reason not to.
Type E (Chromel / Constantan)
Type E has 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 principle application is with old equipment that cannot accept ‘modern’ thermocouples. J types really should not be used above 760°C being an abrupt magnetic transformation will result in permanent decalibration.
Type N (Nicrosil / Nisil)
High stability and effectiveness against high temperature oxidation makes type N suited to high temperature measurements without the cost of platinum (B,R,S) types. Built to be an ‘improved’ type K, it really is gaining popularity.
Thermocouple types B, R and S are all ‘noble’ metal thermocouples and exhibit similar characteristics. Those are the most stable of all the thermocouples, but because of the low sensitivity (approx 10uV/0C) they can be usually only useful for high temperature measurement (>300°C).
Type B (Platinum / Rhodium)
Best for high temperature measurements approximately 1800°C. Unusually type B thermocouples (due to shape of their temperature / voltage curve) offer the same output at 0°C and 42°C. This may cause them useless below 50°C.
Type R (Platinum / Rhodium)
Designed 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)
Suitable for high temperature measurements up to 1600°C. Low sensitivity (10uV/vC) and high cost causes them to be unsuitable for general purpose use. For its high stability type S is utilized because the standard of calibration for your melting point of gold (1064.43°C).
Precautions and Considerations for Using Thermocouples
Most measurement problems and errors with thermocouples result from an absence of understanding of how thermocouples work. Thermocouples can experience ageing and accuracy may vary consequently especially after prolonged exposure to temperatures in the extremities in their useful operating range. Listed below are some of the more widespread problems and pitfalls to be familiar with.
Many measurement errors develop from unintentional thermocouple junctions. Understand that any junction of two different metals can cause a junction. If you want to increase the duration of the leads through your thermocouple, you need to make use 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 should be created from the appropriate 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 yet another consideration). This could make the thermocouple to have a high resistance that make it understanding of noise and might also cause errors due to input impedance of your measuring instrument. An average exposed junction thermocouple with 32AWG wire (.25mm diameter) could have a resistance around 15 ohms / meter. If thermocouples with thin leads or long cables are essential, it really is worth keeping the thermocouple leads short then using thermocouple extension wire (that is much thicker, so has a lower resistance) to operate in between the thermocouple and measuring instrument. It will always be a great precaution to study the resistance of your thermocouple before use.
Decalibration is the procedure of unintentionally altering the makeup of thermocouple wire. The usual cause will be the diffusion of atmospheric particles to the metal at the extremes of operating temperature. Another cause is impurities and chemicals in the insulation diffusing to the thermocouple wire. If operating at high temperatures, look into the specifications in the probe insulation.
The output from a thermocouple is a small signal, so it will be at risk of electrical noise grab. Most measuring instruments reject any common mode noise (signals that are identical on wires) so noise may be minimised by twisting the cable together to help you ensure both wires pick-up the identical noise signal. Additionally, an integrating analog to digital converter may be used to helps average out any remaining noise. If operating in an extremely noisy environment, (like near dexmpky44 large motor) it is actually worthwhile considering using a screened extension cable. If noise pickup is suspected first turn off all suspect equipment and find out in case the reading changes.
Common Mode Voltage
Although thermocouple signal are really small, larger voltages often exist at the input for 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. A normal demonstration of an ‘earthed’ junction can be measuring the temperature of a hot water pipe with a non insulated thermocouple. If there are any poor earth connections a number of volts may exist between the pipe and the earth in the measuring instrument. These signals are again common mode (a similar within both thermocouple wires) so will never cause a problem with most instruments provided they are not too big.
All thermocouples possess some mass. Heating this mass takes energy so will change the temperature you are hoping to measure. Consider as an example measuring the temperature of liquid inside a test tube: the two main potential issues. The first is that heat energy will travel in the thermocouple wire and dissipate for the atmosphere so decreasing the temperature of your liquid round the wires. A similar problem can take place in case the thermocouple is not sufficiently immersed from the liquid, due to the cooler ambient air temperature on 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 could help, mainly because it can cause a steeper gradient of temperature across the thermocouple wire in the junction between the liquid and ambient air. If thermocouples with thin wires are used, consideration needs to be paid to steer resistance. Utilizing a thermocouple with thin wires linked to much thicker thermocouple extension wire often offers the best compromise.