Thermocouple temperature sensor
How to choose a thermocouple?
Choice of thermocouple Thermocouples are the most common temperature sensors. They are inexpensive, interchangeable, robust and can measure a wide range of temperatures. Thermocouples are essentially made up of two dissimilar metals / alloys which, on joining, will generate a weak signal if there is a temperature difference between the hot junction (measuring junction) and the cold junction or the reference junction. The signal depends only on the temperature difference and, as such, a thermocouple cannot be tested correctly if the two junctions are at the same temperature. Although almost any type of metal can be used to make a thermocouple, a number of standard types are used because they have predictable output voltages and large temperature gradients.
It is essential that the thermocouple (cold junction) wiring to the measuring instrument be made of compensation material or extension of the same calibration as the thermocouple. The use of copper wire or other materials will result in loss of EMC and therefore an error. The law of the intermediate metals indicates that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions are at the same temperature. This law is also important in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by welding the two metals, because the solder will not affect the reading. In practice, the thermocouple junctions are made by welding together the two metals; This ensures that the performance is not limited by the melting point of the weld.
What is a thermocouple?
Thermocouples are available as inexpensive "bare" thermocouples that offer fast response times, built into mineral insulated metal probes or tubes. A wide variety of sensors are available, suitable for different measurement applications (industrial, scientific, food temperature, medical research, etc.). When choosing a thermocouple, consider the type of thermocouple, insulation and construction of the probe. All of this will have an effect on the measurable temperature range, accuracy and reliability of the readings.
Thermocouple type K (Chromel / Alumel)
Type K is the "general purpose" thermocouple. It is inexpensive and, because of its popularity, is available in a wide variety of probes. The thermocouples are available in the range -200 ° C to + 1200 ° C. The sensitivity is about 41uV / ° C. Use type K unless you have a good reason not to do it.
Material + Chromel / - Alumel
Color + Yellow / - Red
Thermocouple type J (Iron / Constantan)
The limited range (-40 to + 750 ° C) makes Type J less popular than Type K. The main application is with older equipment that can not accept "modern" thermocouples. J types should not be used above 760 ° C because abrupt magnetic transformation will result in permanent decalibration.
Material + Iron / - Constantan
Color White / Red
Thermocouple type N (Nicrosil / Nisil)
High stability and high temperature oxidation resistance make N type suitable for high temperature measurements without the cost of platinum types (B, R, S). Designed to be "K" improved, it is becoming more and more popular.
Material + Nicrosil /
Nile Color + Orange / - Red
Thermocouple types B, R, and S are all "noble" metal thermocouples and have similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (around 10uV / 0C), they are generally only used for high temperature measurements (> 600 ° C). These noble metal thermocouples all require high purity ceramic protective sheaths intended for use in industrial applications.
Thermocouple type B (Platinum / Rhodium)
Suitable for high temperature measurements up to 1800 ° C.
Type R thermocouple (platinum / rhodium)
Suitable for high temperature measurements up to 1600 ° C. Low sensitivity (10uV / ° C) and high cost.
Thermocouple type S (platinum / rhodium)
Suitable for high temperature measurements up to 1600 ° C. Because of its high stability, type S is used as a standard for the gold melting point (1064,43 ° C).
Precautions and considerations when using thermocouples
Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Thermocouples may suffer from aging and accuracy may vary accordingly, especially after prolonged exposure to temperatures at the ends of their useful operating range. Here are some of the most common issues to consider.
Many measurement errors are caused by unintentional thermocouple junctions. Any junction of two different metals will result in a junction. If you need to increase the lead length of your thermocouple, you must use the type of thermocouple extension cable (eg Type K for type K thermocouples). Use of another type of wire will have a thermocouple junction. The connectors used must be made of the correct thermocouple material and correct polarity must be observed. Shorting the thermocouple wires in the head or terminal connector will create another junction and the instrument will read this temperature and not the hot junction temperature.
To improve response times, thermocouples consist of a thin wire (in the case of platinum types, cost is also a consideration). This can result in high thermocouple resistance that may make it sensitive to noise and may also cause errors due to the input impedance of the meter. A typical exposed junction thermocouple with 32AWG wire (0,25 mm diameter) will have a resistance of approximately 15 ohms / meter. If thermocouples with thin wires or long cables are needed, it is worthwhile to keep the thermocouple wires short, then use a thermocouple extension wire (which is much thicker, so a lower resistance) for operate between the thermocouple and the measuring instrument.
The output of a thermocouple is a weak signal, so it is subject to electrical interference. If your sensor is in a noisy environment (for example, near an electric motor), it is recommended to use a shielded extension cable. If the noise sensor is suspected, turn off all suspicious equipment and check if the reading changes.
There is no possible maintenance function on a thermocouple, but programmed calibration checks are recommended.
- Thermocouples drift in the calibration, but the rate of drift depends on time and temperature.
- In a known temperature source, check the thermocouple output with the thermocouple C degrees from the table.
- The thermocouples or their wiring can make a short circuit or open circuit causing error signals. Another fault condition for data loggers or transmitters is a low insulation resistance between the conductors and the earth, which leads to earthing of the thermocouple loop.
If the thermocouple shows one of the 3 fault conditions, it must be replaced.
More information on thermocouples below!
|Temperature range||Conversion table (T ° / mV)|