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Thermocouples Working Principle Introduction
Thermocouples are electronic instruments formed by two simple metals with different electro-negativity that form an electrical junction. This junction generates a voltage of the μV if heated or cooled due to the thermoelectric effect, they are used as temperature sensors. Thermocouples are mostly used to measure very high temperatures, they are resistant to extreme temperatures thanks to the reaction between two metals, some thermocouples work up to 1700 ° C. Normally, they are usually joints between Nickel, Chrome, Constantin, Iron, Alum, Copper, Platinum and other alloys. In 1821 Seebeck discovered that two metals in contact with each other, create an electrical voltage to the heads if the junction is at a different temperature from the rest of the material. The classic connection of a thermocouple is as follows:
The first junction between the Chrome and the Alumel (aluminium alloy and Nickel) is the measuring junction, this means that the hot joint is positioned near the temperature that should be measured. These two wires are connected at some point in the circuit, usually connected to the copper of the tracks. The circuit then creates a pair of junctions between metals with different electronegativity that forms two thermocouples between Chrome and Copper and the other between Alumel and Copper. These two junctions create a potential difference that depends on the materials and the temperature of the cold junction.
The amplifier or converter (instrument T meter) sees the Voltage (mV measured = Vt1 + Vt2 - Vt3), in this case, the system must measure the cold junction to know Vt2 and Vt3 and then calculate the value of Vt1. Doing this is very simple, in fact, the cold junction is not at extreme temperatures, it is assumed that the tempering works close to the ambient temperature, so it is easy to measure this temperature with an integrated temperature sensor. The thermocouples are coded according to the materials used and are classified with a letter of the alphabet. There are thermocouples of type K, J, C, D, G, T, E, N, P, B, R and S each with different material and classified in such a way that the value of the voltage generated on the cold junction is known, which means compensation. cold junction compensation As seen previously, the voltage measured directly from an instrument connected to a thermocouple is the sum of three factors, that is Vt1, Vt2 and Vt3. Being the known thermocouple, we are able to know the behaviour of V1 when the temperature changes and also the behaviour of Vt2 and Vt3, even if more particularly we are interested in the value of Vt2-Vt3 in order to eliminate it from the measured value. Usually, the value of Vt2-Vt3 is negative, so what we need to do is to add a voltage with the same variation of the difference temperature between Vt2 and Vt3. The situation is therefore the following:
Suppose on the cold junction a sensor LM35 is connected (image above, just to give an idea), this sensor generates a voltage (Vsensor) that depends on the temperature. The measured voltage will be Vmeasured = Vt1 + Vt2-Vt3 + Vsensor. If we make Vsensor = -Vt2 + Vt3 then what we get will be Vmeasured = Vt1, so I measure the tension of the hot joint perfectly. This compensation can be done in different ways and including also the additional sensors in series as shown in the figure above (LM35) or with an adder amplifier. Moreover, it can be performed also via software by measuring the value of the sensor on the cold junction and the value on the hot joint and adding them via software. However, the hardware solutions are better. For each type of thermocouple, there will be a suitable compensation voltage and there are several integrated voltages on the market that generate voltages to compensate for different types of thermocouples. In addition to the cold junction compensation, some thermocouples need to calculate the transfer function, because the voltage value does not linearly represent the temperature (as in LM35 where 10mV each degree Celsius), then a mathematical operation carried out in hardware or software to know the temperature value from the voltage across the thermocouple. Type K Thermocouple Type K thermocouples are the most widespread and economical, the range of measurable temperatures goes from a maximum of about 1100 °C to a minimum of -180 °C even if these values depend from manufacturer to manufacturer. They are built with a junction between Chrome and Alumel (95% Nickel, 2% Aluminum and Manganese and 1% Silicon). It is resistant to acidic environments while suffering in basic environments, and it is enclosed in a protective steel sheath.
Thermocouple Extension & Compensating Cable Colour Codes The table below shows the most commonly encountered colour codes used on thermocouple leads and connectors. These are used to identify the Type and positive and negative wires. It is important to recognise the unfortunate use of the same colours for outer sheaths and connectors on different thermocouple types under different standards. Other colour coding systems also exist but are not shown below.
Thermocouple Temperature Ranges The table below shows the defined temperature ranges for thermocouple types. The IEC standard defines Class Tolerances for initial accuracy over a given temperature range. If a thermocouple is used outside these temperatures it may be lost calibration (permanently lose accuracy) quickly.
Thermocouple Materials & Accuracy Tolerances
Thermocouples & RTDs Accuracy Tolerance Comparison The table below contains the manufacturing tolerances for different thermocouple types and Pt100s classes at different temperatures. These are ± (+/-) values, so a Class 1 Type K thermocouple at 500°C is ±2°C, so could read anything between 498°C and 502°C and be within manufacturing tolerance. But remember, with use of thermocouples will de-calibrate and drift outside these tolerances, which is why calibration is so important.
Thermocouple Extension & Compensating Cable Accuracy Tolerances
What to Consider When Buying a Thermocouple If we could rely on temperature range alone, buying a thermocouple would be a breeze, but that’s not the case, at least should consider three-factor which are most important before shopping for a thermocouple:
You may need a professional’s help to determine which is best for your business, to prevent choosing the wrong type and running into thermocouple failures. A thermocouple normally covers a wide range of temperatures and its output is reasonably linear over portions of that range. Thermocouples of the same type are interchangeable within specified limits of error (this is always must be considered). Contact Bennypass, the Temperature Experts team, for All Your Thermocouple Needs Bennypass has been at the center of many innovations for more than 27 years, consulting for innovators and providing them with custom temperature sensors that meet and exceed the international standard where regular solutions may not be the most appropriate. Contact us at (+39) 3470515328 for a quote today.
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