The different types of thermocouples?

Thermocouples sensors are part of the composition of temperature probes used in many professional sectors. It is a technology that is both easy to use and economical. It also offers a short response time to thermal variations. The different types of thermocouples cover more or less extensive temperature beaches with variable precision. The choice of their junction and their cold welding compensation system makes them usable in different environments.

To identify Thermocouple Best to your field of application and your budget, it is essential to understand how it works and distinguish its typologies.

What is a thermocouple?

Thermocouple is one of the two main types of most widespread temperature sensors.

The thermocouple makes it possible to measure the temperature by calculating a voltage difference between two points. It is a system based on a thermoelectric principle.
The second category, the Temperature sensor with thermistor, uses a thermoreistive principle. It is the Material resistance variation which is used to calculate the temperature difference. The materials in question are metallic, pressed and encapsulated oxides.

The operation of the thermocouple

A thermocouple, or thermoelectric torque, consists of two wires in conductive metals different. The wires are connected by two types of welds, The hot point and the cold point. Also called reference junction. Udoes not create an electrical voltage, When the hot junction has a temperature change. We will then measure the difference in electric potential between the hot welding and the cold weld to measure the temperature. This is called The thermoelectric effect Seebeck, named after the German physicist Thomas Johann Seebeck who theorizes this phenomenon in 1821.

A voltmeter allows you to measure theA voltage generated in the conductive metal wires as a function of the temperature variation. We calculate the potential difference (electro-motor force) between hot welding and cold welding. Afterwards, A converted temperature transmitter This data in Celsius degrees. The conversion of the temperature voltage is made thanks to A reference table And depends on the types of metals that make up thermocouple. Indeed, the thermoelectric sensitivity of each metal is different. A Seebeck coefficient has been attributed to each material according to this sensitivity.

Maintaining the temperature of the reference junction

For the thermocouple probe to provide high precision information, cold welding must be at A stable or measurable temperature. You should know that this reference junction is actually generally composed of two different welds. There is Several methods of compensation for cold welding, used in particular when the ambient temperature can vary. The icy water bath is a simple method that keeps welding at 0 ° C. In practice, it is largely useful in the laboratory but can be complex to implement in an industrial environment. It is also possible to use A second measurement instrument which performs a temperature control of the reference point. The measurement tables are generally established from a cold welding temperature of 0 ° C, it is then necessary to carry out a calculation taking into account the measured temperature.

When the distance between the measurement point and the device of measurement is important, it may be necessary to add an extension. The material of this cable must be a Calibration identical to the type of thermocouple In order to avoid false measurements. A solution is to use the same material directly, which can be expensive, it is a extension cable. It is also possible to use a compensation cable, made from different metals from those of thermocouple.

The types of normalized thermocouples

The design of thermocouples sensors is made from all kinds of metals. But 8 types of thermocouples are mainly used. These are the types of thermocouples covered by The European standard CE 60584.1. This standard lists thermocouples by designating them by a letter and a name in which the positive element is first.

The most used thermocouples types

The type K (Chromel (nickel-chrome alloy) / alumel (nickel-aluminum alloy)) is the most widely used thermocouple in the industry. It covers a wide temperature measurement range, from -200 to 1200 ° C. Its price is inexpensive and its sensitivity is around 41 UV / ° C.
Thermocouple Type J (Iron / Constantan (Copper-Nickel alloy)) is useful in a reductive environment. It measures the temperature on a range from -40 to 750 ° C. It should not be used beyond 760 ° C, at the risk of causing a magnetic transformation resulting in permanent decalibration. The presence of iron making it sensitive to oxidation, it is often offered with a stainless steel sheath.
THE Type t (Copper / Constantan (Copper-Nickel alloy)) is mainly used in the laboratory on a range -185 to 300 ° C. It is very stable on low temperature measurements and cryogeny.
Type E (Chromel (Nickel-Chrome Alloy) / Constann (Copper-Nickel alloy)) is suitable for temperature measurements between -40 and 800 ° C. It is recommended for use in a vacuum medium, inert gas, moderately oxidizing or reducing.

The types of thermocouples reserved for high temperatures

THE Type N (Nicrosil (Nickel-Chrome-Silicium Alloy) / Nisil (Nickel-Silicon Alloy)) benefits from high stability and oxidation resistance. It is used to measure high temperatures, up to more than 1,280 ° C over short periods. It has the advantage of being available at a cheaper price than other types of thermocouples for high temperatures, B, R or S, which contain plate.
THE Types S (Rhodié Platinum 10 % / Platinum) and R (Rhodié Platinum 13 % / Platinum) have close characteristics. They are used for the measurement of high temperatures, that is to say above 1,000 ° C and up to 1,600 ° C. Due to its high stability, type S is used as a stallion for the gold merger point (1,064.43 ° C).
Thermocouple Type B (Rhodié Platinum 30 % / Rhodié Platinum 6 %) is provided for a temperature measurement up to 1,600 ° C continuously and 1,800 ° C in a point.

The choice of a temperature probe Is not limited to the type of thermocouple, the other elements that constitute it must also best meet your needs. Thermometre.fr advisers are at your disposal to advise you on the type of probe you need. They also design with you the tailor -made probe that best suits your application.