Thermocouple sensors are part 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 extended temperature ranges with varying precision. The choice of their junction and their cold junction compensation system makes them usable in different environments.
To identify the thermocouple best suited to your field of application and your budget, it is essential to understand how it works and to distinguish its typologies.
What is a thermocouple?
The thermocouple is one of the two main types of most common temperature sensors.
- The thermocouple allows you to measure the temperature by calculating a voltage difference between two points. This is a system based on a thermoelectric principle.
- The second category, the thermistor temperature sensor, uses a thermoresistive principle. It's here variation in resistance of materials which is used to calculate the temperature difference. The materials in question are metal oxides, pressed and encapsulated.
How the thermocouple works
A thermocouple, or thermoelectric couple, consists of two conductive metal wires different. The wires are connected by two types of solder, the hot spot and the cold spot. Also called reference junction. INno electrical voltage is created, when the hot junction has a temperature change. We will then measure the difference in electric potential between the hot junction and the cold junction to measure the temperature. This is what we call the Seebeck thermoelectric effect, named after the German physicist Thomas Johann Seebeck who theorized this phenomenon in 1821.
A voltmeter allows you to measure thehe voltage generated in the conductive metal wires as a function of temperature variation. We calculate the potential difference (electromotive force) between the hot junction and the cold junction. Afterwards, a converted temperature transmitter this data in degrees Celsius. The conversion of voltage into temperature is done using a reference table and depends on the types of metals that make up the thermocouple. Indeed, the thermoelectric sensitivity of each metal is different. A coefficient de Seebeck was assigned to each material based on this sensitivity.
Maintaining the temperature of the reference junction
For the thermocouple probe to provide high precision information, the cold junction must be at a stable or measurable temperature. You should know that this reference junction is in reality generally composed of two different welds. It exists several methods of compensating the cold junction, used in particular when the ambient temperature may vary. The ice water bath is a simple method that allows you to maintain the weld at 0°C. In practice, it is widely useful in the laboratory but can be complex to implement in an industrial environment. It is also possible to use a second measuring instrument which performs a temperature control of the reference point. As the measurement tables are generally established based on a cold junction temperature of 0°C, a calculation must then be carried out taking into account the measured temperature.
When the distance between the measuring point and the device measurement is large, it may be necessary to add an extension. The material of this cable must be of calibration identical to the thermocouple type to avoid false measurements. One solution is to use the same material directly, which can be expensive, so it is a extension cable. It is also possible to use a compensation cable, made from different metals than the thermocouple.
Types of standardized thermocouples
The design of thermocouple 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 IEC 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 types of thermocouples
- Type K (Chromel (nickel-chrome alloy) / Alumel (nickel-aluminum alloy)) is the most widely used thermocouple in industry. It covers a wide temperature measurement range, from -200 to 1200°C. Its price is inexpensive and its sensitivity is around 41uV/°C.
- The thermocouple type J (Iron / Constantan (Copper-Nickel alloy)) is useful in a reducing environment. It measures temperature over a range of -40 to 750°C. It should not be used above 760°C, as this could cause a magnetic transformation leading to permanent decalibration. The presence of iron makes 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 over a range -185 to 300°C. It is very stable on low temperature measurements and cryogenics.
- Type E (Chromel (Nickel-Chromium alloy) / Constantan (Copper-Nickel alloy)) is suitable for temperature measurements between -40 and 800°C. It is recommended for use in a vacuum, inert gas, moderately oxidizing or reducing environment.
Types of thermocouples reserved for high temperatures
- The type N (Nicrosil (Nickel-Chromium-Silicon alloy) / Nisil (Nickel-Silicon alloy)) benefits from high stability and resistance to oxidation. It is used to measure high temperatures, up to over 1280°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 platinum.
- THE types S (Platinum Rhodium Plated 10% / Platinum) and R (Platinum Rhodium Plated 13% / Platinum) have similar characteristics. They are used for measuring high temperatures, that is to say above 1000°C and up to 1600°C. Due to its high stability, type S is used as a standard for the melting point of gold (1064.43°C).
- The thermocouple type B (Platinum Rhodium plated 30% / Platinum Rhodium plated 6%) is designed for temperature measurement up to 1,600 °C continuously and 1,800 °C peak.
The choice of a temperature sensor is not limited to the type of thermocouple, the other elements which constitute it must also best meet your needs. Thermometre.fr advisors are available to advise you on the type of probe you need. They also design with you the tailor-made probe that best suits your application.
Go further on thermocouples
To go further on the subject of thermocouples, we also recommend these articles:
