Thermocouple measuring range

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Plage de mesure des thermocouples

Many industrial processes require reliable and regular temperature readings. The thermocouple sensor provides a response to this need. It also allows measuring wide temperature ranges in various environments. There are 8 main thermocouple types, each dedicated to a specific measurement range. To delve deeper into the subject of thermocouple measuring range and help you choose your sensors. We will return to the thermocouple operation. We will then see all of its characteristics linked to the measuring range of the thermocouple.

The principle of the thermocouple sensor

The thermocouple sensor is a component of the thermocouple probe, a temperature measuring device. Its operation is based on the Seebeck effect. VSthat is, an electrical voltage can be raised when exposed to a change in temperature. This voltage is then converted into °C or °F via the thermocouple conversion tables.

The thermocouple contains two wires of conductive metals or metal alloys. They connect at one point and form an open loop. The electromotive force (emf) appears when the solder connecting the two wires is in the medium whose temperature we wish to measure. The other ends of the wires allow you to connect to the measuring device. And this at what we call cold spots or cold junctions. Conversion tables assume the cold point to the freezing point temperature of water. That is 0°C or 32°F. If we cannot maintain this temperature, then we must measure this actual temperature. This must then be taken into account to carry out a compensation calculation.

The wide measuring range of the thermocouple makes it a popular sensor

It is the type of temperature sensor most commonly used in industry. It is robust, it offers good resistance to vibrations and good measurement accuracy. Its cost is low and it provides a rapid response following a change in temperature.

It is also popular because it is capable of measuring very low temperatures (like absolute zero) as well as very high ones. Chosen correctly, it allows you to measure temperatures with high precision with minimal response time. The thermocouple is a contact probe, which can measure an ambient temperature as well as a surface temperature.

What determines the measuring range of a thermocouple?

The measuring range refers to the temperature range that a thermocouple can measure. Also called measuring range, it varies depending on the thermocouple type. We are essentially talking about 8 types of thermocouples because they are the most used. They are compatible with the European standard IEC 60584.1. This classification depends on the types of materials used in the manufacture of the probe.

Apart from the materials and measuring ranges of thermocouples, another element is specific to each type. THE conversion curves. These curves indicate the temperature corresponding to the electromotive force recorded by the measuring instrument. The temperature is on the abscissa and the millivolts are on the ordinate. The link between the two does not evolve regularly, forming a curve.

Thermocouple measurement ranges by type

Each type covered by the European standard IEC 60584.1 is designated by a letter and the materials it is composed of, the positive element being cited first.

  • The thermocouple type K (Chromel / Alumel) has a measuring range of 0 to 1100°C in continuous use. In intermittent use, this range extends over a wide temperature range from -200 to 1300°C.
  • The sensor type J (Fer/Constantan) allows measurement to be taken over a range of -20 to 700°C in continuous use and -180 to 750°C in intermittent use.
  • A thermocouple of type T (Copper / Constantan) measures temperature over a range of -185 to 300°C in continuous use and -250 to 400°C in intermittent use.
  • The thermocouple measuring range of the type E (Chromel / Constantan) East from 0 to 800°C in continuous use and from −40 to 900°C in intermittent use.
  • For the type N (Nicrosil / Nisil), continuous use ranges from 0 to 1,150 °C, intermittent use from −270 to 1,280 °C.
  • The thermocouple of types S (Platinum Rhodium plated 10% / Platinum) in continuous use ranges from 0 to 1,550°C. In intermittent use, its measuring range goes from 0 to 1700°C
  • The type R (Platinum Rhodium plated 13% / Platinum) can measure temperature from 0 to 1600°C in continuous use and from 0 to 1700°C in intermittent use.
  • The thermocouple type B (Platinum Rhodium plated 30% / Platinum Rhodium plated 6%) offers a measurement range for continuous use from 100 to 1,600 °C, for intermittent use from 0 to 1,800 °C.

Understanding the thermocouple measuring range to choose a probe

The choice of a type of thermocouple turns to the one that best meets the technical requirements of the project for which it will be used. It's necessary match the optimal measuring range of the thermocouple with the temperature range to be measured.

As we have just seen, each type of thermocouple is associated with a continuous use range and an intermittent use range. These indications mean that the sensor should be used moderately over the intermittent use range. Prolonged use of the thermal probe in this temperature range risks altering it, no longer guaranteeing precise temperature control. The continuous use range is therefore the one to take into consideration as a priority when choosing a temperature probe.

Determining the temperature range to measure is also useful for choose the tools associated with the measuring probe, as the thermocouple recording box. This device, which generally has a digital display, stores temperature data and can take into account several thermocouples simultaneously.

In short, do not hesitate to contact our sales team from Thermometre.fr dedicated to benefit from our professional advice.

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