Thermocouple

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The thermocouple is a temperature sensor with many applications used for contact temperature measurement. It is one of the constituent elements of the thermocouple temperature probe, a measuring device whose main elements are the sensor and a data reading device, generally having a liquid crystal screen. The thermocouple is one of the most used sensors, alongside the RTDs (Pt100, Pt100, Pt1000 etc.) and the thermistor, whose resistance depends on the temperature. The advantages of the thermocouple are its robustness, its affordable price, its reaction speed and its ability to measure over wide temperature ranges.

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How does a thermocouple sensor work?

The operation of the thermocouple is based on a phenomenon called the Seebeck thermoelectric effect. This phenomenon is manifested by the appearance of an electrical voltage within an open circuit composed of two conductive materials of different natures during a temperature variation. The voltage obtained is converted into temperature using the Seebeck coefficient depending on the type of thermocouple. The precautions to take for optimal operation of thermocouples are the choice of the appropriate type according to the application, the type of junction and taking into account electromagnetic disturbances.

How does the thermocouple measure temperature?

The thermocouple sensor is made of two wires of different conductive metals connected together by a junction called a hot spot. Their other end, the cold point, is connected to the measuring device. When the hot spot is exposed to heat or cold, theThe electron density of each metal wire is modified. The electric current which passes through the sensor appears thanks to the temperature difference that exists between the hot point and the cold point. To measure the temperature with a thermocouple accurately, you must either maintain the cold point at zero degrees Celsius or implement what is called cold junction compensation.

What are the types of thermocouple?

There are several types of thermocouples that offer different temperature measurement ranges and adapt to several types of applications. Selecting the appropriate type of thermocouple probe depends on the environment in which it will be used and the budget allocated for the purchase. THE 8 types of thermocouple sensors mainly used are covered by the European standard IEC 60584.1. Some types are made of base metals and are therefore inexpensive. Types B, R and S are made from precious metals such as platinum, which makes their price higher.

What affects thermocouple response time?

The response time of thermocouples varies depending on their design rather than their type. The characteristics that influence the reaction speed of these sensors are multiple: the protection of the probe, the junction method or even the connection cable. The more direct the contact with the environment whose temperature we wish to measure, the faster the response provided by the measuring probes. A thermocouple without a metal case or protective sheath therefore gives a rapid response. Bare wires can be subject to corrosion depending on their material and the environment, so each characteristic must be chosen carefully.

What is a thermocouple conversion table used for?

A table or thermocouple conversion table is a tool used to convert an electromotive force (emf) to a temperature in degrees Celsius or Fahrenheit. It also allows the calibration of the thermal probe by comparing the voltage obtained with the expected temperature. To read thermocouple conversion tables, you need to know the type of thermocouple used, each type having its own Seebeck coefficient. To read the table, we match the potential difference noted with the temperature in degrees Celsius or Fahrenheit.

What are thermocouple curves?

An alternative to presenting the voltage/temperature ratio in table form is the graph. The temperature is on the abscissa and the voltage on the ordinate, the link between the two not being linear, it forms a curve. Each type of thermocouple has a distinct curve which allows the variations between the two data to be visually appreciated. Thermocouple curves, like tables, are based on the voltage measured on the sensor when the cold point is at 0°C. It is therefore necessary to maintain this weld in a bath of agitated ice water or to carry out compensation. This technique consists of leaving the cold point at room temperature, measuring this temperature and converting it into voltage in order to add a corrective coefficient to the final calculation.

What is the measuring range of a thermocouple?

If the thermocouple is the most common type of sensor in the industry, it is largely because it makes it possible to measure a large temperature range. Depending on the type, it can measure temperatures from -200°C to 1800°C. Thermocouples capable of measuring very low temperatures are types K, J, T and N. For high temperatures, types N, S, R and B are used. To choose a thermocouple suitable for a specific application, the temperature range to be measured and the measuring range of the thermocouple type must coincide.

How to calibrate a thermocouple?

So-called calibration or thermocouple calibration is a procedure consisting of check that the voltage indicated on the thermal probe voltmeter corresponds to the correct temperature in the conversion table. For this, the measuring point must be at a temperature that is known with certainty. Tests to verify the accuracy of the probe are carried out at different points of its measuring range. If the results indicate a mismatch between the voltage and the corresponding temperature in the conversion table, a corrective coefficient must be added when using the sensor. Incorrect values ​​can mean that an element of the probe is damaged, such as the sensor itself, the connector or the recorder.

How to carry out tests on a thermocouple?

Testing is one of the essential steps of calibration. It is recommended to test a thermocouple when the probe experiences a change of assignment, regularly depending on the intensity of its use or in the event of suspected error. The procedures generally applied to test a thermocouple follow ASTM (American Society for Testing and Materials) recommendations for the calibration of thermocouples. Two options are possible, calibration by comparison or fixed point. In comparison, we carry out a temperature measurement with the probe to be tested as well as with a reference sensor, which we know to be reliable. At fixed points, the sensor is exposed to the triple point temperature of the water.

What is the response time of thermocouples?

 Response time is the time taken for the sensor to reach 66.6% of the final reading and is the industry standard way of measuring probe response times. Five times the response time quoted is the figure normally required to obtain 100% reading. Response times depend on the substance being measured and, in the case of a liquid or gas, the degree of agitation. This makes it difficult to quote a precise response time without knowledge of the application.

The results shown in this catalog were obtained in a stirred oil bath and may differ from those obtained under other conditions but can be used as a general guide when choosing probes.

What is the difference between the different handles offered?

Where applicable, each probe is supplied with a hexagonal, small round, heavy-duty ribbed or T-shaped handle. To reduce bacterial growth, the probe handles contain an antibacterial additive 'Biomaster'.

  • Hexagonal handle made of nylon and available in black. The maximum temperature is 105°C.
  • Small round handle made of nylon and available in black. The maximum temperature is 105°C.
  • T-shaped handle made of polypropylene and available in black or white. The maximum temperature is 85°C.
  • The sturdy ribbed handle (with color coded caps) is made of polypropylene and available in black or white. The maximum temperature is 85°C.

What types of cable are available?

Straight PVC cord is a general purpose cord and available in lengths up to 100 meters. As standard and where applicable, each probe comes with a one meter straight PVC lead and connector. As an alternative, a 1 meter coiled PU cable is available for standard, portable, type K or T thermocouple probes, replace the first digit (1) of the ordering code with the digit 3. The maximum temperature for PVC and PU is 80°C.

Some industrial and high temperature probes are available with an over-braided cable insulated with stainless steel and fiberglass. As standard and where applicable, each probe is supplied with a two meter stainless steel cable and connector. Maximum temperature 350°C.

Talk to one of our advisors to choose the equipment best suited to your project. We offer a wide choice of temperature probes as well as a tailor-made design service.

If you cannot find the probe you are looking for, contact GUILCOR to obtain a tailor-made probe.

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