In industry or medicine, many processes require the use of a thermostat to take temperature measurements. The thermocouple sensor, robust, usable over wide temperature ranges and offering a short response time, is an excellent solution. It works thanks to the electrical voltage generated by the temperature difference between two parts of the probe. The conversion table is a tool for interpreting this voltage which is also used to calibrate the measuring probe.
What is a thermocouple chart?
A thermocouple table or board is an essential tool for using the thermocouple temperature sensor. The table allows you to convert an electromotive force (emf), measured using a measuring device, into a temperature in degrees Celsius or in degrees Fahrenheit. This conversion is done using the Seebeck coefficient, a variable specific to the nature of the materials constituting the sensor. Indeed, thermoelectric sensitivity varies from one metal to another. It is often the recorder, the box measuring the voltage, which converts directly into temperature. However, it is useful to understand how the thermocouple probe works to use it best.
The thermocouple board is also used to perform the calibration of the thermal probe sensor. Calibrating a thermocouple follows the recommendations of ASTM (American Society for Testing and Materials) which aims to standardize this type of procedure. During calibration, we ensure that the voltage obtained by exposing the thermocouple to a given temperature corresponds to the expected temperature in the table. We repeat the operation several times, in order to test the thermocouple on several temperatures of its measuring range. If the readings are inaccurate, a correction coefficient is applied when using the temperature probe.
How to read a thermocouple conversion chart?
The thermocouple conversion chart can take many forms. It indicates the degrees Celsius or Fahrenheit corresponding to each voltage power in millivolt (mV). When this voltage/temperature ratio is in the form of thermocouple curves, we see that this relationship is not linear and that the shape of the curve varies between the types of thermocouples. To read the table, you need to know the type of thermocouple. The most detailed conversion tables show all possible degrees for a thermocouple type. Other, more synthetic tables compare the voltage of all types for each ten degrees. You can consult a summary thermocouple table on our site by going to the page Thermocouple conversion table.
The type of thermocouple varies depending on the nature of the materials used to design it. Although it is possible to create a thermocouple from many combinations of metals, 8 main types are generally used. They are framed by a European standard and make it possible to cover varied applications. Types E, J, K, N and T thermocouples are made of common metals such as iron, constantan, copper, alumel or chromel. Types B, R and S are made from noble metals, such as platinum, which makes them quite expensive. Each material has its own expansion and conductivity characteristics. Types N, S, B and R have the particularity of being able to measure high temperatures, up to 1,800°C at maximum temperature. Each type of thermocouple has a temperature range optimal.
How to know the potential difference of a thermocouple?
The thermocouple works thanks to the voltage that appears when the two types of sensor solders are exposed to different temperatures. The two conductive metal wires are connected together at the hot spot, or hot weld. This is the part exposed to the environment in which temperature control is carried out. The cold spot is the solder located at the other end of the wires, on the measuring instrument side. You must know that the thermocouple table is based on the voltage measured when the cold junction is at 0°C. To know the potential difference precisely, it is therefore necessary to look at the cold point compensation method.
One of the solutions is simply to maintain the cold junction at 0° C. The method is then to immerse it in a bath of moving cold water. Cooling is a reliable technique, but it can be difficult to implement in practice in certain areas. It can indeed be implemented in a laboratory. In an industrial context, where a large number of thermocouples can be used simultaneously, the method is more restrictive. A second method consists of measure cold spot temperature in real time. While monitoring variations in ambient temperature. It is then necessary to convert this temperature into millivolts and carry out a differential calculation to obtain the real potential difference.
At Thermometre.fr, we offer a wide range of measuring probes to meet the expectations of professionals as closely as possible. Contact your advisor to be guided towards the equipment best suited to your project.
Go further on thermocouples
To go further on the subject of thermocouples, we also recommend these articles:
- Thermocouple probe category
- What are the different types of thermocouples
- Calibrating a thermocouple
- How is temperature measured by thermocouple?
- Thermocouple Response Times by Type
- Thermocouple curves by type
- How to test a thermocouple?
- Thermocouple measuring range
- How does a thermocouple work?
- Thermocouples
