A thermocouple is a sensor that measures temperature. It consists of two different types of metals, joined together at one end. When the junction of the two metals is heated or cooled, a voltage is created that can be correlated back to the temperature. A thermocouple is a simple, robust and cost-effective temperature sensor used in a wide range of temperature measurement processes.
Thermocouples are manufactured in a variety of styles, such as thermocouple probes, thermocouple probes with connectors, transition joint thermocouple probes, infrared thermocouples, bare wire thermocouple or even just thermocouple wire.
THE SEEBECK EFFECT In 1821 Thomas Seebeck discovered the continuous current flow in the thermoelectric circuit when two wires of dissimilar metals are joined at both ends and one of the ends is heated.
Thermocouples are commonly used in a wide range of applications. Due to their wide range of models and technical specifications, but it is extremely important to understand its basic structure, functionality, ranges as to better determine the right thermocouple type and material of thermocouple for an application.
How does a thermocouple work?
When two wires composed of dissimilar metals are joined at both ends and one of the ends is heated, there is a continuous current which flows in the thermoelectric circuit. If this circuit is broken at the center, the net open circuit voltage (the Seebeck voltage) is a function of the junction temperature and the composition of the two metals. Which means that when the junction of the two metals is heated, or cooled, a voltage is produced that can be correlated back to the temperature.
What are the elements of a common thermocouple design?
Thermocouple sensors consist of the wires often called thermoelements, insulation, sheath, end seal and a means of connection (extension wires, connectors, etc.). The wires are connected together on one end to form the “measuring” or “hot” junction.The sensor must be connected to a read-out device that compensates for the reference temperature. If a thermocouple readout is used then it will contain the needed circuitry. If a millivolt meter is used, then either a properly prepared ice bath or some other cold junction compensation is needed.
What is the voltage range of a thermocouple?
The output of a thermocouple depends on the type of thermocouple it is. The normal thermocouple categories include Types J, K, T, E and N which are called “Base Metal” thermocouples, Types R, S and B which are called the “Noble Metal” thermocouples, and Types C and D which are called the “Refractory Metal” thermocouples.The output vs. temperature characteristics for each type are defined in two standards, ANSI/ASTM E230 and IEC 60584. Copies of the output tables can also be found here.
The thermocouple output or temperature tables are often referred to as “Thermocouple Curves”. Thermocouple output is not linear over temperature, this is the reason that the thermocouple equations contain a number of rather long coefficients in order to properly define them.
Thermocouple Time Constant
A time constant has been defined as the time required by a temperature sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to approach 100% of the step change value. An exposed junction thermocouple offers the fastest response. Also, the smaller the probe sheath diameter, the faster the thermocouple response time, but the maximum temperature may be lower. Be aware, however, that sometimes the probe sheath cannot withstand the full temperature range of the thermocouple type.Thermocouple Types
Thermocouples are available in different combinations of metals or calibrations. The most common are the "Base Metal" thermocouples known as Types J, K, T, E and N. There are also high temperature calibrations - also known as Noble Metal thermocouples - Types R, S, C and GB.Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.
Although thermocouple calibration dictates the temperature range, the maximum range is also limited by the the diameter of the thermocouple wire. That is, a very thin thermocouple may not reach the full temperature range.
K Type Thermocouples are known as general purpose thermocouple due to its low cost and temperature range.
| Common Thermocouple Types Temperature Ranges | |||
|---|---|---|---|
| Calibration | Temperature | Standard | Special |
| J | 0° to 750°C (32° to 1382°F) | Greater of 2.2°C or 0.75% | Greater of 1.1°C or 0.4% |
| K | -200° to 1250°C (-328° to 2282°F) | Greater of 2.2°C or 0.75% | Greater of 1.1°C or 0.4% |
| E | -200° to 900°C (-328° to 1652°F) | Greater of 1.7°C or 0.5% | Greater of 1.0°C or 0.4% |
| T | -250° to 350°C (-418° to 662°F) | Greater of 1.0°C or 0.75% | Greater of 0.5°C or 0.4% |
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