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Using Thermocouples in Temperature Measurement

26/09/2018
A thermocouple is a sensor used to measure temperature. Thermocouples are made with two wires of different metals, joined together at one end to form a junction. The junction is placed on the surface or in the environment that's being measured. As the temperature changes, the two dissimilar metals begin to deform, causing a change in resistance. Naturally, a thermocouple outputs a millivolt signal, therefore, as the resistance changes, the change in voltage can be measured. Thermocouples are desirable because they're extremely low cost, simple to use, and are capable of providing accurate readings.
Thermocouple

Known for their versatility as temperature sensors, 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.

Due to their wide range of models and technical specifications, it is extremely important to understand its basic structure, functionality, ranges as to better determine the right 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.

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.
 
Common Thermocouple Temperature Ranges
Calibration Temperature
Range
Standard Limits
of Error
Special Limits
of Error
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
(-328° to 662°F)
Greater of 1.0°C
or 0.75%
Greater of 0.5°C
or 0.4%

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 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.

 

 

Choosing a Thermocouple

Because a thermocouple measures in wide temperature ranges and can be relatively rugged, thermocouples are very often used in industry. The following criteria are used in selecting a thermocouple:
- Temperature range
- Chemical resistance of the thermocouple or sheath material
- Abrasion and vibration resistance
- Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter)
 

What is the response time of a thermocouple?

A time constant has been defined as the time required by a 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 response, 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.

How do I know which junction type to choose?

Sheathed thermocouple probes are available with one of three junction types: grounded, ungrounded or exposed. At the tip of a grounded junction probe, the thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. In an ungrounded probe, the thermocouple junction is detached from the probe wall. Response time is slower than the grounded style, but the ungrounded offers electrical isolation.

Choosing the right type and style of thermocouple

Beaded Wire Thermocouple
A beaded wire thermocouple is the simplest form of thermocouple. It consists of two pieces of thermocouple wire joined together with a welded bead. Because the bead of the thermocouple is exposed, there are several application limitations. The beaded wire thermocouple should not be used with liquids that could corrode or oxidize the thermocouple alloy. Metal surfaces can also be problematic. Often metal surfaces, especially pipes are used to ground electrical systems The indirect connection to an electrical system could impact the thermocouple measurement. In general, beaded wire thermocouples are a good choice for the measurement of gas temperature. Since they can be made very small, they also provide very fast response time.
Thermocouple Probe
A thermocouple probe consists of thermocouple wire housed inside a metallic tube. The wall of the tube is referred to as the sheath of the probe. Common sheath materials include stainless steel and Inconel®. Inconel supports higher temperature ranges than stainless steel, however, stainless steel is often preferred because of its broad chemical compatibility. For very high temperatures, other exotic sheath materials are also available. View our line of high temperature exotic thermocouple probes.

The tip of the thermocouple probe is available in three different styles. Grounded, ungrounded and exposed. With a grounded tip the thermocouple is in contact with the sheath wall. A grounded junction provides a fast response time but it is most susceptible to electrical ground loops. In ungrounded junctions, the thermocouple is separated from the sheath wall by a layer of insulation. The tip of the thermocouple protrudes outside the sheath wall with an exposed junction. Exposed junction thermocouples are best suited for air measurement. Certain thermocouple probes allow you to connect directly to a PC via USB connectivity.
Surface Probe
Measuring the temperature of a solid surface is difficult for most types of temperature sensors. In order to assure an accurate measurement, the entire measurement area of the sensor must be in contact with the surface. This is difficult when working with a rigid sensor and a rigid surface. Since thermocouples are made of pliable metals, the junction can be formed flat and thin to provide maximum contact with a rigid solid surface. These thermocouples are an excellent choice for surface measurement. The thermocouple can even be built in a mechanism which rotates, making it suitable for measuring the temperature of a moving surface. Type K is ChrOMEGA™/AlOMEGA™.
Wireless Thermocouple
Bluetooth® wireless transmitters that connect with smartphones or tablets to log and monitor temperature measurements. These transmitters measure different sensor inputs, including but not limited to temperature, pH, RTD, relative humidity. The data transmission is performed via Bluetooth wireless technology to a smart phone or tablet with the app installed. The app will allow the smartphone to pair and set up multiple transmitters.
 
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Perta Mahaka Indonesia
Ruko Gaharu Residence, Jl Kramat III Blok C No. 10, Tapos, Depok, 16454
Jawa Barat - Indonesia
(+6221) 874 5456
(+6221) 874 5456