THERMOCOUPLE & RTDs

Acrolab Thermocouples measure temperatures during industrial operations. The versatility that our Thermocouples provide is remarkable and a lot depends on how our temperature sensors are configured for the operation being undertaken. Thermocouples have many industrial applications and at Acrolab we have several different categories.

  • Plastics Thermocouples

  • Industrial Thermocouples

  • Food, Dairy and Pharmaceutical Thermocouples

  • Explosion Proof Thermocouples

Our goal it to assist customers in determining the correct thermocouple for their application. This is all part of the services we provide as Thermal Solution Providers.

Types of Thermocouples

Acrolab Thermocouples come with different combinations of metals, wire and functional elements. Each must be calibrated for the required application. Every thermocouple has a specific operating temperature range, which is set mainly by the diameter of the internal thermocouple wire. Therefore, the temperature range is a combination of materials used along with the configuration or type of thermocouple.

Base Metal thermocouples (Types J, K, T, E and N) are the most common while Metal thermocouples (Types R, S, B, M, C and W) have higher temperature calibrations. General purpose Thermocouples, J and K Types, are the most common because they function at low temperature ranges and are very cost effective.

The voltage output of a thermocouples is correlated to the temperature being measured by the thermocouple. Our table indicates the metals used in construction, temperature ranges for each thermocouple type, and provides basic application information.

Thermocouple Material Specifications

ANSI CALIBRATION CODEPOSITIVE LEGNEGATIVE LEGTEMP. RANGE OF PROT.TC**APPLICATION INFORMATION
JIronConstantan *32 to 1400°F
(0 to 760°C)
Suitable for vacuum, reducing or inert atmospheres, oxidizing atmospheres with reduced life. Iron oxidizes rapidly above 1000°F (538°C) so only heavy gauge wire is recommended for high temperature. Bare elements should not be sulphurous atmospheres above 1000°F (538°C)
KChromel *ALumel *32 to 2300°F
(0 to 1260°C)
Recommended for continuous oxidizing or neutral atmpspheres. Mostly used above 1000°C (538°C). Subject to failure if exposed to sulphur. Preferential oxidation of chromium in positive leg at certain low oxygen concentrations causes "green rot" and large negative calibration drifts most serious in the1500-1900°F (816-1038°C) range. Ventilation or inert-sealing of the protection tubes can prevent this.
TCopperConstantan *-300 to 700°F
(-184 to 371°C)
Useable in oxidizing. Reducing or inert atmospheres as well as vacuum. Not subject to corrosion in moist atmospheres. Limits are published for sub-zero temperature ranges.
EChromel *Constantan *32 to 1600°F
(0 to 871°C)
Recommended for continuously oxidizing or inert atmospheres. Sub-zero limits of error are not established. Highest thermoelectric output of common calibrations.
R

S
Platinum 13% Rhodium
Platinum 10% Rhodium
Platinum
Platinum
100 to 3100°F
(538 to 1482°C)
Protection tube and ceramic insulators. Continued high temperature usagesRecommended for high temperature. Must be protected with a non-metalliccause grain which can lead to mechanical failure. Negative calibration drift caused by rhodium diffusion to pure leg as well as from rhodium volatilization. Type R is generally used in industry while Type S is general used in laboratories.
BPlatinum 30% RhodiumPlatinum 6% Rhodium1600 to 3100°F
(871 to 1705°C)
Similar to type R & S but output is lower. Also less susceptible to grain growth and drift.
MNickel18% Nickel Molybedenum32 to 2250°F
(0 to 1287°C)
High temperature applications in inert or vacuum atmospheres. Useful in many hydrogen applications. Continuous cycling causes excessive grain growth.
C5% Tungsten Rhenium (W-5Re)26% Tungsten Rhenium (W-26Re)32 to 4200°F
(0 to 2315°C)
Very high temperature applications in inert or vacuum atmospheres. Preferred over Tungsten/26%, Tungsten Rhenium because it is less brittle at low temperatures.
W3% Tungsten Rhenium (W-3Re)25% tungsten Rhenium (W-25Re)32 to 4200°F
(0 to 2315°C)
The ductility of W3R3 leg is superior to pure Tungsten, but not as good as W5Re. This combination has the highest output if the 3 common Tungsten Rhenium calibrations from 1860 to 4200°F.
NNicrosil *** 14.5% Chromium 1.4% Silicon 0.1% Magnesium Balance Nickel4.2% Nisil *** 0.1% Silicon Magnesium Balance Nickel32 to 2300°F
(0 to 1260°C)
Can be used in applications where Type K elements have shorter life and stability problems due to oxidation and the developement of "green rot".
NONEPlatinel * 5355Platinel * 767432 to 2300°F
(0 to 1260°C)
Noble metal combination which approximates Type K curve bus has much improved oxidation resistance. Should be treated as any noble metal calibration.

5 Questions to Help Determine the Right Thermocouple

  1. Evaluate the application environment for the thermocouple.
  2. Anticipate temperature ranges the thermocouple must manage.
  3. Consider the thermocouple sheath material and any chemical resistance required.
  4. Determine if abrasion or vibration resistance is part of the process.
  5. Specify installation details based on compatibility with operational equipment and diameter availability.

Normally, a metallic tube will house the thermocouple wire that is commonly referred to as the sheath or housing. Stainless steel is often used for sheath construction because it can handle a wide range of chemicals, as well as a broad range of temperatures. When the thermocouple is expected to experience very high temperatures, more exotic sheath materials are required.

As you know, temperature sensors have difficulty measuring the temperature of a solid surface because the entire measurement area of the sensor needs to be touching the surface. As a result, thermocouples can be constructed from flexible materials to make full contact with rigid surfaces. Sometimes thermocouples are built with a rotating mechanism for making contact with a moving surface.

Thermocouple probes have tips that are either grounded, ungrounded or exposed. These 3 styles are readily available depending on your application.

  • Thermocouples with grounded tips are in contact with the housing wall and provide the fastest response time for readings, however electrical ground loops can be an issue.
  • Ungrounded thermocouples have a layer of insulation that keeps the thermocouple from touching the probe housing wall. This offers electrical isolation but has a slower reading response time then a grounded style.
  • Exposed thermocouples are just that, having the thermocouple wire extended beyond the housing wall. These are commonly utilized for taking air temperatures.

Selecting Resistance Temperature Detectors (RTD’S), Thermistors & Infrared Devices Instead of a Thermocouple

Obviously, it is important to consider different sensors that can be utilized for your application. In general, thermocouples are a robust and inexpensive means to gather temperature measurements over a fairly wide range. However, thermocouples are not as accurate as RTD’s and Thermistors, and these sensors also tend to be more stable. Keep in mind that RTD’s can have occasional self-heating issues because they utilize an internal electric current.
Again, much depends on the application requirements. Thermistors are considered to be the most accurate senor but have a limited temperature operating range and are susceptible to self-heating.

For situations that require taking high-temperature measurements, infrared sensors can be the best choice. They do not need to be in direct contact with the measured surface, allowing for application in high-temperature situations. However, not being in contact with the surface area means they are not as accurate as those in direct contact. One needs to keep in mind the emissivity of surface areas when selecting infrared sensors.

Thermocouple FAQ’S

1 - What is a Thermocouple?

This device measures temperatures during industrial operations. They consist of a linear sensor within a sheath housing and a probe at the tip, usually having two (2) different metal wires connecting the thermometer within the thermocouple. The operating temperature range for the thermocouple will vary depending on the style (Types J, K, T, E & N – OR – R, S, B, M, C & W).

3 - What are the temperature ranges and accuracies of different thermocouples?

Thermocouple construction; metals & alloys utilized, sensor construction, housing material (sheath) & diameter, and thickness of the thermocouple wire all factor into the accuracy and temperature range for a thermocouple. Review our table above for the specific details to properly match your application (measuring liquid, solid, or gas) with the appropriate thermocouple type.

2 - How does a thermocouple work?

Thermocouples are built on the SEEBECK effect discovered in 1821. Thomas Seebeck discovered a current flow that was continuous when two wires of different metals were connected in a thermoelectric circuit. This discovery is the basis of every thermocouple, which consists of dissimilar metals connected at both ends to generate a continuous current that can be related to a specific temperature. Obviously, there are different applications, for heating, cooling or interrupting the circuit and taking specific voltage measurements.

4 - What is better, a grounded or ungrounded thermocouple probe?

All depends on the application, and your objectives. If you have questions, please contact the Acrolab team and we will assist in the evaluation process. We have a full engineering department that is very experienced to help you with these types of decisions.