The 3 Most Common Temperature Sensors: Thermocouples, RTD's and Thermistors

This post explains the basic operation of the three most common temperature sensing elements - thermocouples, RTD's and thermistors.

Thermocouple (image courtesy of Duro-Sense)

A thermocouple is a temperature sensor that produces a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and maintained, the voltage produced by the sensing junction can be measured and directly applied to the change in the sensing junctions' temperature.

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, fairly accurate, have a fairly linear temperature-to-signal output curve, come in many “types” (different metal alloys) for many different temperature ranges, and are easily interchangeable. They require no external power to work and can be used in continuous temperature measurement applications from -185 Deg. Celsius (Type T) up to 1700 Deg. Celsius (Type B).

Common application for thermocouples are industrial processes, the plastics industry, kilns, boilers, steel making, power generation, gas turbine exhaust and diesel engines, They also have many consumer uses such as temperature sensors in thermostats and flame sensors, and for consumer cooking and heating equipment.

Wire-wound RTD (image courtesy of Wikipedia)

RTD’s (resistance temperature detectors), are temperature sensors that measure a change in resistance as the temperature of the RTD changes. They are normally designed as a fine wire coiled around a bobbin (made of glass or ceramic), and inserted into a protective sheath. The can also be manufactured as a thin-film element with the pure metal deposited on a ceramic base much like a circuit on a circuit board.

The RTD wire is usually a pure metal such as platinum, nickel or copper because these metals have a predictable change in resistance as the temperature changes. RTD’s offer considerably higher accuracy and repeatability than thermocouples and can be used up to 600 Deg. Celsius. They are most often used in biomedical applications, semiconductor processing and industrial applications where accuracy is important. Because they are made of pure metals, they tend to more costly than thermocouples. RTD’s do need to be supplied an excitation voltage from the control circuitry as well.

Thermistor (image courtesy of Wikipedia)

The third most common temperature sensor is the thermistor. Thermistors work similarly to RTD’s in that they are a resistance measuring device, but instead of using pure metal, thermistors use a very inexpensive polymer or ceramic material as the element. The practical application difference between thermistors and RTD’s is the resistance curve of thermistors is very non-linear, making them useful only over a narrow temperature range.

Thermistors however are very inexpensive and have a very fast response. They also come in two varieties, positive temperature coefficient (PTC - resistance increases with increasing temperature), and negative temperature coefficient (NTC - resistance decreases with increasing temperature). Thermistors are used widely in monitoring temperature of circuit boards, digital thermostats, food processing, and consumer appliances.

For more information, contact Duro-Sense by calling 310-533-6877 or visit https://duro-sense.com.

What is the Difference Between Thermocouple Wire and Thermocouple Extension Wire?

Thermocouple Wire

Insulated thermocouple wire is single pair wire that can be made into a thermocouple. More specifically, it is the wire from the sensing junction, to the point of cold junction compensation. Thermocouple wire is made from two dissimilar metals conductors that are joined (welded) together at the sensing end. Different metal wires are used for the various types of  thermocouple types (J, K, T, E, etc). These dissimilar metals produce a millivolt electrical signal at the cold junction that is proportional to the change in temperature at the sensing end.  Thermocouple wire has two grades; standard and Special Limits of Error (SLE). Special Limits of Error wire is made of the same metals as standard wire, but uses a higher grade with increased accuracy and higher expense than standard thermocouple wire.

Thermocouple extension wire.

Thermocouple wire is almost always insulated. Insulation varies from low temp (Nylon, FEP Teflon, TFE Teflon, PFA Teflon, Kapton) to high temp (fiberglass, Hi-Temp fiberglass, Refrasil, Ceramic Fiber).  Thermocouple wire is typically sold in 1000' spools, but can be sold in small quantities as well.

Thermocouple extension wire is a single pair wire that cannot be made into a thermocouple, but is used to carry the signal from a thermocouple to the recorder, controller, or instrument reading the signal. Extension grade wire is used to carry a signal representing the higher temperature seen by the sensing location, but extension wire itself cannot be generally exposed to those higher temperatures. Extension wire cannot be used to make a thermocouple, but thermocouple wire can be used as extension wire. Insulation is typically PVC, but other option are available.

Multi-pair extension wire is simply more than a single pair in the same jacket.  It is extension only and is usually available in 2,4,6,8,12,16,24 pairs. It is used primarily when a contractor has to run multiple runs of wire. It allows them to run one piece of wire rather than multiple individual runs.

For more information on all varieties of thermocouple wire, contact Duro-Sense by calling 310-533-6877 or visiting https://duro-sense.com.

Duro-Sense Sheath-Pak Mineral Insulated Thermocouples

Duro-Sense Sheath-Pak mineral insulated thermocouples are rugged, high temperature sensors made from a drawn tube filled with magnesium oxide and the thermocouple conductor wire.

See the document below for a quick review of these products, or download the Sheath-Pak Mineral Insulated Thermocouple PDF from this link.

https://duro-sense.com
310-533-6877

Duro-Sense Sheath-Pak Mineral Insulated Thermocouples from Duro-Sense Corporation

Choice of the Thermocouple Materials

Of the approximately 300 different types of temperature measuring thermocouples that have been identified and studied, only a few types, having the more favorable characteristics, are in general use. There are eight types of thermocouples that have been standardized, because they are the ones most commonly used industrially. In the United States each type is identified by a letter. This practice was originated by the Instrument Society of America (ISA) and adopted in 1964 as an American Standard to eliminate the use of proprietary names. The standards of the American National Standards Institute (ANSI-MC96.1, 1982) and the American Society for Testing and Materials (ASTM 230-87) utilize the reference tables from National Institute of Standards and Technology Monograph 125 as the basis for standardization. As noted in the ANSI and ASTM standards, the letter designations actually identify the tables and may be applied to any thermocouple that has a temperature-emf relationship agreeing within the tolerances specified in the standards with that of the table, regardless of the composition of the thermocouple. Substantial variations in composition for a given letter type do occur, particularly for types J, K, and E.

• Type B = platinum- 30% rhodium/platinum-6% rhodium - 0 to 1820°C ^*
• Type E = nickel-chromium alloy/a copper-nickel alloy -270 to 1000°C^*
• Type J = iron/another slightly different copper-nickel alloy -210 to 1200°C^*
• Type K = nickel - chromium alloy/nickel - aluminum alloy -270 to 1372°C
• Type N = nickel-chromium-silicon alloy nickel-silicon alloy -270 to 1300°C^*
• Type R = platinum- 13% rhodium/platinum -50 to 1768°C^*
• Type S = platinum- 10% rhodium/platinum -50 to 1768°C^*
• Type T = copper/a copper-nickel alloy -270 to 400°C^*

* temperature range as per NIST Table I: Thermocouple Types Definitions.

Certain combinations of alloys, such as Type J and K, have become popular as industry standards. Thermocouple type selection is driven by cost, availability, melting point, chemical properties, stability, and output. Different type thermocouples are best suited for different uses/applications. Thermocouple types are usually selected on the basis of the temperature range and accuracy needed. Other selection criteria include the chemical inertness of the thermocouple material and whether it is magnetic or not. 

For more information about thermocouples, contact Duro-Sense by calling 310-533-6877 or visiting https://duro-sense.com.

Thermocouple Selection Criteria

Thermocouple shape, assorted
hardware, lead wire insulation
and sheath material are all variable. 

It would be difficult to chart a career course in the industrial process control field without being exposed to thermocouples. They are the ubiquitous basic temperature measuring tools with which all process engineers and operators should be familiar. Knowing how thermocouples work, how to test them, is essential. Sooner or later, though, you may be in charge of selecting a thermocouple for a new application. With no existing part in place for you to copy, what are the selection criteria you should consider for your process?

Thermocouple sensor assemblies are available with almost countless feature combinations that empower vendors to provide a product for every application, but make specifying a complete unit for your application quite a task. Let's wade through some of the options available and see what kind of impact each may have on temperature measurement performance.

Thermocouple with terminal
block and no head.

Thermocouple Type: Thermocouples are created using two dissimilar metals. Various metal combinations produce differing temperature ranges and accuracy. Types have standard metal combinations and are designated with capital letters, such as T, J, and K. Generally, avoid selecting a type that exhibits your anticipated measurements near the extremes for the type. Accuracy varies among thermocouple types, so make sure the accuracy of the selected type will be suitable.

NIST Traceability: This may be required for your application. The finished thermocouple assembly is tested and compared to a known standard. The error value between the thermocouple shipped to you and the standard are recorded  and certified. The certified sensor assembly will be specially tagged for reference to the standard.

Junction Type: If your sensor will be contained within a tube or sheath, the manner in which the actual sensor junction is arranged is important. The junction can be grounded to the sheath, electrically insulated from the sheath (ungrounded), or protruded from the sheath (exposed). If your process environment may subject the sensor assembly to stray voltages (EMF), it may be wise to stay away from a grounded junction, even though it provides fast response to a change in temperature. Exposed junctions provide very quick response, but are subjected to potential damage or corrosion from surrounding elements. The ungrounded junction provides protection within the enclosing sheath, with a slower response time than either of the other two junction types. When using ungrounded junctions, keep the mass and diameter of the sheath as small as might be practical to avoid overdamping the sensor response.

Probe Sheath Material: This applies to assemblies installed in a tube or sheath which houses and protects the sensor junction and may provide some means of mounting. Material selections include a variety of stainless steel types, polymers, and metals with coatings of corrosion resistant material to suit many applications. Make sure the sheath material, including any coatings, will withstand the anticipated temperature exposure range.

Thermocouple with handle and
sharpened end for piercing.

Probe Configuration: Sheath tube diameter and length can be customized, along with provisions for bends in the tube. Remember that as you increase the mass around the junction, or increase the distance of the junction from the point of measurement, the response time will tend to increase.

Fittings and Terminations: There are innumerable possibilities for mounting fittings and wiring terminations. Give consideration to ease of access for service. How will the assembly be replaced if it fails? Are vibration, moisture, or other environmental factors a concern? What type of cable or lead wires would be best suited for the application?

Mounting hardware is very importamt
in thermocouple selection.

Your options are so numerous, it is advisable to consult a manufacturer's application engineer for assistance in specifying the right configuration for your application. Their product knowledge and application experience, combined with your understanding of the process requirements, will produce a positive outcome in the selection procedure.

Duro-Sense Corporation
https://duro-sense.com
310-533-6877

Temperature Sensor Basics: Thermocouples

Industrial style thermocouple
(Duro-Sense)

Industrial thermocouples are used for a very broad range of temperature sensing applications. They are inexpensive, accurate, and can be fabricated in many forms to meet the requirements of the process. They operate on the "Seebeck Effect" which is the phenomena of dissimilar metal conductors producing a measurable voltage difference between two substances.

Thermocouples are used widely in industrial processes in industries such as power generation, primary metals, pulp and paper, petro-chemical, and OEM equipment. They can be fabricated in protective wells, and can be housed in general purpose, water-tight, or explosion-proof housings.

Thermocouple types - such a type J, type K, type R, and type S - refer to the alloy combinations used for the conductors and are based on standardized color designations.

The following video provides a basic visual understanding of thermocouple wire, how a T/C junction is determined, and also discusses thermocouple connectors, polarity and some aspects of construction (such as grounded vs. ungrounded vs. open tip).

Get to Know Duro-Sense

Here's a short video to learn more about Duro-Sense Corporation. Hope you enjoy.

Thermocouple Basics

Type K thermocouple diagram

A thermocouple is a temperature sensor that produces a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and maintained, the voltage produced by the sensing junction can be measured and directly applied to the change in the sensing junctions' temperature.

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, fairly accurate, have a fairly linear temperature-to-signal output curve, come in many “types” (different metal alloys) for many different temperature ranges, and are easily interchangeable. They require no external power to work and can be used in continuous temperature measurement applications from -185 Deg. Celsius (Type T) up to 1700 Deg. Celsius (Type B).

Welcome to the Duro-Sense Blog

Welcome! We hope (over time) you find this blog interesting to visit and it becomes a trusted resource for all-things-temperature-measurement.

We plan on weekly educational and informative blog posts about innovative temperature sensor solutions, insight to how sensors work, and new products that solve tough engineering challenges.

Specific products we'll be discussing are:

• Thermocouples
• RTDs
• Thermowells
• Sensor Accessories

Please come back often!