Thermocouples for Process Measurement

Industrial Thermocouples

A thermocouple is a device which functions as a temperature sensor. Two wires, made of dissimilar metals, are joined together at both ends. The thermocouple is, at this point, a thermoelectric circuit with current flowing in relation to the temperature at each junction.  The temperature of one junction is known and constant, called the cold junction. The other junction is placed in contact with the media or object to be measured. The closed circuit of the thermocouple subsequently gets broken at the center, and the open circuit voltage from the hot junction is a direct function of the junction temperature. Heating or cooling the sensing junction of the two metals produces a voltage directly correlating to the temperature.

There are a number of metal combinations used to produce industrial thermocouples. Each combination has a respective set of performance attributes, operating temperatures, tolerances, and cost that may deliver best performance for a particular use.

Common Thermocouple Types and Composition

• Type J (Iron / Constantan)
• Type K (Chromel / Alumel)
• Type T (Copper / Constantan)
• Type E (Chromel / Constantan)
• Type N (Nicrosil / Nisil)
• Type B (Platinum / Rhodium)
• Type R (Platinum / Rhodium)  

Generally, thermocouple sensor assemblies, often a tube enclosing the thermocouple junction, can be grounded, ungrounded, or exposed. The wires constituting the tip of a grounded probe are attached to the inside of the probe wall, allowing for maximal heat transfer through the probe wall to the junction while maintaining a physical separation between the junction and the measured media or object. The difference between grounded thermocouple probes and ungrounded thermocouple probes is that in ungrounded probes, the junction is not attached to the probe wall. For this reason, ungrounded probes can exhibit slower response time, but do provide electrical isolation of the junction from the housing. An exposed thermocouple will have the junction protruding from the probe tube or perforations in the tube to allow actual contact between the measured material and the junction. While this provides very rapid response, the junction is exposed to damage and wear.

Various Thermocouple Styles

Thermocouples are useful for process measurement due to their size, response, ruggedness, accuracy, cost, and wide range of available form factors. Chemical and petroleum refineries, as well as other fluid processing operations, can use multiple thermocouples to control, limit, test, log, and monitor process-related temperatures. Temperature is one of the most measured parameters in industry. Understanding the temperature range being measured, the accuracy level required, and the related process vibration concerns of each industry will aid in selecting the proper thermocouple for an application. When properly matched to the application requirements, thermocouples are a durable and cost-effective process measurement option.

Download your temperature sensor selection catalog here.

Contact Alliance Technical Sales with your temperature sensing requirement. Their application engineers will help you select the best sensor for the job.

Match the Right Temperature Sensor Configuration to the Application

Using a temperature sensor properly configured for
the application will result in enhanced process performance
Image courtesy Smart Sensors, Inc.

There are more temperature controlled operations than any of us could count in a lifetime. Each one exhibits an exclusive set of performance requirements and design challenges. Matching the means of temperature measurement, the control loop characteristics, and heat delivery method to the application are essential to achieving successful operation.

Step one is to measure the process temperature. This sounds simple until you start researching products and technologies for measuring temperature. Like the temperature controlled operations mentioned previously, there are more than you can count in a lifetime. To filter the possible candidates for temperature sensing devices, consider these aspects of your application and how well a particular sensor may fulfill your requirement.

• Response Time - How rapidly the sensor will detect a change in process temperature is a function of how the sensor is constructed and how it is installed. Most temperature sensors are enclosed or encapsulated to provide protection for the somewhat vulnerable sensing element. Greater mass surrounding the sensing element will slow sensor response. Whether the slower response time will adversely impact process operation needs to be considered. More consideration is due to the manner in which the temperature sensor assembly is installed. Not all applications involve a fluid in which the sensor assembly can be conveniently immersed, and even these applications benefit from careful sensor placement.
• Accuracy - Know what your process needs to be effective. Greater levels of accuracy will generally cost more, possibly require more care and attention to assure the accuracy is maintained. Accuracy is mostly related to the type of sensor, be it RTD, thermocouple, or another type.
• Sensitivity - Related to the construction, installation, and type of sensor, think of sensitivity as the smallest step change in process temperature that the sensor will reliably report. The needs of the process should dictate the level of sensitivity specified for the temperature sensor assembly.

Let's look at a very simple application.

Heat tracing of piping systems is a common application throughout commercial and industrial settings experiencing periods of cold weather. Electric heat trace installations benefit from having some sort of control over the energy input. This control prevents excessive heating of the piping or applying heat when none is required, a substantial energy saving effort. A temperature sensor can be installed beneath the piping's insulation layer, strapped to the pipe outer surface. One sensor design option available to improve the performance of the sensor is a surface pad. The surface pad is a metal fixture welded to the sensing end of a temperature sensor assembly. It can be flat, for surface temperature measurements, or angled for installation on a curved surface, like a pipe. The increased surface contact achieved with the surface pad promotes the conduction of heat to the sensor element from the heated pipe in our example. This serves to reduce and improve the response time of the sensor. Adding some thermally conductive paste between the pad and the pipe surface can further enhance the performance. While the illustration is simple, the concepts apply across a broad range of potential applications that do not allow immersion of the temperature assembly in a fluid.

A simple modification or addition of an option to a standard sensor assembly can deliver substantially improved measurement results in many cases. Share your temperature measurement requirements and challenges with a process measurement specialist. Leverage your own process knowledge and experience with their product application expertise.

Surface Temperature Sensors and Transmitters for Industrial Applications from Alliance Technical Sales, Inc.

Technical Reference for Thermocouples and Reistance Temperature Detectors (RTD)

One of many industrial
temperature sensor
configurations
Smart Sensors, Inc.

Temperature measurement is probably employed in process control more than any other physical property measurement. Methodology for temperature measurement is well established, as is the industry providing instruments and devices for acquiring temperature data from almost any facet of any process. If you are even peripherally involved in process measurement and control, having a solid understanding of how thermocouples and RTDs work is a requisite to solving problems or servicing customers.

One manufacturer of a comprehensive line of thermocouple and RTD assemblies, Smart Sensors, Inc., produced a technical manual with all you need to know about temperature sensors for process measurement and control. The manual is included below for easy reference. It covers:

• Thermocouple theory
• RTD and thermocouple specification criteria
• Cable specifications for both sensor types
• Comparison of thermocouple and RTD attributes
• Thermowell and protection tube specification and selection
• Specifying temperature sensors for hazardous areas
• Reference data tables for both sensor types
• Practices for improving temperature measurement
• Calibration

The tech manual should be on the shelf or cloud drive of anyone involved in accomplishing, interpreting, or maintaining temperature measurement. The configuration options for temperature sensor assemblies are extensive. Reach out to a product application specialist and combine your process knowledge with their product application expertise to develop effective solutions to temperature measurement challenges.

Technical Reference for Thermocouples and Reistance Temperature Detectors (RTD) from Alliance Technical Sales, Inc.

Temperature Sensors for Process Measurement - Thermocouple, RTD, Thermistor

Simple RTD, thermocouple, thermistor
straight tube assembly
Courtesy Smart Sensors, Inc.

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

A thermocouple is a temperature sensor producing 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 constant, the voltage produced by the sensing junction can be measured and a corresponding temperature derived.

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, exhibit appropriate accuracy for many applications, have a fairly linear temperature-to-voltage 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.

Coil wound RTD element
(image courtesy of Wikipedia)

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

Thin-film RTD element
(image courtesy of Wikipedia)

The RTD wire is usually a pure metal such as platinum, nickel or copper because these metals have a predictable change in resistance as their temperature changes. RTDs 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 higher accuracy is important. Because they are made of pure metals, they tend to more costly than thermocouples. RTDs do need to be supplied an excitation voltage from the control circuitry.

The third most common temperature sensor is the thermistor. Thermistors work in a similar fashion to RTDs, in that they are a resistance based 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 response curve of thermistors. It is very non-linear, making thermistors useful over a narrower temperature range than RTDs.

Thermistor bead with wires
(image courtesy of Wikipedia)

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.

Temperature sensors are available in an almost infinite number of assemblies and configurations to accommodate every conceivable application. Share your application with a product specialist and take advantage of their application knowledge and experience.