Precision RTD's (Resistance Temperature Detectors)

Duro-Sense RTDs, thermowells, and accessories provide high quality solutions to the aerospace, aviation, process control, medical, R&D, power generation, alternative energy, plastics, primary metals, high-tech and OEM industries.

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Advantages and Disadvantages of Thermocouples

Industrial thermocouples in ceramic
protection tubes (Duro-Sense)

Temperature sensors, inherent to their design or operating principle, are often the weakest link in the control loop. Fragility, noise sensitivity, and material deterioration have to be considered when applying temperature sensors. Generally speaking, when it comes to temperature sensing technology, RTDs are more accurate and stable, but are also fragile; thermistors have greater temperature sensitivity, but their electrical properties can change overtime; thermocouples (TC) are rugged, inexpensive, and operate over a wide temperature range, but they do have drawbacks that need to be understood for successful implementation.

The thermocouple's primary disadvantage is their weak output signal and their susceptibility to electrical noise. The mV signal generated is so small it requires conditioning, namely amplification and linearization.  The good news is this conditioning is built-in to and provided by the TC's corresponding controller, indicator or transmitter.  Calibration drift due to oxidation, contamination, or other physical changes to the alloys is another problem associated with thermocouples, requiring a facility to implement periodic recalibration procedures. Lastly, thermocouples require alloy-matching thermocouple extension wire when running lead wires any distance from sensor to instrument. This adds cost, as thermocouple extension wire is more expensive than standard wire.

Despite these concerns, thermocouples are widely used in industry because of their relative low cost, ruggedness, wide selection of operating ranges, and versatility in size, shape, and configuration. They have been used in millions of process control applications for over a generation, and provide an excellent balance of performance, cost, and simplicity. Successful application depends on knowing thermocouple's strengths and weaknesses, and consulting with an applications expert prior to specifying or installing any temperature sensor is always recommended to ensure the sensor's longest life and best performance.

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.

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!