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Concepts and Methods for Temperature Measurement

Posted November 04, 2013 12:00 AM by CR4 Guest Author

How are you able to measure temperatures? Temperature can be measured with a wide variety of sensors. Each one of them infers temperature by sensing a change in a physical characteristic. There are six different methods which an engineer is likely to come in contact with: change-of-state devices, liquid expansion devices, bimetallic devices, infrared radiators, resistive temperature devices (thermistors and RTD's), and thermocouples.

Thermocouple Temperature Measurement Sensors

These generally consist of two wires or strips that are made from different metals joined at one end. Changes of temperature at this junction create a change in the EMF, or electromotive force, between the opposite ends. As the temperature goes up, the thermocouple output EMF will rise, although not always linearly. Thermocouple sensors are regarded as among the best temperature measurement devices, along with the RTD's.

RTD or Resistance Temperature Devices

These devices will capitalize on the idea that the electrical resistance of the material will change when the temperature rises. Two main forms are metallic devices (generally known as RTD's) and thermistors. Just like their name says, the metallic devices will rely on only a change of resistance in the metal, with this resistance rising linearly with the temperature. Thermistors are mainly a resistance change within a semiconductor made of ceramic; the resistance will drop non-linearly with the rise of temperature.

Infrared Temperature Measurement Devices

These sensors are unique in being non-contracting devices. They generally infer the temperature by measuring the amount of thermal radiation that a material is emitting. Though less accurate than thermocouple or RTD sensors, infrared devices still provide information regarding the change in temperature. These are generally only used as a backup method for measuring temperature.

Bimetallic Temperature Measurement Devices

These devices will take advantage of the variations in rate of the thermal expansion between various metals. Two different metals are bonded as one. When they are heated, one side expands a little more than the other, and the bending result is translated into a reading of temperature by mechanical linking to a point. These types of devices are portable and don't require any supply of power. However, they are not generally as accurate as RTD's or thermocouples and they don't lend themselves readily to recording of temperature.

Fluid Expansion Temperature Measurement Device

These kinds of devices, typically the thermometer you find at home, come in two categories: the organic-liquid type and the mercury type. There are also some versions available that use gas rather than liquid. Mercury is known as an environmental hazard, so there are some regulations against the shipping of it. The fluid-expansion devices require no power, do not pose as a hazard for explosion, and are stable after many uses. On the other hand, they don't generate information that is easily transmitted or recorded, and they won't make good point or spot measurements.

State Change Temperature Measurement Devices

These types of sensors generally consist of liquid crystals, lacquers, crayons, pellets or labels that change appearance when a specific temperature has been reached. They are usually used, for example, with steam traps. When a trap exceeds the specified temperature, a small white dot on the label of the sensor that is attached to the trap will change to black. The time of response is usually several minutes, therefore these devices cannot respond to quick temperature changes. Accuracy is also much lower than that of the other sensors. Moreover, except in the case of liquid-crystal displays, the change in the state is irreversible. Even so, these types of sensors may be very helpful when an individual needs a confirmation that the temperature of a piece of material or equipment has not exceeded a specific level, for example for legal or technical reasons during shipping of a product.

Editor's Note: This article, written by Camia Sidle, explains the different methods of temperature measurement that are often used. When you are in need of calibration services, ensure that you choose a service and a type of temperature measurement that is best for your business, for your equipment and for your price.

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#1

Re: Concepts and Methods for Temperature Measurement

11/04/2013 1:00 PM

What about semiconductor (diode) junction voltage?

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#2

Re: Concepts and Methods for Temperature Measurement

11/05/2013 12:22 AM

Good. Add some picture as well. Omega website may have good links.

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#3

Re: Concepts and Methods for Temperature Measurement

11/05/2013 8:11 AM

Some years back I was involved in the measurement of the temprature of a coal fired power station boiler using the change in speed of sound caused by differing temperatures across the firing chamber. The company was Codel and a paper was published at the time. I may have a paper copy somewhere if there is any interest.

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#4

Re: Concepts and Methods for Temperature Measurement

11/21/2013 12:01 AM

which is more sensitive rtd or thermocouple

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Re: Concepts and Methods for Temperature Measurement

11/21/2013 12:47 AM

Thermistors are most sensitive one. However you can't use then at very high temperatures. Thermocouples are cheap and fast response type and can sense 0.1C change.

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#6

Re: Concepts and Methods for Temperature Measurement

12/31/2013 12:40 PM

A fairly new technology being explored for high-temperature measurement uses optical interferometry with sapphire crystals. A pulse of light is fired through a fiber-optic cable to a sapphire disk located at the measurement point, with a double reflection of light (one off the front face of the crystal, the other off the rear face) returning through the fiber optic cable. Since temperature affects both the speed of light in sapphire as well as the thickness of the sapphire crystal, the phase shift between the two reflected light waves ends up being a linear representation of crystal temperature.

The benefits of sapphire sensors include extremely high temperature measurement limits (up to 1800 degrees C, 3300 degrees F claimed) with long life, and high resistance to chemical attack which plagues metallic thermocouples.

A prime application of this technology is in the monitoring and control of slagging coal gasifiers, where high temperatures and corrosive chemical compounds make thermocouple-based temperature measurement troublesome at best.

A short DOE paper on this technology may be found here:

http://www.netl.doe.gov/technologies/coalpower/advresearch/pubs/ARSS-002_4P.pdf

Rosemount even sells one of these units now:

http://www2.emersonprocess.com/en-us/brands/rosemount/temperature/high-temperature-profiling-and-non-intrusive-sensors/sapphire/pages/index.aspx

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Re: Concepts and Methods for Temperature Measurement

12/31/2013 12:57 PM

Whoops -- I typed too soon: the Rosemount link is merely for a thermocouple sensor with sapphire thermowell, not a true sapphire sensor. The product description is very misleading: first referring to a "sapphire sensor" and then later to a sapphire protection tube with a metallic thermocouple as the actual sensing element.

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#8

Re: Concepts and Methods for Temperature Measurement

02/17/2014 5:24 AM

I Like this post...very useful information provided..

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