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Continuous Temperature Measurement

Temperature is the measure of average molecular kinetic energy within a substance. The concept is easiest to understand for gases under low pressure, where gas molecules randomly shuffle about. The average kinetic (motional) energy of these gas molecules defines temperature for that quantity of gas. There is even a formula expressing the relationship between average kinetic energy (Ek) and temperature (T) for a monatomic (single-atom molecule) gas:


a formula expressing the relationship between average kinetic energy (Ek) and temperature (T) for a monatomic (single-atom molecule) gas



= Average kinetic energy of the gas molecules (joules)

   k = Boltzmann’s constant (1.38 × 1023 joules/Kelvin)

   T = Absolute temperature of gas (Kelvin)

Thermal energy is a different concept: the quantity of total kinetic energy for this random molecular motion. If the average kinetic energy is defined as ,  then the total kinetic energy for all the molecules in a monatomic gas must be this quantity times the total number of molecules (N) in the gas sample:

the total kinetic energy for all the molecules in a monatomic gas must be this quantity times the total number of molecules (N) in the gas sample

This may be equivalently expressed in terms of the number of moles of gas rather than the number of molecules (a staggeringly large number for any realistic sample):

the total kinetic energy for all the molecules in a monatomic gas must be this quantity times the total number of mole (n) in the gas sample


   Ethermal = Total thermal energy for a gas sample (joules)

   n = Quantity of gas in the sample (moles)

   R = Ideal gas constant (8.315 joules per mole-Kelvin)

   T = Absolute temperature of gas (Kelvin)


Heat is defined as the exchange of thermal energy from one sample to another, by way of conduction (direct contact), convection (transfer via a moving fluid), or radiation (emitted energy); although you will often find the terms thermal energy and heat used interchangeably.

Temperature is a more easily detected quantity than heat. There are many different ways to measure temperature, from a simple glass-bulb mercury thermometer to sophisticated infrared optical sensor systems. Like all other areas of measurement, there is no single technology that is best for all applications. Each temperature-measurement technique has its own strengths and weaknesses. One responsibility of the instrument technician is to know these pros and cons so as to choose the best technology for the application, and this knowledge is best obtained through understanding the operational principles of each technology.


Bi-Metal Temperature Sensors - Solids tend to expand when heated. The amount that a solid sample will expand with increased temperature depends on the size of the sample, the material it is made of, and the amount of temperature rise.Click here to read more...

Filled-Bulb Temperature Sensors - Filled-bulb systems exploit the principle of fluid expansion to measure temperature. If a fluid is enclosed in a sealed system and then heated, the molecules in that fluid will exert a greater pressure on the walls of the enclosing vessel. By measuring this pressure, and/or by allowing the fluid to expand under constant pressure, we may infer the temperature of the fluid. Class I and Class V systems use a liquid fill fluid (class V is mercury). Click here to read more...

Thermistors and Resistance Temperature Detectors (RTDs) - One of the simplest classes of temperature sensor is one where temperature effects a change in electrical resistance. With this type of primary sensing element, a simple ohmmeter is able to function as a thermometer, interpreting the resistance as a temperature measurement. Click here to read more...

Thermocouples - RTDs are completely passive sensing elements, requiring the application of an externally-sourced electric current in order to function as temperature sensors. Thermocouples, however, generate their own electric potential. In some ways, this makes thermocouple systems simpler because the device receiving the thermocouple’s signal does not have to supply electric power to the thermocouple. The self-powering nature of thermocouples also means they do not suffer from the same “self-heating” effect as RTDs. In other ways, thermocouple circuits are more complex than RTD circuits because the generation of voltage actually occurs in two different locations within the circuit, not simply at the sensing point. This means the receiving circuit must “compensate” for temperature in another location in order to accurately measure temperature in the desired location. Click here to read more...

Non-Contact Temperature Sensors - Virtually any mass above absolute zero temperature emits electromagnetic radiation (photons, or light) as a function of that temperature. This basic fact makes possible the measurement of temperature by analyzing the light emitted by an object. Click here to read more...

Temperature Sensor Accesories - One of the most important accessories for any temperature-sensing element is a pressure-tight sheath known as a thermowell. This may be thought of as a thermally conductive protrusion into a process vessel or pipe that allows a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe. Thermowells are critically important for installations where the temperature element (RTD, thermocouple, thermometer, etc.) must be replaceable without de-pressurizing the process. Click here to read more...

Process/Instrument Suitability - The primary consideration for selecting a proper temperature sensing element for any application is the expected temperature range. Mechanical (bi-metal) and filled-system temperature sensors are limited to relatively low process temperatures, and cannot relay signals very far from the point of measurement. Click here to read more...



Beckerath, Alexander von; Eberlein, Anselm; Julien, Hermann; Kersten, Peter; and Kreutzer, Jochem, WIKA-Handbook, Pressure and Temperature Measurement, WIKA Alexander Wiegand GmbH & Co., Klingenberg, Germany, 1995.

Darling, Charles Robert, Pyrometry – A Practical Treatise on the Measurement of High Temperatures, E. & F.N. Spon, Ltd, London, 1911.

Fribance, Austin E., Industrial Instrumentation Fundamentals, McGraw-Hill Book Company, New York, NY, 1962.

Irwin, J. David, The Industrial Electronics Handbook, CRC Press, Boca Raton, FL, 1997.

Kallen, Howard P., Handbook of Instrumentation and Controls, McGraw-Hill Book Company, Inc., New York, NY, 1961.

Lipt´ak, B´ela G., Instrument Engineers’ Handbook – Process Measurement and Analysis Volume I, Fourth Edition, CRC Press, New York, NY, 2003.

“Model 444 Alphaline Temperature Transmitters”, Document 00809-0100-4263, Revision AA, Rosemount, Inc., 1998

“Radiamatic Detectors and Accessories”, Specification document 23-75-03-03, Honeywell, Inc., Fort Washington, PA, 1992.

“Temperature - Electromotive Force (EMF) Tables for Standardized Thermocouples”, Pyromation, Inc.

“Temperature Measurement – Thermocouples”, ISA-MC96.1-1982, Instrument Society of America, Research Triangle Park, NC, 1982.

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