Temperature Measurement: Temperature Definition

What is Temperature? Temperature Definition

By definition, Temperature is a physical property of a material that manifests the thermal energy content of that material. Temperature expresses the relative measures of how hot or cold an object is with respect to some other object.

  • The hotness or coldness of a piece of plastic, wood, metal, or other material depends upon the molecular activity of the material.
  • Kinetic energy is a measure of the activity of the atoms which make up the molecules of any material. Therefore, the temperature is a measure of the kinetic energy of the material under measurement.

Units of Temperature: Temperature Scales: Temperature measurement units

To standardize on the temperature of objects under varying conditions, several temperature scales have been devised.

  • Degree Celsius/ Centigrade Scale (C): A temperature scale defined by 0 Deg C at the ice point and 100 Deg C at the boiling point of water at sea level.
  • Fahrenheit Scale (F): A temperature scale defined by 32 Deg F at the ice point and 212 Deg F at the boiling point of water at sea level.
  • Kelvin Scale (K): The unit of absolute or thermodynamic temperature scale based upon the Celsius with 100 units between ice point and boiling point of water. 0 Deg C=273.15 K.
  • Rankine Scale (R): An absolute temperature scale based upon the Fahrenheit scale with 180 Deg between the ice point and boiling point of water. 0 Deg F=459.67 Deg R.

Conversion between Temperature Scales

The following image provides the formulas/equations for conversion to one temperature scale to the other.

Conversion to Temperature Scales
Fig. 1: Conversion to Temperature Scales

Temperature Measurement

Temperature measurement is done using various thermometers or pyrometers or temperature transmitters. The various devices used for temperature measurement are listed below:

  • Mechanical Thermometers: Glass Thermometer, Bimetallic Temperature Gauge
  • Electronic Thermometers: Thermocouple, RTD, Thermistors, Integrated Circuitry Sensors
  • Non-Contact Pyrometers
  • Intelligent Temperature Transmitters

Temperature Measurement by Mechanical Thermometers

Liquid Filled Thermometers (Fig. 2, A):

Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the temperature. To increase the sensitivity, there is usually a bulb of mercury at the end of the thermometer which contains most of the mercury; expansion and contraction of this volume of mercury are then amplified in the much narrower bore of the tube.

The space above the mercury may be filled with nitrogen or it may be a vacuum to increase the temperature range.

Typical Mechanical Thermometers
Fig. 2: Typical Mechanical Thermometers

Industrial Mercury Thermometer (Fig. 2, B):

Some manufacturers use a liquid alloy of gallium, indium, and tin, galinstan, as mercury replacement.

The industrial thermometer is used for a range of 0-538 Deg C

At this upper limit, the space above the mercury column must be under great pressure with an inert gas to prevent the evaporation of mercury from the top of the column.

Filled type thermometersOpens in a new tab. (Fig. 2, C) with thermo-well are used as a locally mounted temperature indicator, with extended capillary where necessary.

Bimetallic Thermometer (Fig. 3):

Fundamental Principle:

  • Metals change the volume with Temperature
  • This coefficient of change is not the same for all metals.

Most industrial bimetal thermometers use a helical coil which can be designed to fit into a stem more easily. The element is surrounded by a protective wall.

Bimetal type or filled type dial thermometers with thermo-well is used for local temperature indication.

Various Bimetallic thermometers
Fig. 3: Various Bimetallic thermometers

Temperature Measurement by Non-Contact Pyrometers

IR Pyrometers (Fig. 4):

  • Fundamental Principle: Radiation emitted by Hot Body
  • IR Pyrometers detect the dominant wavelength of the radiation received from a hot body.
  • As temperature increases, the dominant wavelength of hot body radiation gets shorter.
  • Range : 0-3890 DegC
  • The IR Pyrometer must be carefully aimed to avoid focusing on reflected solar radiations
  • When the temperature of a hard to reach the area is required (Temperature of turbine blades inside Turbine Engines), IR Pyrometers are used.

Limitation: Changes in emissivity of the surface and by contamination and dirt buildup.

IR Pyrometers
Fig. 4: IR Pyrometers

Temperature Measurement by Intelligent Temperature Transmitters (ITT)

  • The incorporation of a microprocessor has improved the performance of Temperature Transmitters. These improvements include the near elimination of errors due to ambient temperature variations.
  • An ITT usually provides a linear output with all its sensing elements and can also be reconfigured to accept any thermocouple, RTD, or millivolt sensors.
  • The ITT automatically selects the right cold junction compensation to match the selected thermocouple.
  • ITT is capable to handle dual RTD elements, allowing for temperature averaging, difference measurement, or automatic RTD sensor switchover if the primary sensor fails in a redundant installation.
Intelligent temperature transmitters
Fig. 5: Intelligent temperature transmitters
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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

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