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TEMPERATURE: Units & Methods of measuring temperature

Updated: Feb 4, 2021

Introduction:

The hotness or coldness of a material rests upon the molecular activity of the material. Accordingly, temperature is a measure of the kinetic energy of the material under measurement. Kinetic energy is a measure of the activity of the atoms which make up the molecules of any material.

Units Of Temperature

To systematize on temperature of objects under varying conditions, several scales have been formulate:-

Degree Celsius/ Centigrade Scale (C):- A temperature scale explained by 0°C at the ice point and 100°C at the boiling point of water at sea level.

Fahrenheit Scale (F):- A temperature scale described by 32° at the ice point and 212 ° 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°C=273.15 K

Rankine (R): - An absolute temperature scale based upon Fahrenheit scale with 180° between the ice point and boiling point of water. 0° F = 459.67°R.

METHODS OF MEASURING TEMPERATURE

· Mechanical Thermometers: Glass Thermometer, Bimetallic Temperature Gauge

· Electronic Thermometers: Thermocouple, RTD, Thermistors, Integrated Circuitry Sensors

· Non-Contact Pyrometers

· Intelligent Temperature Transmitters

MECHANICAL THERMOMETERS:

Calibrated marks on the tube, favors the temperature to be known by the length of the mercury within the tube, which diversifies according to the temperature. To increase the sensitivity, there is regularly a bulb of mercury at the end of the thermometer which involves most of the mercury; expansion and contraction of this volume of mercury is then augmented in the much narrower bore of the tube.

The space above the mercury is loaded with nitrogen or vacuum to reinforce the temperature range

Some manufacturers use a liquid alloy of gallium, indium, and tin, galinstan, as a replacement for mercury. The industrial thermometers instrumentalized for a range of 0-538 °C At this top limit the space above the mercury column must be under great pressure with an inert gas to restrict evaporation of mercury from the top of the column.

Filled type thermometers with thermo-well are adopted as locally mounted temperature indicator, with prolonged capillary where unavoidable.

Fundamental Principle:

• Metals transform volume with Temperature

• This coefficient of change is not same for all metals.

Most industrial bimetal thermometers use a helical coil which can be devised to fit into a stem more quickly. The element is enveloped by a protective well.

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

NON-CONTACT PYROMETERS:

IR Pyrometers:

· Fundamental Principle : Radiation expended by Hot Body

· IR Pyrometers spot the dominant wavelength of the radiation taken in from a hot body.

· As temperature increases, the dominant wavelength of hot body radiation gets shorter.

· Range : 0-3890°C

·The IR Pyrometer must be correctly targeted to escape focusing on reflected solar radiations

·When the temperature of a hard to reach 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.

Intelligent Temperature Transmitters (ITT):-

· The incorporation of a microprocessor has upgraded the performance of Temperature Transmitters. These improvements consist of 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 are 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.