How are the temperatures below zero degrees celsius measured

Mercury thermometer is used to measure temperatures above zero degree Celsius. Mercury expands upon heating and is a good thermal conductor. It is also a bright liquid and thus convenient for temperature measurement. However, mercury cannot be used because it freezes and stops flowing at minus 38.87 degrees Celsius. Measurement of subzero temperatures are necessary in many areas. These include weather , processes such as freeze drying to make milk powder, immunology where vaccines have to be preserved and so on.

Temperatures below zero degree Celsius (that is, the minus scale) or 32 degrees fahrenheit can be measured by resistance thermometers. They work on the premise that the resistance of materials changes with temperature. Resistance decreases as the temperature is reduced in the case of metals (positive temperature coefficient), while the resistance increases with decrease of temperature in the case of semiconductors (negative temperature coefficient).

Platinum resistance thermometers have a range of -200 to 500 °C(-328 to 932 °F). For measurement of even lower temperatures, Germanium and Silicon diode (semiconductor) thermometers are used. Thermocouples can also be used for measuring temperatures below zero degrees C.

Thermocouples are based on the principle of thermo-electric effect which states that when two dissimilar metals (or their alloys) are joined to each other at their two ends and given a difference of temperatures at these two junctions, then a thermo-electric current starts flowing through the loop.

Consequently, if one of the metal wires is cut and a voltmeter connected in the gap, then a voltage (emf) is developed which is almost proportional to the temperature difference between the two junctions. The emf is read out and converted to the corresponding temperature.

RTDs vs thermocouples

The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTDs) and thermocouples. Choice between them is usually determined by four factors.

  • temperature: If process temperatures are between -200 to 500 °C (-328 to 932 °F), an industrial RTD is the preferred option. Thermocouples have a range of -180 to 2,320 °C (-292 to 4,208 °F), so for temperatures above 500 °C (932 °F) they are the only contact temperature measurement device.
  • response time: If the process requires a very fast response to temperature changes—fractions of a second as opposed to seconds (e.g. 2.5 to 10 s)—then a thermocouple is the best choice. Time response is measured by immersing the sensor in water moving at 1 m/s (3 ft/s) with a 63.2% step change.
  • size : A standard RTD sheath is 3.175 to 6.35 mm (0.1250 to 0.250 in) in diameter; sheath diameters for thermocouples can be less than 1.6 mm (0.063 in).
  • accuracy and stability requirements: If a tolerance of 2 °C is acceptable and the highest level of repeatability is not required, a thermocouple will serve. RTDs are capable of higher accuracy and can maintain stability for many years, while thermocouples can drift within the first few hours of use.