Mechanical Thermometer

What is a mechanical thermometer?

Several different types of mechanical thermometers are commonly used: 

Bimetal Thermometer

Two different metals with different coefficients of thermal expansion are bonded together. As a change in temperature occurs the unequal expansion to the two metals will cause the bimetal strip to curl. If one end of the metal strip is fixed than the other end will be displaced in response to temperature changes. Bimetal strips can be fabricated into coils, spirals and disks.

Liquid Expansion Thermometer

The volumetric expansion of liquids and solids can be used for temperature measurement. Mercury and alcohol filled thermometers work under this principle.

Gas Thermometer

A gas filled thermometer operating under the principles of the ideal gas law (PV = nRT) can be used for temperature measurement. If pressure is held constant the change in gas volume can be used to indicate temperature.

How does a mechanical thermometer work?

A strip made from permanently-laminated rolled sheets, made from metals having different coefficients of expansion ("bimetal"), will bend because of any temperature changes. The bending is roughly proportional to the change in temperature. For bimetallic strips, two different measurement systems have been developed: helically wound or spirally wound.

Through the mechanical deformation of the bimetal spring in either of these spring forms, on any change in temperature a rotational movement occurs. If one end of the bimetal measuring system is fixed securely, the other will rotate a pointer shaft. The scale ranges are between -94 and +1,112 °F (-70 and +600 °C) with an accuracy class of 1 and 2 in accordance with EN 13190.

Mechanical thermometer applications

  • Mold Temperature
  • Oven Insulation
  • Pulp and Paper Machinery
  • Refineries and Petrochemical
  • Product Temperature
  • Hydraulics
  • Duct Leakage
  • Supply/Return Register
  • Air Balance/Stratification
  • Super Heat and Sub-cooling
  • Valves and Steam Traps

How to select a mechanical thermometer?

  • Select a compliant material of case and internals. The case can be constructed with a screw to fit window of glass or a Lexan with a gasket. Select the case with seal compliant to your application, for example IP65 and chemical resistant.
  • Select the size and type of connection and length of the probe if required.
  • Select the dial size and range of temperature to be measured. The range should fit the temperature range over or under range tolerance of 50% of scale. Select the required accuracy: for example, +1% full range span per ASME B40.3. For damping the case can be filled with an inert gel to minimize pointer oscillation.

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