Temperature Measurement by Filled Thermal Systems

Principle of Filled Thermal System

This works on the following principle: Liquid or Gas expands when heated and this change in volume is used for measurement.

A filled thermal system is basically a pressure gage (generally bourdon type) with a small-bore tubing connected to a bulb acting as a temperature sensor. The complete system is gas-tight and filled with gas or liquid under pressure.

Filled System selection Criteria

The selection is typically application-oriented.

  • Ambient temperature compensation.
  • Scale graduations.
  • Bulb size and tubing lengths.
  • Bulb material.
  • Over-range capacity.
  • Torque requirements.
  • Selection of filling fluid/gas.

Impact due to Ambient Temperature

Ambient temperature compensation-

  • Thermal expansion will have a small impact on the reading.
  • If the range is narrow or bulb is small and capillary long then full compensation is required.
  • Compensation is done either using a bimetallic compensator or by duplicating the spiral and capillary.
  • A good practice is to have a large bulb diameter.
Impact due to Ambient Temperature
Fig. 1: Impact due to Ambient Temperature

Liquid filled system

  • Here the complete system is filled with a liquid (other than mercury) and operates on the principle of liquid expansion.
  • The filling liquids are – Inert Hydrocarbon like Xylene (C8H10), which has a coefficient of expansion 6 times that of mercury and makes smaller bulbs possible.
  • The criteria are that the pressure inside the system must be greater than the vapor pressure of the liquid to prevent bubbles of vapor from forming inside the spiral.
  • Also, the filling liquid should not solidify.

Vapor filled system

  • Here the filling medium is in both liquid and gaseous form.
  • The interface between the two must occur in the bulb and this will move slightly with temperature affecting the pressure.
  • The bulb will be mostly filled with gas while the capillary and spiral would contain liquid.
  • This is not recommended when the ambient temp. is near to the measured temp.
Vapor filled system
Fig. 2: Vapor filled system

Gas-filled system

  • The operating principle for the gas-filled systems is that in a perfect gas confined to a constant volume the pressure is proportional to the absolute temperature.
  • Nitrogen is the normal filling media. However, for ranges above 427 Deg.C, this is avoided, as it interacts with steel, which is the bulb material.
  • For Low temperatures, Helium is used.

Mercury filled system

  • This is a very common method for measurement.
  • The response is fast and accurate.
  • The ambient temperature compensation is less of a problem, as mercury is incompressible.
Typical images of filled systems
Fig. 3: Typical images of filled systems
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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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