Barometric type of temperature measurement relies on the basis of pressure measurement. To suit the differing applications, the temperature sensitive bulb is available in many different shapes and sizes. The temperature that is displayed is a result of the pressure in the filled system. However, the pressure in the system is a result of the temperature around the bulb and the temperature around the rest of the system.
Working:
Filled thermal systems, consist of bulbs connected through capillary tubing to pressure or volume sensitive elements. They work on the expansion of a fluid with temperature. As the temperature increases, the fluid is pushed further along the capillary, and the pressure in increased. The Bourdon type is commonly used to measure the pressure.
In industrial applications, filled thermal systems not using glass are more common. These work on the same principle, where a bulb is filled with fluid which extends along a metal capillary tube. A pressure measuring device is used to detect the amount of expansion of the fluid.
Types of Barometric pressure measurement:
Liquid filled systems (Class I):
Liquid expansion systems have narrow spans, small sensors and give high accuracy. They also have the ability to provide temperature compensation using either an auxiliary capillary or bimetallic techniques.
Liquid filled systems have the advantage over Mercury in that the expansion of the fluid is about six times that of a Mercury systems.
The normal operating minimum for this type of sensing is from -75°C to -210°C, with the maximum being up to 315°C.
Vapour filled systems (Class II):
This form of measurement is based on the vapour-pressure curves of the fluid and measurement occurs at the transition between the liquid and vapour phases. This transition occurs in the bulb, and will move slightly with temperature, but it is the pressure that is affected and causes the measurement.
If the temperature is raised, more liquid will vaporise and the pressure will increase. A decrease in temperature will result in condensation of some of the vapour, and the pressure will decrease.
Gas-filled systems (Class III):
Nitrogen is quite commonly used as it doesn’t react easily and is inexpensive, although it does have temperature limitations. At low temperatures and above 400°C, helium should be considered
As the volume is kept constant, the pressure varies in direct proportion to the absolute temperature. This type of measurement is quite simple and low cost. The range of operation is determined by the initial filling pressure.
Mercury filled systems (Class V):
Mercury expansion systems are different from other liquid filled systems because of the properties of the metal. Mercury is toxic and can affect some industrial processes and for this reason is used less in filled systems. The high liquid density also limits on the elevation difference between the sensor and instrument.
Mercury filled systems provide the widest range of operation, which range from the freezing to boiling point of the metal, ie from -40°C to 650°C.
Advantages:
- Simple operation
- Robust
- Inexpensive
- No power source required
- Easily maintained
- Reasonably accurate
Disadvantages:
- Bulky
- Slow response time
- Wide spans only
- Subject to gauge pressure problems
- Non linear
