Full system thermometer or pressure gauge thermometer

Full system thermometer or pressure gauge thermometer

The filled system essentially consists of:

Pressure pipe:

It is composed of copper, monel or stainless steel. It is full of the liquid or gas or vapor and liquid pressure. Hence, this thermometer is known as a filled system. Liquids are filled at absolute pressure and temperature.

Capillary tube:

A capillary tube connects the filled tube to the receiving element.

Receiving Item:

The receiving element is a pressure sensor and can be a Bourdon tube, bellows or diaphragm. The free end of the Bourdon tube is connected to a pointer on a temperature-calibrated scale.

They are usually of 3 types:

1. Liquid

2. Gas and

3. Liquid vapor pressure filled system.

Fluid filled system:

The pressure bulb is filled with mercury, ethyl alcohol and toluene. The pressure is immersed in the well (open) where the temperature is to be measured. A filled liquid with a high coefficient of expansion expands. The liquid fills the capillary tube and Bourdon tube. Pressure is converted to displacement, the displacement moves the pointer on the temperature scale.

The equation:

Vt = Vo(1+KT)


Vt = final volume of liquid.

Vo = Initial volume of liquid.

K = Coefficient of thermal expansion of the liquid (constant).

T = Change in temperature.

Here, the final volume expansion of the liquid on immersion in the well is linearly related to Vo, the initial volume of the filled liquid. The measurement temperature range in the liquid-filled system is from -90’C to 310’C.

Gas system:

It basically works on the principle of Charle’s law. An ideal gas of a given weight at constant volume produces an absolute pressure that is directly proportional to the absolute temperature of the gas under ideal conditions.

Pv = RT


P = absolute pressure.

V = specific volume of the gas.

R = gas constant.

T = absolute temperature.

The relationship between pressure and temperature is linear.

The filled gas expands as the temperature rises, the pressure is sensed by the receiving element. The change in displacement of a Bourdon tube gives an indication of temperature. The gas-filled system works by expanding or contracting depending on changes in temperature.

Nitrogen and helium gases are used as fill gases. This system operates in a temperature range of – 240’C to 850’C. Accuracy is plus or minus 0.5% to 1%.

A system full of liquid vapor pressure:

A liquid vapor filled system operates from the vapor pressure of the liquid that partially fills the system.

Vapor pressure is measured by a receiving element with a temperature-calibrated scale.

The vapor pressure depends on the free surface area of ​​the liquid. The pressure bulb is dipped to measure the temperature, the liquid in the filled system boils and evaporates. The liquid vapor expands and fills the capillary tube and Urdons tube, giving an indication of the temperature. When the temperature to be measured is low, the vapor condenses, causing the pressure in the filled system to decrease, the Bourdon tube is clamped and indicates the low temperature.

Commonly used liquids are methyl alcohol, sulfur dioxide, ether, toluene, butane, propane, and hexane.

The relationship between vapor pressure and temperature is non-linear:

P = log(ab/t)


A,b = constant.

P = vapor pressure.

T = temperature.

The scale used is non-linear with wider graduations at higher temperature and narrow graduations at lower temperature. The operating temperature range of this thermometer is from -10’C to 300’C.


It is more economical, versatile, widely used.

Sturdy construction, easy maintenance.

Can be used for remote indication.

It is stable in operation.

This system provides sufficient power to drive the steering mechanism.


Compensation required for changes in ambient (surrounding) temperature and long capillary tube.

For accuracy, the pressure bulb should be large.

In the event of an error, the entire system must be replaced.

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