Instrumentation

Pressure Gauge

Hi Friends, this lesson will contain knowledge about pressure gauge, its types and working principle.

Before pressure gauge we will see what is gauge.

Gauge

Gauge is used to measure or indicate any process quantity or variable.

Pressure Gauge

Pressure gauge is used to indicate the pressure of any process line, body or tank.

  • Dial Size:- Dial size is the diameter of dial of a pressure gauge. Pressure gauges are available in different dial sizes according to application. Dial sizes are measured in inches.
  • Range:- Range of a pressure gauge is range of pressure indicated by a pressure gauge. Pressure gauges are available in different ranges.
  • Pressure gauges are used for a variety of industrial and application-specific pressure monitoring applications. Uses include visual monitoring of air and gas pressure for compressors, vacuum equipment, process line.

Pressure Gauge Operating Principle

  • Pressure gauges use a bourdon tube-sensing element generally made of a copper alloy (brass) or stainless steel for measuring pressures 15 PSI and above. Bourdon tube gauges are widely used in all branches of industry to measure pressure and vacuum.
  • Digital gauges use an electronic pressure sensor to measure the pressure and then transmit it to a digital display readout.
  • The construction is simple yet rugged and operation does not require any additional power source. The C-shaped or spirally wound bourdon tube flexes when pressure is applied producing a rotational movement, which in turn causes the pointer to indicate the measured pressure. These gauges are generally suitable for all clean and non-clogging liquids and gaseous media.

Pressure units conversion

We have studied different units of pressure in the first chapter. we have a unit conversion chart below which will help to convert a unit of pressure into another.

Commonly used bourdon tubes.

The following are the commonly used Bourdon tubes used in industries:

  • Spiral –Low range 10 –100kpa
  • C type –Medium range  100 – 5000kpa
  • Helical – High range 5000 – 20000kpa

Purpose of using hair spring

  • A spring is used for come back the pointer when pressure is released.
  • Purpose of hair spring is to push pointer back to zero when there is no pressure in tube and to reduce play between gears. It is also called as backlash spring.

Reasons for gauge failure

Two common reasons for gauge failure are

  1. pipe vibration
  2. water condensation

How to overcome these failures

Glycerin filled

Glycerin is filled to reduce the pipe vibrations. Glycerin is used in pressure gauge to damping the effect of vibration and pulsation.

A rubber is shown in the figure, with the help of this rubber glycerin is filled.

Siphons are used

A pressure gauge siphon is a simple device used to protect a pressure sensor from high-temperature media, such as steam. It can also be used to reduce the potentially damaging effects of rapid pressure changes.

Types of pressure gauges

Pressure gauges are of three types.

  1. Pressure Gauge.
  2.     Vaccum Gauge.
  3. Compound Gauge

Pressure Gauge:

Pressure Gauge is used to indicate pressure.

Vaccum Gauge:

Vaccum gauge is used to indicate vaccum.

Compound Gauge:

Compound Gauge is used to indicate both pressure and vaccum.

While selecting a pressure gauge for a process, consider the following points:

  • Characteristic of the process (corrosive or non-corrosive)
  • Operating pressure
  • Maximum process pressure
  • The gauge range should be a minimum of twice the process operating pressure.

Advantages of Bourdon tube pressure gauge:

These Bourdon tube pressure gauges give accurate results.
Bourdon tube cost low.
Bourdon tube is simple in construction.
They can be modified to give electrical outputs.
They are safe even for high pressure measurement.
Accuracy is high especially at high pressures.

Limitations of bourdon tube pressure gauge:

They respond slowly to changes in pressure
they are subjected to hysteresis.
They are sensitive to shocks and vibrations.

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