An Introduction to Pressure Gauges ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

### An Introduction to Pressure Gauges

Custom Search

A pressure gauge is a pressure sensor that is used to indicate the pressure of a given process or system. A pressure gauge usually refers to a self-contained indicator that converts the detected process pressure into the mechanical motion of a pointer. Depending on the reference pressure used, they could indicate absolute, gauge, and differential pressure. “Gauge” pressure is defined relative to
atmospheric conditions. Differential pressure is the pressure between two points in a system while Absolute pressure is defined relative to full vacuum.

Pressure gauges for gauge pressure are very popular while those for absolute pressure are also in use but are not as popular as gauge pressure gauges. The relationship between gauge pressure and absolute pressure is given below:

Absolute pressure = Gauge Pressure + Atmospheric Pressure

Gauge pressure is indicated by adding a ‘g’ to the unit of pressure in use. For example, if Pounds Per Square Inch is used, then the pressure unit is ‘pounds per square inch gauge’ abbreviated psig

When using SI units i.e. pascal, it is proper to add ‘gauge’ to the units used, such as ‘Pa gauge’. When pressure is to be measured in absolute units, the reference is full vacuum and the abbreviation for ‘Pounds Per Square Inch Absolute’ is psia.

Areas of Application
Pressure gauges are practically everywhere as they are applied in almost every field of human endeavour. They are widely used in the process industries, food and beverage industry (in this case sanitary gauges are desirable), Automobile and Aviation industries just to name a few. Several types of gauges are in use in these industries. We have air pressure gauge, oil pressure gauge, aircraft gauges, differential pressure gauges etc.

Designs of Pressure Gauges
Dry Pressure Gauges
By dry gauges we mean mechanical pressure gauges with pressure sensors such as the Bourdon Tube, Helical , capsules, bellows, diaphragm and spring depending on the make that are not filled with any liquid. They are made up of delicate links, pivots, and pinions that are often sensitive to both condensation and vibration

Liquid Filled Pressure Gauges
These types of gauges are filled with viscous oil such as silicone or glycerine. The
life of the filled gauge is longer because it has fewer moving parts; its oil filling helps to lubricate the internal parts of the gauge and dampens pointer vibrations. The presence of oil in these pressure gauges prevent humid air from entering and condensing inside these gauges making them to last even longer. Filled gauges are commonly used in systems where there is a lot of vibration.

Features of Pressure Gauges
Pressure gauges come in various varieties and with increasing fascination. Typical features include:
• Digital readouts (for better visibility)
• Illuminated dials
• Temperature compensation (To correct for ambient temperature variations)
• Differential gauges (for indicating differential pressure)
• Duplex gauges (for dual pressure indication on the same dial)
• Pressure gauges with a visual indication as well as with a 4 -20mA output, etc.
Common Terms Associated with Pressure Gauges
Gauge Pressure
Indicates pressure from a single source and uses ambient pressure as zero in other words if a pressure gauge indicates a pressure of zero, it means the pressure is atmospheric.

Absolute Pressure
The pressure measured above a perfect vacuum. It is the pressure indicated by an ordinary gauge plus the atmospheric pressure.

Differential Pressure
The difference between two independent, but related pressures.

Compound Pressure Gauges
Indicate pressures above & below ambient pressure or positive & negative pressure.

Duplex Gauges
Utilizes two independent sensing elements each connected to different but related pressure sources.

Accuracy of Pressure Gauges
In ASME B40.1(Accuracy for Pressure Indicating Dial Type, Elastic Element Gauges) Pressure gauges are classified according to their precision, from grade 4A (permissible error of 0.1% of span) to grade D (5% error) as shown below:

 Grade Lower 25%(±) Middle 50%(±) Upper 25%(±) 4A 0.1 0.1 0.1 3A 0.25 0.25 0.25 2A 0.5 0.5 0.5 1A 1 1 1 A 2 1 2 B 3 2 3 C 4 3 4 D 5 5 5

Note that the error figures quoted in the table above are exclusive of friction forces. Note also that the accuracy of a pressure gauge may be expressed as percent of span or percent of indicated reading. Percent of span is the most common method. Percent of indicated reading is usually limited to precision test gauges and unless specifically indicated, it may be assumed that an accuracy of ±1% means ±1% of span

Uses of Gauges with Each Accuracy Grade by Application Type.
ASME Grades 2A, 3A & 4A: Used for precise measurements in the laboratories and workshops
ASME Grades A & 1A: Used for industrial pressure measurement on machinery and production lines.
ASME Grades B, C & D: Used for simple monitoring applications without precise requirements.

Pressure Gauge Installations
Gauges are typically installed with certain accessories. A typical gauge is installed with two valves: a block valve between it and the process, which allows blocking while removing or performing maintenance. The second valve is often added for one of two reasons: draining of condensate in vapor service (such as steam), or, for higher accuracy applications, to allow calibration against an external pressure source. A typical pressure gauge installation is shown below:
Other accessories that accompany pressure gauges include:
Syphons
Syphons (pipe coil) are used in steam service installations to protect the pressure gauge against temperature damage. A typical installation is shown below:

Snubbers or Pulsation Dampeners
Snubbers or Pulsation dampeners are used to absorb pressure shocks and fluctuations. A typical installation is shown below:
Diaphragm or Chemical Seals
Diaphragm or Chemical seals are used to protect the gauge from plugging up in viscous or slurry service, and to prevent corrosive process materials from reaching the pressure sensor inside the gauge. A typical installation is shown below: