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Pressure
Pressure is the amount of force applied to a surface or distributed over it.
Measured as force per unit area.
Formula
British units – pounds per square inch
Metric – newtons per square metre
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To monitor and control the process
The pressure of steam in a boiler is measured for ensuring safe operating condition of the boiler
Pressure measurement is done in continuous processing industries such as manufacturing and
chemical industries
Pressure measurement helps in determining the liquid level in tanks and containers
Pressure measurement helps in determining the density of liquids
In many flow meter (such as venturi meter, orifice meter, flow nozzle, etc.,) pressure
measurement serves as an indication of flow rate
Apart from this, pressure measurement is also required in day-to-day situations such as
maintaining optimal pressure in tubes of vehicle tyres
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Bourdon Tube
It uses flexible element as sensor. As the pressure changes, the flexible element moves and this motion is used to rotate the pointer in front of dial.
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Safe for high pressure measurement
Adaptable to transducer designs for electronic instruments
Available at wide variety of ranges
Disadvantages
Subjected to hysteresis
Low spring gradient
Can be in form of flat, corrugated or dished plates
Choice depends on strength and amount of deflection desired
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Metallic Diaphragm
It consist of a thin flexible diaphragm made of materials such as brass or bronze.
A pointer is attached to the diaphragm .
The force of pressure against the effective area of the diaphragm causes a deflection of the diaphragm.
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Slack Diaphragm
Diaphragm with large area produce large change in force cause small change in pressure
Slack diaphragm can be made of rubber or other flexible materials
Range 0.01 – 0.40 torr
Accuracies – 1 to 2%
Adaptable to absolute and differential pressure measurement
Good linearity
Difficult to repair
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Bellows
Used for the measurement of absolute pressures (normal as well as low pressure ).
The bellows are made of an alloy which is ductile, has high strength and retains its property over long use.
There are used in two forms.
In one arrangement, pressure is applied to one side of the bellows & the resulting deflection is counter-balanced by a spring. This arrangement indicates the gauge pressure.
In another differential arrangement, the differential pressure is also indicated.
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Disadvantages
Availability of metals and work-hardening of them is limited
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Snubbers
Used where sudden pressure shocks or rapidly fluctuating pressure are expected.
Such Cases installed between gauge and the process.
Porous
Piston
Adjustable
Installed between gauge and the process
Coil siphon for horizontal installation
Pigtail siphon for vertical installation
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Gauge saver
Overpressure protector are intended to protect the pressure gauge against the effect of pressure that exceed the maximum pressure rating of the pressure gauge.
Gauge Saver shuts off the system with quick and positive action of built in push-rod device
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Manifold
To isolate the DP transmitter from the process for maintenance and calibration
To ensure that DP transmitter is not over-ranged
Manifold Types
2-valve manifold
3-valve manifold
5-valve manifold
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2-Valve Manifold
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3-Valve Manifold
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5-Valve Manifold
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Hook-Up
Hook-up drawing is a detailed drawing showing typical installation of instrument in a correct manner so that instrument operates properly (gives accurate indication and  prevent any issued which could potentially  affect the measurement such as liquid trap in gas impulse line)
Hook-up drawing indicates tubing slopes, position of instrument in reference to process tapping point, scope break between instrument vs. piping.
Hook-up drawing also gives information the requirement of bulk material for each installation. It also details its specification (size, type and material) and the quantity.
There are two types of hook-up drawing:
Process Hook-Up
Pneumatic Hook-Up
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Selection Criteria
Gauge Process
Gauge Size
Does the medium contain solids?
Is a diaphragm seal required?
What material of tube & socket is appropriate? Brass, 316 Stainless steel, Monel, etc.?
Will there be heavy pulsation or vibration in the application?
Are there any temperature extremes to take into account?
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DIN-50049 - Documents on material testing
ASME B40.1 - Document on pressure range and Accuracy
ASME PTC 19.2 - Pressure measurements
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Thank You