pressure measuring devices

Measurements of

Pressure

Presented by:- Gaurav kumarDeepak kumar

Pressure

Pressure (P) expresses the magnitude of normal force(F) per unit area(A) applied on the surface.

SI units :- Pascal (N/m²)Other units:- bar(=10⁵ pa),atm (=101.3kpa), mm of hg or ( torr = 0.133322kpa)

Bourdon tube pressure gaugePrinciple:- The bourdon tube works on a

simple principle that a bent tube will change its shape .

As pressure is applied internally, the tube straightens and returns to its original form when the pressure is released .

The tip of the tube moves with the internal pressure change and is easily converted with a pointer onto a scale.

Advantages:- Inexpensive Wide operating rangeFast response Good sensitivityDirect pressure measurements

Disadvantages:-Hysteresis on cycling Sensitive to temperature variationsLimited life when subjected to shock and vibrations

Applications:-These devices should be used in air if calibrated for air

and in liquid if calibrated

Diaphragm Gauge of pressure measurements

The diaphragm is a flexible disc, which can be either flat or with concentric corrugations and is

made from sheet metal with high tolerance dimensions.

The diaphragm can be used as a means of isolating the process fluids, or for high pressure applications.

It is also useful in providing pressure measurement with electrical transducers (LVDT).

It is used as primary pressure transducers in many dynamic pressure measuring devices.

Diagram:-

Types of diaphragms :- Diaphragm

Flat diaphrag

m

Corrugated diaphragm

diaphragm:-

consider, pw= deflection developed due to pressure difference, P(ext.)=applied pressure (measured pressure),P(ref)= atmospheric pressure, a= radius of diaphragm t= thickness, E= young’s modulus, γ= poison’s ratio, ∆p= difference in pressure,Now deflection can be given by; W= {γ for metal= 0.3-0.5)Let k =3a⁴(1-γ²)/ 16Et³, therefore, w= k*∆p

Advantages :-

Good for low pressureInexpensiveWide rangeReliable and provenUsed to measure gauge, atmospheric and

differential pressure.

Linear variable differential transformer(LVDT)This type of pressure measurement relies on

the movement of a high permeability core within transformer coils . the movement is transferred from the process medium to the core by use of a diaphragm , bellows or bourdon tube.

Working principle :- The LVDT operates on the inductance ratio

between the coils. The primary coil is located between two secondary coils and is energized with an alternating current . Equal voltage are induce in the secondary coil if the core is in the center when the core is moved from the center position the result of the voltages in the secondary winding will be different.

Conversion of ac to dc