lecture#02,pressure instrumentation

8/13/2019 Lecture#02,PRESSURE Instrumentation

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Lecture # 02M. Aamir Shehzad


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Placing of a device in the field having Sensing, Indicating,

Transmitting, Recording or Controlling capabilities

(of physical quantities or conditions, performance,

position, direction, and so forth) for the system.


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Classification according to physical quantities:-

1. Pressure instruments

2. Flow instruments

3. Temperature instruments

4. level instruments

Heating

Element

Water Bath

Temperature


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Physical Quantities related to Basic Instrumentation:-

1. PRESSURE

2. FLOW

3. LEVEL

4. TEMPERATURE


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Pressure (P) is the force per unit area applied in a directionperpendicular to the surface of an object.Also

Pressure is an effect that occurs when a force is applied on a

surface.

Mathematically:

where:

P: is the pressure,

F: is the normal force,

A: is the area of the surface on contact


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1) Absolute pressure: Pressure above perfect vacuum or zeroabsolute.

2) Atmospheric pressure: Pressure exerted by earthsatmosphere.Atmospheric pressure at sea level is 14.7psia or 29.9 inches ofmercury absolute. Barometric Pressure ????

3) Differential pressure: It is difference between two pressures i,eit is the pressure measured with respect to another pressure, and is

expressed as the difference between the two values. Thisrepresents two points in a pressure or flow system, and is referredto as the deltap,or p.

4) Gauge pressure: Pressure above atmospheric pressure.

5) Static pressure: Force per unit area exerted on a wall by a fluidat rest or flowing parallel to a pipe wall. It is also called linepressure.


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6) Velocity pressure: Pressure exerted by the speed of flow. It isalso called velocity head or impact pressure.

7) Hydrostatic pressure: Pressure below a liquid surface exerted

by the liquid above.

8) Total pressure: All pressures, including static, acting in alldirections.

9) Vacuum: Pressure below atmospheric pressure. i,e. pressurebetween total vacuum and normal atmospheric pressure.Pressures less than atmospheric pressure are often referred to asnegative gauge, and indicated by an amount belowatmospheric pressure. As an example, 5 psig corresponds to9.7 psia.

10) Total vacuum: Itis zero pressure or lack of pressure, as wouldbe experienced in outer space, and is very difficult to achieve inpractice. Vacuum pumps can only approach a true vacuum.


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Conditions affecting liquid pressure:

Surface pressure: any pressure acting on the surface istransmitted throughout the liquid and contributes to the pressure atany location beneath the surface.

Depth:pressure is proportional to depth below the surface.

Density: pressure is proportional to the density (or relativedensity) of the liquid.


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lecture#02,pressure instrumentation

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