Pressure Instrumentation I-Control and Instrumentation-Lecture Slides, Slides for Electronic Measurement and Instrumentation

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This lecture was delivered by Prof. Sonu Vamsi at Agra University. This lecture is part of lecture series for course Control and Instrumentation. Its main points are: Pressure, Instrumentation, Measurement, Force, Exerte...
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Pressure Instrumentation

Javaid Khurshid

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How do you work?

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Pressure Measurement

Pressure is defined as force exerted on a unit

surface area.

• Mathematically, we have: P = F/A

where P=Pressure(Pa), F=Force(N) and A=Area(m2)

• Basic unit of pressure in SI units is Pascal (Pa).

– Pascal is defined as force of 1 Newton (N) per square

meter (m2). That is: 1 Pa = 1 N/m2

• Since the Pascal (Pa) is a very small unit (1

Pa = 1.45 x 10-4 PSI)

– it is more common to use units of kPa and MPa,

when we deal with the pressures in typical industrial

process applications.

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Absolute, Gauge and Differential

Pressures • Pressure measurements can be stated as either

– gauge,

– absolute or differential.

Gauge pressure is the unit we encounter in everyday work (e.g. tire ratings are in gauge pressure).

• A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e. gauge pressure is in reference to atmospheric pressure – that pressure above atmospheric pressure).

• Gauge pressure is denoted by a (g) at the end of the pressure unit (e.g. kPa(g)).

Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit (e.g. kPa(a).)

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An absolute pressure indicator would indicate

atmospheric pressure when completely bled down -

it would not indicate scale zero.

The relationship between absolute pressure and

gauge pressure is:

Absolute Pressure = Gauge Pressure + Atmospheric

Pressure

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• The standard value of atmospheric pressure is

taken as the atmospheric pressure at sea level,

which is 101.3 kPa.

• Note a reading of less than 101.3 kPa(a) indicates

a vacuum condition.

– For example, a typical condenser pressure is 5 kPa(a) or a

vacuum of 96.3 kPa.

• Differential Pressure is the difference of two pressures.

• In order to produce a standard (4 - 20 mA) electronic

signal which represents the pressure in a process,

the pressure must be sensed and a physically

detectable motion or force in proportion to this

applied pressure must be developed.

• To sense the process, we use a pressure sensor.

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Pressure Measurement Devices

• Some common sensors include:

– Bourdon Tubes

– Bellows

– Diaphragms

– Capsules

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Bourdon Tube

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Reference: http://matec.org/ps/library3/secure/modules/101/LA3/M101LA3.html

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Bellows

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Diaphragms

When a force acts

against a thin

stretched diaphragm,

it causes a deflection

of the diaphragm with

its centre deflecting

the most.

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Pressure Capsules

• If it is fixed at the

air inlet, it can

expand like a

balloon.

• It expands in both

ways whereas in

Diaphragm it

expands in one

direction.

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Pressure Capsules

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DIFFERENTIAL PRESSURE DEVICES

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Differential Pressure Devices • Used to measure Differential Pressure (that

is, the difference between a high pressure input and a low pressure input) and hence called DP transmitters or DP cells.

• A differential pressure capsule is mounted inside a housing.

• One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted out of the housing.

• A sealing mechanism is used where the force bar penetrates the housing.

• This seal also acts as the pivot point for the force bar.

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A Typical DP Cell

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• Provision is made in the housing for high

pressure fluid to be applied on one side of the

capsule and low pressure fluid on the other.

• Any difference in pressure will cause the capsule

to deflect and create motion for the force bar.

• The top end of the force bar is connected to an

electronic motion detector, which via an

electronic system, will produce a 4 - 20 mA

signal that is proportional to the force bar

movement.

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DP Capsule Construction • The DP capsule is formed by welding two metallic

(usually stainless steel) diaphragms together.

• To provide over-pressurization protection, a solid plate

with diaphragm matching convolutions is mounted in the

center of the capsule.

• Silicone oil is used to fill the cavity between the

diaphragm for even pressure transmission. Most DP

capsules can withstand static pressure of up to 14 Mpa

(2000 psi) on both sides of the capsule without any

damaging effect.

• However, the sensitive range for most DP capsules is quite

low. Typically they are sensitive up to only a few hundred

kPa of differential pressure.

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DP Transmitter

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DP Transmitter • A DP transmitter is used to measure the gas

pressure (in gauge scale) inside a vessel.

• In this case, the low pressure side of the

transmitter is vented to atmosphere, and the

high pressure side is connected to the

vessel through an isolating valve.

• The isolating valve facilitates the removal of the

transmitter

• The output of the DP transmitter is

proportional to the gauge pressure of the gas

in the tank, i.e., 4 mA when pressure is 20 kPa

and 20 mA when pressure is 30 kPa.

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Strain Guage

• The strain gauge is a device that can be affixed to

the surface of an object to detect the force applied

to the object.

• One form of the strain gauge is a metal wire of

very small diameter that is attached to the surface

of a device being monitored.

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• For a metal, the electrical resistance will increase

as the length of the metal increases or as the

cross sectional diameter decreases.

• When force is applied as indicated in Figure, the

overall length of the wire tends to increase while

the cross-sectional area decreases.

• The amount of increase in resistance is

proportional to the force that produced the change

in length and area. The output of the strain gauge

is a change in resistance that can be measured

by the input circuit of an amplifier.

• Strain gauges can be bonded to the surface of a

pressure capsule or to a force bar positioned by

the measuring element.

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Strain Gauge DP Cell Shown in figure is a

strain gauge that is

bonded to a force

beam inside the DP

capsule.

The change in the

process pressure

will cause a resistive

change in the strain

gauges, which is

then used to

produce a 4-20 mA

signal.

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