2 pressure measurement 28032011

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INDUSTRIAL INSTRUMENTATIONS

Pressure & Level Measurement



PRESSURE MEASUREMENT (RECAP««)

Introduction (definitions)

Classification and working principles of pressure

gauges

y Manometer (U-tube, well type, inclined type and

etc.)

y Elastic elements (Bourdon, diaphragm, bellow, strain

gauges and etc.)

y

Problem discussion



PRESSURE & LEVEL MEASUREMENT

Pressure measurement

y High Pressure Measurement,

y Low Pressure Measurement,

y Effect of Connecting tubing,

y Calibration & Testing

y Summary

Level measurement

y Introduction

y

Displacery Float

y Pressure gages

y Balance method

y Summary

28/03/2011



PRESSURE MEASUREMENT

HIGH PRESSURE MEASUREMENT

For pressure > 1000 atm

The pressure is being measured by the change in

electrical resistance of a manganin (alloy of Cu, Ni, Mn)

or gold-chrome wire, with hydrostatic pressure, due to

bulk compression effect.

The coil is enclosed in a flexible bellows filled with

kerosene, for transmitting the pressure to the coil.

A

B

Insulators

Wire

p




HIGH PRESSURE MEASUREMENT

Change in resistance of the wire between A and

B is measured by Wheatstone bridge,

y For manganin sensitivity is 2.5 x 10-11 ;/;-Pa,y For gold-chrome sensitivity is 9.85 x 10-12 ;/;-Pa

y Though the gold chrome is less sensitive but it is

preferred over manganin as it is less sensitive to the

temperature.




LOW PRESSURE MEASUREMENT

Vacuum pressure is measured as Torr and

micron

1 Torr = 1 mm of Hg

1 micron = 10-3 Torr Manometers and elastic element gauges can be

used upto 0.1 Torr.

Below 0.1 Torr the gauges are used as follows:

y McLeod gauge,y Thermal conductivity gauge or Pirani gauge,

y Ionisation gauge,

y Knudsen gauge




LOW PRESSURE MEASUREMENT ² MCLEOD GAUGE

Modified mercury manometer and in the range of

0.01 ² 0.001 microns,

Principle: Compressing a known volume of low-

pressure to a high pressure and measuring theresulting volume change.

p unknown pressure

A Area of cross-section

Vc = Ay Volume of gas in capillary

pc pressure of gas in thecapillary c after

compression

VF Volume of capillary and

bulb till F




LOW PRESSURE MEASUREMENT ² MCLEOD GAUGE

Using Boyle·s law:




LOW PRESSURE MEASUREMENT ²

THERMAL CONDUCTIVITY GAUGE OR PIRANI GAUGE

Comprised of a platinum filament inside a chamber,

Wire forms an arm of Wheatstone bridge,

The temperature of the wire, for a given magnitude of

current, depends on the rate of heat dissipation, which in

turn depends on the conductivity of the surroundingmedium and hence its pressure,

Thus with change in pressure of the medium, the

temperature and hence the resistance of the wire changes,

which is measure using Wheatstone bridge.




LOW PRESSURE MEASUREMENT ² IONISATION GAUGE

Used for measuring very low pressure (i.e., � 1 micron)

Gauge is comprised of triode vacuum tube.

Heated cathode emits electrons which are accelerate by the

positively charged grid. As the electrons move towards the

grid, they ionize the gas molecules through collisions.

The plate is maintained at a negative potential so that

positive ions collect there, producing a plate current i1.

The electrons and negative ions are collected by the grid,

producing grid current i2. It is found that the pressure of

the gas is given by

where K is the sensitivity of the gauge




LOW PRESSURE MEASUREMENT ² K NUDSEN GAUGE

Knudsen gauge gives an absolute

measurement of pressure p

(independent of gas composition).

It covers a rang of pressure from

10-8 to 10-2 Torr.

Gas pressure is calculated as

follows:

where p is gas pressure, F the

force and K is a constant




LOW PRESSURE MEASUREMENT ² K NUDSEN GAUGE

Gauge chamber is comprised of fixed plates F1 and F2

and heated to a maintained temperature at T.

Near these plates, there is a restrained movable vane

V, such that the gap between the vane and the fixedplate is less than the mean free path of the gas whose

pressure is to be measure.

The vane is at gas temperature T0.

The gas molecules rebound from plates F1 and F2 withgreater momentum than from V, thus imparting a net

force to V which may be measured by measuring the

angular displacement of mirror M.




DYNAMIC EFFECT OF TUBING

A connecting tube is used between the pressure

measuring instrument and the location where the

fluid pressure is measured.

Such tube can cause a lag under dynamicconditions.




DYNAMIC EFFECT OF TUBING

The lag is calculated as:

where X is time constant of the first order relation

representing the lag of the system

y In order to reduce X,

l, A and V should be small and d should be large




CALIBRATION & TESTING ² STATIC CALIBRATION

Dead weight tester.

y The pressure in the tester is built up till the weights areseen to float, when the fluid gauge pressure equals thedead weight divide by the piston area.

For moderate pressure measurement manometers areused.

For low pressure McLeod gauge are used.




CALIBRATION & TESTING ² DYNAMIC CALIBRATION

Special tester based on the f requency response or

transient type of tests are used:

A harmonic steady state test apparatus:

y

The shaker which may be of electrodynamic andmechanical type, is used to create sinusoidal variations in

pressure at desired frequencies.

y The reference transducer should have a flat frequency

response over the test frequencies and the performance of

the test transducer can be compared against the same.




CALIBRATION & TESTING ² DYNAMIC CALIBRATION

Transient tests: shock tube or steel balls (by

dropping on an elastic membrane of the pressure

transducer),

y

In shock tube the bursting of a thin diaphragm,subjected to gas pressure, creates a nearly step

pressure. The diaphragm is burst either by pressure

differential or an external probe.

Diaphragm

HP end LP end

Pressure transducers




SUMMARY

Pressure measurement device Pressure range

Manometer

Bourdon gaugeElastic diaphragm with LVDT or

capacitance or resistance strain

gauge transducer

Piezo-electric transducer

Hydrostatic compression gauge

McLeod gaugePirani gauge

Ionisation gauge

Knudsen gauge

10 to 106 Pa

103

to 5 x 108

Pa100 to 108 Pa

104 to 108 Pa

100 to 105 atm

10

-5

to 1 Torr10-4 to 1 Torr

10-12 to 10-3 Torr

10-8 to 10-2 Torr




PROBLEM

A McLeod gauge has a volume of bulb, capillaryand tube down to its opening equal to 90 cm3 anda capillary diameter of 1 mm. Calculate thepressure indicated by a reading of 3 cm.

A vacuum gauge is to use a differentialtransformer combination, which has a resolutionof 2.5 microns. The diaphragm is to beconstructed of steel (V = 7.9 x 10-6 kg/mm3, E =2.07 x 15 N/mm2, v = 0.3), with a diameter of 15cm. Calculate the diaphragm thickness so thatthe vacuum, which may be sensed and thefrequency limit of the instrument.

2 pressure measurement 28032011

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