Pressure Measurement
1- Introduction:-
Pressure:- Perpendicular Force per unit area.
F
2- Definition : - P =
A
Where :-
P = Pressure ( N/m2).
F = Force ( N ).
A = Area (m2).
3- Solid, Liquid & Gas Pressure Behavior:-
Solid:- P=0
i) Weight:-
W
P=0 W P=0
P=
A
P
ii) Weight & Applied Force:-
P=0 F
W W+A
P=0 P=0
P=
A
P
Pressure of A liquid column.
P = δgh = N/m2 P= 0
Where:-
δ = Density of the liquid. ( Kg/m3 ).
g = Gravitational acceleration.
( m/sec2 ) = 9.81 m/sec2
h = Height of liquid. ( meters ).
PT
Liquid:-
P3=δgh3
(i) Weight.
P2=δgh2
P1 > P 2 > P 3
P1=δgh1
P3 = 0
(i) Weight & Applied Force:- P
This is according to the PT3=P+P3
PASCAL’s law.
PT2=P+P2
PT1=P+P1
This is according to the Pascal’s Law.
Pascal’s Law:- An external pressure exerted on a fluid
is transmitted uniformly through out the volume of the fluid.
Example:- Hydraulic Pressure.
P1 = P2
P1 = F1/A1
P2 = F2/A2
F1 = P1/A1
F2 = P2A2
F1 = P1 A1
F2 = P1 A1
=> P1 = P2
A2 > A1
=> F2 > F1
So, by Appling small force, we get large force for
pressing purpose. (Hydraulic Pressure).
Gases:- The internal pressure of a gas
is the same in all directions.
Pe
Applied external pressure:-
PT = Pi + Pe
Pt
Ideal gas equation:-
P1 V1 P2 V2
=
T1 T2
4- Pressure Units:-
N/m², Pa, KPa, MPa, Psi, Kg/cm2, atm, Torr,
In H2O, In Hg, mmH2O, mmHg, bar, mbar.
5- Units Conversion :-
6- Pressure Scales :-
(i) Gauge Pressure Scale.
(ii) Absolute Pressure Scale.
Pab = Pg + Patm
Where:-
Pab = Absolute pressure scale.
Pg = Gauge Pressure scale.
Patm = Atmospheric pressure.
7- Pressure Measuring Devices.
(i) Manometers:-
→ Simple & accurate.
→ Used as standard instrument.
Working principle:-
Change in pressure causes a column of liquid to rise or
fall in the tube.
A- Well Type Manometer.
B- Inclined Type Manometer.
C- U-Tube Type Manometer.
A - Well-Type Manometer.
→ Reservoir Marked transparent tube.
→ Open-tube manometer exposes the liquid to the
atmospheric pressure & indicates gauge pressure.
→ Closed-tube manometer indicates absolute pressure.
B - Inclined Manometer:-
→ Advantage:-
Smaller pressure change will cause greater movement
of liquid in the column.
C - U-Tube Manometer:-
→ Bent u-tube is the reservoir
→ Specially used for differential pressure.
(ii) Bourdon Tube Pressure Sensors:-
A - C-Type Pressure Gauge.
B - Spiral-Type Pressure Gauge.
C - Helical-Type Pressure Gauge.
A- C-Type Pressure Gauge:-
→ Bourdon tube parts ------- etc.
→ Pressure applied at one open end.
→ Other end is closed & free to move.
→ Works on Pascal’s Law.
→ Movement effecting factors:-
Length of C Radius Cross-sectional area Design of
cross-sectional Wall thickness Material.
B- Spiral-Type Pressure Gauge.
→ A series of C-bourdon tubes jointed end to end.
→ Greater movement & requires no amplification.
→ Increases sensitivity & accuracy.
C- Helical-Type Pressure Gauge.
→ More better than spiral type for above parameters.
(iii) Diaphragm Pressure Gauges:-
A- Single Diaphragm Pressure Gauge.
B- Double Diaphragm Pressure Gauge
OR
Capsule Pressure Gauge.
C- Multi Capsule Pressure Gauge.
A- Single Diaphragm Pressure Gauge.
→
(iv)- Bellow Element Pressure Gauge:-
(v)- Strain Gauge (Pressure Sensor):-
(vi)- Piezoelectric Crystal Pressure Gauge (Sensor):-
Strain Gauge Pressure Sensor.
Piezoelectric Crystal Pressure Sensor.
Temperature Measurement
1- Definition:-
2- Temperature Effects On Other Measurements:-
3- Temperature Units:-
4- Units Conversion:-
A:- Mechanical Temp. Measuring Devices.
(1) Bimetallic Thermometer:-
→ A strip of two metals.
→ Different thermal expansion coefficients.
→ One end fixed.
→ Other end free & attached with pointer.
(ii) Liquid-In-Glass Thermometer:-
→ Small bore in glass tube.
→ Thin walled glass bulb.
→ Mercury or Alcohol.
→ Ranges – 120 To 320 0C.
→ N2 Gas filled at 2 to 20 bars to increase range up to 6000C.
→ Not used as field instrument.
(iii) Filled System Thermometer:-
→ Consists on thermal bulb, capillary tube & bourdon tube.
→ Filled with liquid, gas or vapour.
→ Error compensation.
→ Ranges 200 to 570 0C.
B- Electrical Temp. Measuring Devices.
(i) Resistance Temp. Detectors (R.T.D).
→ Temp. change affects a metal’s resistance.
→ Different metal have different temp. resistance coefficients.
→ Pt , Cu , Ni.
→ Electrical, Mechanical, Chemical, properties.
→ Construction & working.
→ Error compensation.
→Advantage & disadvantages.
(ii) Thermocouples.
→Working principle.
→Construction.
→Reference tables.
a:- Cold Junction Compensation.
(i) Ref. junction at 0 oC.
(ii) Ref. junction at constant temp.
(iii) R.T.D at ref. junction.
(iv) Mechanical compensation.
b:- Cares About Thermocouple.
→ Shielded cables
→ Tension free leads.
→ Protection from contamination.
(iii):- Thermister.
→ Similar construction to R.T.D’s
→ Semi-conductor element (sensor).
(iv):- Pyrometer.
→ Direct contact with process is difficult.
→ Measures the amount of radiant energy which an object
emits
(a):- Optical / Brightness Pyrometer.
→ Indicates temp. by measuring emissivity.
i.e., ability to transmit radiant energy depends on object’s
temperature.
→ Object is viewed through a lens & filter.
→ The object appears as a back ground behind the lamp
filament.
→ Filament brightness is adjusted by changing lamp current.
→ On filament disappearing, the lamp current is proportional
to radiant energy.
(b):- Ratio Pyrometer.
→ Also based on object’s emissivity to measure temp.
→ Blue wave length radiant energy is passed through a blue
filter & measured by ratio pyrometer.
→ Then, red wave length radiant energy is passed through red
filter.
→ Compared the blue & red wave length radiant energies.
→ The ratio of the two radiant energies produces the measure
of the object’s temperature.
Flow Measurement
1- Definition:- Mass or volume / unit time from a particular cross
section.
2- Units:- Kg/sec, Ib/sec, Liters/sec, Gallons/min.
3-Viscosity:- Resistance to flow resulting from internal of friction
particles of a fluid.
4-Types Of Flow:-
(i) Laminar Flow:-
→ Higher viscosity & lower viscosity.
→ Particles travel in parallel layered paths.
→ Move more slowly near the wall of the pipe.
(ii) Turbulent Flow:-
→ Lower viscosity & higher velocity.
→ Particles exhibit high-frequency fluctuations.
→ More accurate flow measurement.
) Transitional Flow:-
→ Exhibits the properties of both laminar & turbulent flow.
→ Tends to oscillate between both states.
5- Reynolds Number:-
→ Factor used to predict which flow behavior a particular
system will exhibit.
→ R. No. < 2000 => Laminar flow.
→ R. No. > 4000 => Turbulent flow.
→ R. No. Between 2000 - 4000 => Transitional flow.
6- Effect Of Temp. On Flow:-
→ As temp. increases density decreases.
→ Flow rate decreases.
7- Classification Of Flow Measurement.
A- Differential Pressure Flow Measurement.
Flow rate α √ ∆P
α √ (P1- P2)
a- Closed Channel Flow.
(i) Restrictional flow measurement.
1- Orifice plate.
2- Integral orifice plate.
3- Venturi tube.
4- Flow nozzle.
b- Non-Restrictional Flow Measurement.
1-Pitot t0ube.
B-Non-Differential Pressure Flow Measurement.
OR
Volumetric Flow Measurement.
(i) Vertex shedding flow-meter.
(ii) Magnetic flow-meter.
(iii) Ultrasonic flow-meter.
(iv) Positive displacement flow-meter.
a- Rotary van flow-meter.
b- Rotating disk flow-meter.
c- Lobed impeller flow-meter.
(v)- Rota-Meter.
(vi)-Turbine Flow-Meter.
(vii)-Mass Flow-Meter.
(viii)-Coriolis Mass Flow-Meter.
LEVEL MEASUREMENT.
(A) - Direct Level Measurement.
(B) - Indirect Level Measurement.
(A):- Direct Level Measurement.
(a) - Sight and Gauge Glasses.
(b) - Dipsticks and weighted lines (strings).
(c) - Float-type devices.
(d) - Displacement devices (Torque Tube).
(B):- Indirect level measurement.
(a) - Hydrostatic head pressure level measurement.
(b) - Electrical level measurement.
(c) - Solid level measurement.
(d) - Non-invasive level measurement.
(a):- Hydrostatic head pressure level measurement.
(1) - Open tank.
(i) - Differential pressure measurement.
P X 27.7
H=
S.G
H = Height in inches.
P = H. Head pressure in PSI.
S.G = Specific Gravity.
(ii) - Air bubble System.
(2) - Pressure closed tanks.
(i) - Dry leg method.
(ii) - Wet leg method.
(iii) - Hydrostatic tank gauge (H.T.G).
(b):- Electrical Level Measurement.
1 - Capacitance Method.
2 - Resistance Method.
(c):- Solids Level Measurement.
1- Rotating wheel.
2- Vibrating paddle.
(d):- Non-Invasive level measurement.
1- Ultrasonic method.
2- Sonic method.
3- Fiber optic method.
