Vacuum Fundamentals

1. SCTN (MPH2C05 )
UNIT I: PHYSICAL SYNTHESIS
TECHNIQUES

2. Why is a Vacuum Needed?
To move a particle in a (straight) line over a large distance

3. Why is a Vacuum Needed?
Contamination
(usually water)
Clean surface
Atmosphere (High)Vacuum
To provide a clean surface

4. Vacuum Measurement – Units / Ranges:
Conversion Table

5. PUMP OPERATING RANGES
Rough Vacuum
High Vacuum
Ultra High
Vacuum
Venturi Pump
Rotary Vane Mechanical Pump
Rotary Piston Mechanical Pump
Sorption Pump
Dry Mechanical Pump
Blower/Booster Pump
High Vac. Pumps
Ultra-High Vac. Pumps
10-12 10-10 10-8 10-6 10-4 10-2 1 10+2
P (mbar)

6. Gauge Operating Ranges
10-12 10-10 10-8 10-6 10-4 10-2 1 10+2
P (mbar)
Rough Vacuum
High Vacuum
Ultra High
Vacuum
Bourdon Gauge
Thermocouple Gauge
Cold Cathode Gauge
Capacitance Manometer
Hot Fil. Ion Gauge
Residual Gas Analyzer
Pirani Gauge
Spinning Rotor Gauge
McLeod Gauge

7. Methods of Vacuum Measurement: Direct
• By a direct measurement of the force per unit area.
• limited for the pressures larger than ~10-3 Pa (~10-4 mbar) and requires an
electrically amplification.
• their reading is independent of the gas species.

8. Methods of Vacuum Measurement: Indirect
• By measurement of a quantity that is proportional to pressure, e.g., the
molecular density, the impingement rate of molecules, the thermal
conductivity, etc.
• The signal depends for a given pressure on the gas species so it may not
be possible to convert the signal into a correct pressure reading if the gas
composition of a mixture is not known exactly. their reading is independent
of the gas species.

9. U-Tube manometer or Torricelli tube (atmospheric to 0.2
mbar)
• Important and desirable properties of the manometric liquids are:
• High chemical stability
• Low viscosity (High viscosity causes transmission lags)
• Low capillary constant (for better capillary action)
• Low coefficient of thermal expansion (Thermal expansion causes changes in zero reading)
• Low volatility
• Low vapour pressure

10. Measuring pressure / U tube manometer I.
• Pipe flow
• Butterfly valve
• Average the pressure on pressure
taps around the perimeter
D
H
> g
p
B
p
J
B J
p p

1 2 ( )
ny ny m
p gH p g H h g h
  
    D  D
The manometers balance equation:
1 2 ( )
m ny
p p g h
 
   D
ny <<m
(For example> measuring air
with water)
Or
(Measuring water with mercury)
m
p g h

D  D
( )
m ny
p g h
 
D   D
Notice that ( )
p f H
D 

11. D
Measuring pressure / U tube manometer II.
Density of the measuring fluid mf (approximately)
The manometers balance equation:
Density of the measured fluid: ny (For example air)
  3
1000
víz
kg
m
  3
840
alkohol
kg
m
  3
13600
higany
kg
m
plevegő = pair -atmospheric pressure [Pa] ~105Pa
R - specific gas constant for air 287[J/kg/K]
T - atmospheric temperature [K] ~293K=20°C
( )
m ny
p g h
 
D   D
3
19
1
m
kg
,
RT
p
levegő
levegő
levegő 


mercury
water
alcohol

12. Measuring pressure / U tube manometer III.
Example: the reading:
The exactness ~1mm: The absolute error:
How to write the correct value with the
absolute error(!)
The relative error:
10 1
h mm mm
D  
10
h mm
D 
1
h mm

D  
Disadvantages:
• Reading error (take every measurement twice)
• Exactness~1mm
• With a small pressure difference, the relative error
is large
Advantages:
• Reliable
• Does not require servicing
%
,
mm
mm
h
h
10
1
0
10
1



D
D


13. Bourdon Tube Gauge (from several atmospheres down to 0.1 mbar)
Advantages:
•These pressure gauges give accurate
results
•They costs low.
•They are simple in construction.
•They can be modified to give an better
electrical outputs.
•They are safe even for high pressure
measurement.
•Accuracy is high especially at high
pressure.
Disadvantages:
•They respond slowly to changes in pressure
•They are subjected to hysteresis
•They are sensitive to shocks and vibrations
•Amplification is a must as the displacement
of the free end of the bourdon tube is low
•It cannot be used for precision
measurement

14. McLeod Gauge (~13 mbar to 10-6 mbar)
• Uses the same principle as manometer.
• Because of pressure dividing technique,
its range can be extended up to 10-4
mbar.
• It works on the principle of Boyle’s Law.
• It consists U tube with a glass bulb B of
known volume in the left arm.
• The right arm is branched into a capillary
tube, to monitor the minute changes in
pressure.
• The capillary aa’ is marked with a zero
reference point. It culminates back into
the right arm.
• Both Seat and reservoir is equivalent to a
piston.
Zero ref. point

15. p1
• Initially, the apparatus is filled with mercury up
to the indicated level.
• Let the vacuum pressure to be measured be
p1. It is applied on the right arm as shown in
the figure.
• In this situation, the pressure at any point in
the system is p1.
• With the application of piston load, the
mercury level in the apparatus rises.
• When the mercury crosses the junction, a
known volume of gas is trapped inside bulb
and tube.
• Let this volume of the gas be V1 as shown in
the figure.
• Therefore, initial condition is p1 and V1
p1

16. • With the further application of piston load, the
mercury rises to fill up both the arms.
• The load is applied until the mercury level in the
capillary tube reaches the zero reference point.
• The mercury levels in the arms are adjusted to
suit to
• Applied vacuum in right arm
• Compressed gas in left arm
p1
• In this condition, the volume of the gas in left arm
is read directly from the available scale.
• That is, the difference in the mercury levels in
capillary and left arm represents volume and
pressure of gas in left arm.
• Let a be the cross sectional area of the tube, we
have final pressure pf = p1+h and volume Vf = ah
p1
h

17. • Hence, applying the Boyle’s Law to the left
arm, we have
p1
h piVi  pf Vf
p1V1  p1  hah
pV  p ah  ah2
1 1 1
ah2
p1 
V1  ah
ah2
p1 
V1
The advantages are
• The gauge reading is independent of gas.
• It serves as a reference standard to calibrate other low pressure
gauges.
• There is no need of any zero error corrections.
The disadvantages are
• The gas should obey the Boyle’s law.
• It does not give a continuous output.

18. Pirani gauge – A Thermal conductivity Gauge (~133 mbar to 10-4 mbar)
• The Pirani Gauge is a type of Thermal Conductivity Gauges.
• Invented in 1906 by Marcello Pirani.
• A conducting wire gets heated when electric current flows through it. The rate at which
heat is dissipated from this wire depends on the conductivity of the surrounding media.
• If the density of the surrounding media is low, its conductivity also will be low causing the
wire to become hotter for a given current flow, and vice versa.
• A better alternative to the Pirani gauge is the thermocouple gauge, which works on the
same principle of detecting thermal conductivity of the gas by a change in temperature.
In the thermocouple gauge, the temperature is sensed by a thermocouple rather than by
the change in resistance of the heated wire.
1. A pirani gauge chamber which encloses a
platinum filament.
2. A compensating cell to minimize variation
caused due to ambient temperature changes.
3. The pirani gauge chamber and the
compensating cell is housed on a wheat
stone bridge circuit.

19. Pirani gauge – Construction
• A filament coil E of platinum wire ( 0.1mm diameter wire, 56cm in length, wound in an open
helix diameter 8mm, resistance at 20 C of 24.6 ohms) welded to F (0.7mm diameter
stainless steel wire).
• Wire D is supported by a cement (Ceramabond 569) on a short length of glass tube.
• The 0.7mm SS wires is cemented into the capillary tubes using Araldite standard Epoxy.

20. • Three modes of operation
1. Constant current : the Pirani-Hall Gauge and
involves feeding the wire filament with a constant
current and measuring the voltage across it,
which increases with falling pressure as the
temperature of the wire increases.
2. Constant resistance: requires the incorporation
of the filament in a micro-calorimetric bridge
such that the current is varied automatically to
keep the bridge balanced and the product of the
voltage and filament current are used to compute
the power dissipation which is approximately
proportional to the pressure over the usable
range. This measurement mode is capable of the
best precision, but involves more complicated
electronics and some computation to exploit it.
3. Constant voltage : is the best for general
purpose use and requires only very simple
electrical circuitry as the diagram shows. R1 & R2 are 1 watt 100 ohm resistors which
form the reference arm of the bridge. Rs & R4 complete the bridge and R4 is adjusted to
give 0.66V out of balance voltage when the gauge is at 1 atmosphere pressure. This
represents the lowest wire temperature. As the pressure is reduced, the filament
temperature rises to about 500 degrees C at 0.001 mB, and the out of balance voltage
indicated by the meter falls to about 0.07V.

21. Operation of Pirani gauge
1. A constant current is passed through the filament in
the pirani gauge chamber. Due to this current, the
filament gets heated and assumes a resistance which
is measured using the bridge.
2. Now the pressure to be measured (applied pressure) is
connected to the pirani gauge chamber. Due to the
applied pressure the density of the surrounding of the
pirani gauge filament changes. Due to this change in
density of the surrounding of the filament its
conductivity changes causing the temperature of the
filament to change.
3. When the temperature of the filament changes, the
resistance of the filament also changes.
4. Now the change in resistance of the filament is
determined using the bridge.
5. This change in resistance of the pirani gauge filament
becomes a measure of the applied pressure when
calibrated.
Note: [higher pressure → higher density → higher conductivity → reduced
filament temperature → less resistance of filament] and vice versa.

22. Advantages of Pirani gauge
1. They are rugged and inexpensive
2. Give accurate results
3. Good response to pressure changes.
4. Relation between pressure and resistance is linear for the
range of use.
5. Readings can be taken from a distance.
Limitations of Pirani gauge
1. Pirani gauge must be checked frequently.
2. Pirani gauge must be calibrated from different gases.
3. Electric power is a must for its operation.