High vacuummeasurement

Process ControlProcess Control
andandandand
InstrumentatInstrumentation
Dr. Debasis Sarkar
D t t f Ch i l E i iDepartment of Chemical Engineering
Indian Institute of Technology Kharagpur

Pressure MeasurementPressure Measurement

HighHigh--Vacuum MeasurementVacuum Measurement
1. McLeod Gage
2 Ionization gage2. Ionization gage
3. Thermocouple gage
4. Pirani gage
Thermal conductivity gage
5. Knudsen gage

McLeod Gage:McLeod Gage:
To vacuum
Sealed capillary
Reference
capillary
Zero line
McLeod Gage

McLeod Gage:c eod Gage
Example:
A M L d h V 100 3
To vacuum, p
A McLeod gage has VB = 100 cm3
and a capillary diameter of 1 mm.
Calculate the pressure indicated
by the reading of 4 cm.by e ead g o c
What error would result if we use :
Reading: 4 cm
Instead of
2
B
ay
p
V

VB = 100 cm3
B
2
cyVay
p  
Mercury
i
B B
p
V ay V ay reservoir

Advantages of the McLeod Gage:
 It is independent of the gas composition It is independent of the gas composition
 Used as standard to calibrate other low pressure gages
 No need to apply any corrections to the McLeod Gage readingspp y y g g
Limitations of McLeod Gage/Precautions to be Taken:
 The gas must obey the Boyle's lawg y y
 Presence of condensable vapor causes pressure readings to be low, so moisture
traps should be provided
 The measuring tube should have small diameter, but capillary effect can produce
significant uncertainty
 It cannot give continuous readings thus steady-state condition must prevail for It cannot give continuous readings, thus steady-state condition must prevail for
useful measurement

Ionization Gage:
An electron passing through ap g g
potential difference will
acquire a kinetic energy that is
proportional to the potential
difference
=>
difference
If an electron strikes a gas
molecule, the electron may
knock out an electron from the
gas molecule leaving it
positively charged
Gas molecule
Ion
Number of positive ions
formed is dependent on
number of gas molecules per
- +
Ion
Electron
unit volume  pressure

Ionization Gage:
Working Principle:Working Principle:
Ionization gage measure vacuum by measuring the current produced by
ionized gas molecules. The gas molecules are ionized as a stream of electrons
collide with them.
An electron passing through a potential difference will acquire a kinetic energy
that is proportional to the potential difference If this energy is large and thethat is proportional to the potential difference. If this energy is large and the
electron collides with a gas molecule, the electron may knock out a secondary
electron from the gas molecule. Thus the gas molecule will be a positively
charged ion.
Number of positive ions (ion current) depends on electron current (no. of
electrons emitted by the cathode) and number of gas molecules. For a given
gas and a constant electron current, ion current become a direct measure ofg ,
number of gas molecules per unit volume that is pressure.

Ionization Gage:

Ionization Gage:
Three types of Ionization gages:Three types of Ionization gages:
Hot cathode ionization gage: consists of a heated filament (cathode), a grid
(upto 10-10 torr) with negative potential (ion collector), and an
anode (electron collector).
Cold cathode ionization gage: No heated filament to produce electrons.
(upto 10-5 torr) Uses a high electric field (~4kV) between(upto 10 torr) Uses a high electric field ( 4kV) between
cathode and anode to draw electrons out. High
(Also known as magnetic field (~1500 gauss) causes the
Penning gage) electrons to move towards the anode.
E iExpensive gage.
Alphatron vacuum gage: Relatively less expensive.
Uses suitable alpha-emitterp

Cold Cathode Ionization Gage (Penning gage):

Th l C d ti it GThermal Conductivity Gage: Thermocouple Gage & Pirani gage
Working Principle:
Thermal conductivity of a gas is independent of pressure at normal pressure. But
at low pressure, thermal conductivity of a gas depends on pressure (decreases
with pressure)
Heat loss from a heated conducting wire (or hot thin metal surface) is dependent
on thermal conductivity of the surrounding gas. Thus, equilibrium temperature of a
heated conducting wire (or hot thin metal surface) is a function of pressureheated conducting wire (or hot thin metal surface) is a function of pressure.
Range: 10-4 to 1 Torr
C lib ti d d b i d
Filament
Calibration depends on gas being used
To
vacuum
Cold surface
(glass tube)

Th l GThermocouple Gage:
For a given gas and heating current, the temperature assumed by the hot surface
depends on pressure. Temperature of the surface is measured by a thermocouple.
Cold surface
(glass tube) Hot surface
(thin metal strip)
Range: 10-4
to 1 Torr
(thin metal strip)
Adjust heating
current
(to heat
the hot
Measureme
nt may be
affected by
ambient
High
i d
mV
the hot
surface)
ambient
temperature
Surface of
impedence
microvolt
meter
Thermocouple
low
emissivity is
used to
reduce
Pressure
reduce
effect of
radiation

Th l GThermocouple Gage:
For a given gas and heating current, the temperature assumed by the hot surface
depends on pressure. Temperature of the surface is measured by a thermocouple.

Pirani Gage:
Uses same principle as Thermocouple gage
Function of heating and measuring temperature
bi d i i l l tare combined in a single element
Generally more accurate and expensive than Thermocouple gage
Range: 10-4 to 1 Torr
Gage has to be calibrated for individual gas
Temperature compensation is provided

Pi i GPirani Gage:
Pirani gage also uses the same principle as Thermocouple gage. But unlike
thermocouple gages, they don’t measure the wire temperature directly. Insteadp g g , y p y
they use the fact that the resistance of a conducting wire changes with the wire’s
temperature.
The heated wire (measuring element: Pirani gage) is connected as one leg of aThe heated wire (measuring element: Pirani gage) is connected as one leg of a
Wheatstone bridge. An exactly identical element is connected as another leg which
is exposed to ambient temperature and works as a compensator.
If the circuit is initially balanced, then application of
pressure to the measuring element will cause
unbalance in the circuit.
This is because the sensor wire changes its
resistance with change in pressure that changes
the wire’s temperature.

Pirani Gage:
Compensating element
Two modes of operation:
1. Output current is
function of pressure
Sealed
and
evacuated
2. Keep the
temperature of
resistance constant
To
by adjusting the
current passing
through the element.
Then, change in
vacuum
Measuring element
current is function of
the pressure.

Pirani Gage:

Pirani Gage: • The Pirani is a dedicated
low vacuum gauge device
• The resistance of the hot
wire changes with the rate
of heat loss (conduction)of heat loss (conduction)
to the gas
• The Wheatstone bridgeThe Wheatstone bridge
then measures the change
in resistance of the hot
wire
• Pirani’s are rugged and
generally reliable and
rarely need attentionrarely need attention

Knudsen Gage:
It is an absolute gage in the range of 10-8 to 10-3 torr
Independent of gas composition
More suitable for laboratories
Depends on momentum transfer principle

Knudsen Gage:
The gap between the fixed
plates and the movable
To vacuumFixed heated plates (TF)
vanes must be less than
mean free path of the gas
molecule
Gas molecules rebound
To vacuumFixed heated plates (TF)
Range:
10 8 to 10 3 torr Gas molecules rebound
from the heated plates with
greater momentum than
from the cooler movable
vane – this gives a net
f th i
M
10-8 to 10-3 torr
force on the spring
restricted movable vane.
The force can be measured
by the deflection of they
movable vane
Movable vane (TV)
( / ) 1F V
KF
p
T T

Scale
Temperature of fixed plates: TF
Temperature of movable vane: TV
TF > TVLight source

SolidSolid--State Pressure SensorState Pressure Sensor
Solid-state pressure sensor is based on integrated circuit technology
Finds extensive application in the range of 0 to 100 kPa (0 to 14.7 psi)
The basic sensing element is small wafer of silicon that acts as a diaphragm. The
deflection of the wafer is sensed by a strain gage grown directly on the silicon
wafer. Even signal condition circuitry (for temperature compensation and linearg y ( p p
pressure-voltage output) is also grown directly on the wafer.
To pressure source, P1
Connections to
wafer
Can measure:
 G
External
connections
Housing
 Gage pressure
 Absolute pressure
 Differential pressure
P2 = atmospheric

SolidSolid--State Pressure SensorState Pressure Sensor
Important Characteristics:
 Hi h iti it 10 100 V/kP High sensitivity: 10 – 100 mV/kPa
 High accuracy and reliability
 Fast response: response time on the order of 10 msp p
 Linear voltage versus pressure within the specified operating range
 Temperature compensation for broad range of temperature: 0°C to 60°C
 Easy to use: often with only three connections: DC power supply (~ 5V), ground,
and the sensor output voltage

Range of Vacuum SensorRange of Vacuum Sensor

Selection of Pressure SensorSelection of Pressure Sensor
Strongly influenced by intended application

High vacuummeasurement

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