Low pressure measurement- instrumentation

1. LOW-PRESSURE
MEASUREMENT
SUBMITTED BY:
PRODIP KUMAR SARKER
DEPARTMENT OF ME,RUET, BANGLADESH
1

2. Introduction
 measurement of low pressures which are not usually
accessible to the conventional gages.
 absolute pressures below 1 torr ( 1 mmHg, 133 Pa)
2

3. Measuring Vacuum Method
MEASUREMENT BELOW ATMOSPHERIC PRESSURE 10−3- 10−9
TORR.
MECHANICAL TYPE – MCLEOD GAUGE.
THERMAL TYPE – PIRANI GAUGE AND THERMOCOUPLE.
Knudsen Gauge
IONIZATION TYPE – HOT CATHODE AND COLD CATHODE
RADIATION VACUUM GAUGE – ALPHATRON , QUARTZ REFERENCE
3

4. THE MCLEOD GAUGE
 vacuum gauge with same principle as manometer.
 Range: 10-4 Torr.
 multiple compression technique.
 P1 = ah^2/(V1-ah)
where
P1 = Pressure of gas at initial condition
V1 = Volume of gas at initial Condition.
a = cross – sectional area of measuring
capillary tube
h = height of the compressed gas in the
measuring capillary tube
4

5.  Operation of McLeod Vacuum
gauge:
The pressure to be measured (P1) is applied to the top of the reference column.
The mercury level in the gauge is raised by operating the piston. When this is
the case(condition – 1), the applied pressure fills the bulb and the capillary.
Now again the piston is operated so that the mercury level in the gauge
increases.
When mercury level reaches the cutoff point, a known volume of gas (V1) is
trapped in bulb and measuring capillary tube. The mercury level is further
raised by operating the piston so the trapped are compressed. This is done until
the mercury level reaches the “Zero reference Point” marked on the reference
capillary (condition – 2). In this condition, the volume of the gas in the
measuring capillary tube is read directly by a scale besides it. That is, the
difference in height ‘H’ of the measuring capillary and the reference capillary
becomes a measure of the volume (V2) and pressure (P2) of the trapped gas
5

6. PIRANI THERMAL-CONDUCTIVITY
GAUGE
6
 fine wire of tungsten or platinum
 about 0.002 cm in diameter.
 temperature range (7-400)▫C
 heating current is between (10-100) mA.
 Range: 10−3
Torr – 1 Torr
Pirani gauge
Fig: schemetic of pirani gauge

7. Working principle
 When the pressure changes, there will be
a change in current. For this, the voltage V
has to be kept constant.
 The resistance R2 of the gauge is measured,
by keeping the gauge current constant.
 The null balance of the bridge circuit is
maintained by adjusting the voltage or current.
7

8. KNUDSEN GAUGE
 An absolute gauge in the range of 10−8
-10−3
torr
 Independent of gas composition
 More suitable for laboratories
 Depends on momentum transfer principle
 P= 4F
𝑇𝑔
𝑇−𝑇𝑔
Where
T= temp. Of heated plates
𝑇𝑔= temp. Of vens at the temp of gas
8

9. WORKING PRINCIPLE
Two vanes along with a mirror are mounted on a thin-filament suspension.
Near these vanes are plates. Heaters are installed so that temperature of
plates is higher than that of the surrounding gas. The molecules striking the
vanes from the hot plates have a higher velocity than those leaving the vanes
because of the difference in temperature.
Thus, there is a net momentum imparted to the vanes which may be
measured by observing the angular displacement of the mirror. Total
momentum exchange is a function of molecular density, which is related
to pressure and temperature of the gas.
9

10. THE IONIZATION GAUGE
 measure pressure down to about 2 torr
 p=
1
𝑠
𝑖 𝑝
𝑖 𝑔
Where
S = sensitivity of gauge
( a typical value for nitrogen is S= 20 torr−1
)
𝑖 𝑝 = plate current
𝑖 𝑔 = grid curent
10

11. WORKING PROCEDURE
An ionization gage is setup to have a heated cathod that emits electrons, a
grid that accelerates the electrons, and a plate that collects positive ions.
As the electrons move toward the grid, they produce ionization of the gas
molecule.
It is found that the pressure of the gas is proportional to the ratio of plate
current to grid current.
11

12. THE ALPHATRON
 The Alphatron is a radioactive ionization gage.
 A small radium source serves as an
alpha-particle emitter. These particles
ionize the gas inside the gage enclosure
 no of ions directly proportional to
 gas pressure
 the degree of ionization is determined
by measuring the voltage output
FIG: schemetic of alphatron
 Range: 10−3
- 103
torr
12

13. Summary: 13
Fig: summary of applicable range of pressure gauges

14. THANK YOU !!!
14