1. VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS
Module 2:
Total Pressure Measurement in Vacuum
Second Edition
Section2
Direct Measurement of Pressure

2. Direct Measurements of Pressure
Introduction
Direct measurements are those which measure the real force, F, exerted by a gas at a pressure p
If the area over which this force is exerted is known, then this is a measure of pressure
One way of doing this is to use a liquid – traditionally mercury – in a simple U-tube, to measure the difference in pressures between two volumes of gas, each connected to one arm of the U-tube
If one of the volumes is sealed and evacuated, then the pressure in that volume will simply be the vapour pressure of mercury
The pressure as measured may conventionally be described in terms of mm of mercury or Torr (1 Torr = 133 Pascal)

3. Direct Measurements of Pressure
Manometers Using Liquids
The classical gauge is a simple liquid filled U-tube manometer, which in its most sophisticated forms can form a primary pressure standard
Conventionally, mercury is used as the liquid
The accuracy of such gauges is determined by the accuracy of measuring the level of the mercury in each tube
Two laboratory forms of this gauge will be illustrated
The first, shown in the next slide, is the truncated manometer

4. Direct Measurements of Pressure
Manometers Using Liquids
This is a more convenient and compact adaptation of the classical U-tube manometer
The accuracy is about 20 Pa

5. Direct Measurements of Pressure
Manometers Using Liquids
The second laboratory form of the U-tube manometer, shown in the next slide, is the inclined manometer

6. Direct Measurements of Pressure
Manometers Using Liquids
In this form of the gauge, accuracy is improved at the expense of a reduced measurement range
The accuracy is about 2 to 3 Pa

7. Direct Measurements of Pressure
The McLeod Gauge
To measure lower pressures a McLeod Gauge (illustrated in the next few slides) may be used
In this gauge, a known volume of gas at the unknown pressure P is trapped and then compressed by a known factor (determined by the construction of the gauge)
The compression is achieved using a mercury piston
If the gas obeys the ideal gas laws, then Boyle-Mariotte’s Law can be applied and the pressure p deduced
It is important to note that if condensable vapours are present in the gas or if gases adsorb on the walls of the compression capillary, significant errors are introduced
The McLeod gauge can measure pressures down to 10-2 Pascal with an accuracy of about 5%
For ideal gases, the McLeod gauge is an absolute gauge

8. Direct Measurements of Pressure
The McLeod Gauge
At the beginning, the pressure in the bulb is the same as that in the system, to which the gauge is attached through the open tube at the upper left of the figure

9. Direct Measurements of Pressure
The McLeod Gauge
The mercury (red) is raised until the side arm is cut off
The volume V of the gas in the bulb is accurately known and the pressure is P
If the gas behaves as an ideal gas, then raising the mercury level further will compress the gas and the relationship
PV = const
will hold

10. Direct Measurements of Pressure The McLeod Gauge
The level of the mercury is now raised so that the meniscus in the reference capillary is at the fixed reference point O
The pressure is then given by
Where  is the density of mercury
a is the cross section of the capillary
g is gravitational acceleration
V is the initial volume of the gas
h is as shown
The pressure scale is thus given by a square law

11. Direct Measurements of Pressure The McLeod Gauge
If the gas is then compressed still further the pressure P is given by
Where  is the density of mercury
g is gravitational acceleration
h is as shown in the diagram
The ratio is the compression ratio which is fixed by the construction of the instrument
This results in a linear scale of pressure

12. Direct Measurements of Pressure
The McLeod Gauge
The next slide is an animated diagram of the operation of the McLeod gauge
It will pause for a few seconds between each stage
The animation begins with Pressure Equalisation
The first pause demonstrates Gas Trapping in the compression capillary
The second pause is with the gauge in the Square Law pressure measurement regime
The animation ends with the gauge in the Linear pressure measurement regime
The animation will start automatically on advancing to the next slide
To bypass the animation
Click Here

13. Direct Measurements of Pressure
The McLeod Gauge

14. Direct Measurements of Pressure
Mechanical Gauges
The next two gauges use the deflection of a mechanical element under the force exerted by a difference in pressure
Such gauges need to be calibrated, but are independent of gas species being measured
The Bourdon tube gauge (shown on the next slide) uses a thin bent tube (sometimes a spiral) the inside of which connected to the pressure to be measured.
Atmospheric pressure on the outside forces the tube to bend or uncurl as the pressure inside is varied

15. Direct Measurements of Pressure
Mechanical Gauges
Here, the lever and pointer system is directly attached to the bent tube (shown in red)

16. Direct Measurements of Pressure
Mechanical Gauges
In the diaphragm gauge, a thin membrane is deflected in proportion to the pressure difference across it
One side, the reference side, is usually at atmospheric pressure
In a variation, the reference side is held at a very low pressure.

17. Direct Measurements of Pressure
Mechanical Gauges
The rack and pinion is directly attached to the mechanical diaphragm which deflects under the difference in pressure P1-P2
The diaphragm usually takes the form of a corrugated disc

18. Direct Measurements of Pressure
Mechanical Gauges
Mechanical gauges such as Bourdon tube gauges and diaphragm gauges can measure pressures from above atmosphere to about 100 Pa with good repeatability
Accuracies are limited by hysteresis and variations in atmospheric pressure
Their major advantage is that they are cheap, self contained and reliable

19. Direct Measurements of Pressure
Capacitance Diaphragm Gauges
The Capacitance manometer or Capacitance Diaphragm Gauge is a variant on the simple diaphragm gauge
Here the diaphragm forms one plate of a capacitor, the other plate being fixed
One side of the diaphragm is at a reference pressure, either atmosphere or a very low pressure
As the diaphragm deflects under a pressure difference, the capacitance varies. This can be measured using ac electrical modulation techniques
The reading is, in most cases, independent of the gas species being measured

20. Direct Measurements of Pressure
Capacitance Diaphragm Gauges
The gauge needs to be calibrated, but the reading can be very reproducible
The main source of error is temperature variation in the gauge, so high accuracy gauges operate at a modest elevated temperature (~40oC)
For highest accuracy work, thermal transpiration effects need to be considered
High quality gauges can measure down to better than Pa with accuracies of 0.2%

21. Direct Measurements of Pressure
Capacitance Diaphragm Gauges
Here we can see a schematic of a capacitance diaphragm gauge showing how the diaphragm moves towards the fixed electrode when P2<P1

22. Direct Measurements of Pressure
Capacitance Diaphragm Gauges
In a recent development of diaphragm gauges, piezoresistive sensors are attached to the diaphragm
As the diaphragm deflects, the sensors are stretched or compressed and their resistance changes
An electrical bridge circuit may be used to determine these changes in resistance
Since these sensors can be made as an integrated unit by micromachining techniques they can be manufactured relatively cheaply
An example is shown in the next slide

23. Direct Measurements of Pressure
Capacitance Diaphragm Gauges
Grown Piezoresistive Sensor
Bridge Measurement Schematic

24. VISUAL AIDS for instruction in VACUUM TECHNOLOGY AND APPLICATIONS
Module 2:
Total Pressure Measurement in Vacuum
Second Edition
End of Section2
Direct Measurement of Pressure