Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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260

Chapter

3.1:

The

Measurement

of Low

Pressures

Fig.

10"*'

10"" 107 10? 10"? 10"^ 10"» 107 10? Torr

CAPACITANCE DIAPHRAGM

I ^ I .

MANOMETElt

-V

BOURDON TUBE

RADIOACTIVE GAUGE

CONVECTION GAUGE

THERMISTOR

PIRANI

THERMOCOUPLE

QUARTZ

COLO CATHODE (PENNING)

SCHULZ - PHELPS

DOUBU INVERTED MAGNETRON

[COLD CATHODE MAGNETRON ft INVERTED MAGNETRON

REDHEAD MAGNETRON

HOT CATHODE - MAGNETIC FIELD

MASS SPECTROMETER

H-

760 Torr

(1 Atn)

10-»3 10"" 10"* 10"^ 10

Pressure Ranges for Various Gauges

3.1.2

DIRECT READING GAUGES

Direct reading gauges can be divided into those that use a liquid wall and those

that use a solid wall. In either case the gauge measures the pressure by reading the

force exerted on a surface by thermally agitated molecules and atoms.

3.1.2.1

Liquid-Wall Gauges

There are essentially two common gauges that use a liquid wall: the liquid col-

unm manometer and the McLeod gauge.

3.1.2 Direct Reading Gauges

261

Fig.

DIRECT READING MERCURY MANOMETERS

760MM

Hg = 760

TDRR

AT ATMOSPHERIC PRESSURE

TD VACUUM PUMP •

•

IMM

Hg = 1

TDRR

AT VACUUM DF IMM Hg

AT ATMOSPHERIC

PRESSURE

AT REDUCED

PRESSURE

EVACUATED-

WHEN

THE

REDUCED PRESSURE

THE

SAME

THE PRESSURE

THE

SEALED TUBE

THEN

THE

HIEGHT DIFFERENCE

h = 0

U-TUBE MANOMETER

Mercury Manometers

Perhaps the simplest vacuum gauge is the liquid column manometer. This gauge

(see Figure 3) consists

a straight or U-shaped glass tube exhausted and sealed

at one end and filled partly with mercury

or a

low-vapor-pressure liquid such

diffusion pump oil.

As the

space above

the

mercury pool

evacuated

the

straight tube, the length

the column

mercury decreases.

the case

the

U-tube,

the free end

evacuated the two columns tend

assume

the

same

height. The difference in height is a measure of

the

pressure at the open end. If the

liquid is mercury, the pressure is directly measured in torr (mm

Hg).

is lim-

ited to pressures no lower than ~1 torr (133 Pa).

the liquid is

low-density oil,

the U-tube is capable

measuring

pressure as low as —0.1 torr. Although this

gauge is an absolute direct reading instrument, the use of toxic mercury or creep-

ing low-density oils that will

time contaminate the vacuum system, make

it a

poor choice.

262

Chapter

3.1:

The Measurement of Low Pressures

Rg.4.

CDMPRESSIDN

RATIO « 10*7

TD VACUUM

PUMP

Hg INLET

lOMM

Hg ON

GAUGE

10"^

MM ACTUAL

10-6

X 10*7 = lOMM

TD VACUUM

PUMP

Hg INLET

McLeod Gauge

Due to the pressure limitation of the U-tube manometer the McLeod gauge [2]

was developed. Using Boyle's law, this gauge greatly increased the range of vac-

uum measurement (see Figure 4). It uses a mercury manometer in which a vol-

ume of gas is compressed before measurement. The sensitivity of the McLeod

gauge may be increased by increasing the compressible volume or by decreasing

the diameter of the compression capillary. However, if the compression volume is

too large, the weight of the mercury required to fill it sets a limit. If the capillary

is made too small, the mercury will stick to the walls. These restrictions challenge

the glass-blower's art

[3].

This device when used in a liquid-nitrogen-trapped sys-

tem by an experienced person, can be used as a primary standard. If

in Figure 4

the gas is at 10"^ torr and there is a compression ratio of 10^, the difference

between the colunms will be 10 mm. The fragility of the glass and toxicity of

the mercury argue against use by any but those skilled and careful individuals

interested in a relatively inexpensive standard. Unless precautions such as liquid

3.1.2 Direct Reading Gauges 263

nitrogen trapping are used any condensible vapors present will form a liquid

on compression, giving a false low reading.

3.1.2.2

Solid-Wall Gauges

There are two major solid-wall gauge types—capsule and diaphragm.

BOURDON GAUGES

The capsule-type gauges depend on the deformation of the capsule with changing

pressure. The common pressure gauges on compressors and gas cylinders use this

principle to measure pressures above atmospheric to several thousand psi. The

sensitivity of this type of gauge is low at pressures below atmospheric. One vari-

ant, the Bourdon gauge (Figure 5), enjoys commercial popularity. In this case the

capsule is in the form of a thin-walled tube bent in a circle with the open end at-

tached to the vacuum system. The atmospheric pressure deforms the tube, and an

attached pointer mechanism is deflected giving, in this case, a linear indication of

the pressure that is independent of the nature of the gas. Various manufacturers

supply gauges capable of measuring pressures as low as 1 torr. However, chang-

ing atmospheric pressure causes inaccuracies in the readings. Capsule gauges are

rugged, inexpensive, and simple to use. Also, the capsule can be made of materi-

als inert to corrosive vapors. Compensated versions of the capsule and Bourdon

gauge have been developed that circumvent the inaccuracy caused by changing

atmospheric pressure [4].

DIAPHRAGM GAUGES

If compensated capsule or diaphragm mechanisms are married to suitable elec-

tronic measuring circuits, increased performance is achieved. One such arrange-

ment is the capacitance diaphragm gauge (capacitance manometer).

Fig.

ELLIPTICAL

SHAPED TUBE

Bourdon Gauge

264

Fig.

Chapter

3.1:

The Measurement of Low Pressures

CAPACITANCE

PRDBE

FLEXIBLE-

DIAPHRAGM

REFERENCE

VACUUM

CAPACITANCE DIAPHRAGM GAUGE

STRAIN

GAUGE-

FLEXIBLE—

DIAPHRAGM

REFERENCE

VACUUM

STRAIN GAUGE DIAPHRAGM GAUGE

Diaphragm Gauge

Figure 6 shows the salient features of the capacitance diaphragm gauge. A flex-

ible diaphragm forms one plate of a capacitor, and a fixed probe the other. A

change in pressure deforms the flexible diaphragm that changes the capacitance,

which is converted to a pressure reading. The sensitivity, repeatability, and sim-

plicity of this gauge enables this type of direct reading gauge to be used with ap-

propriate heads as a standard from 10"^ torr to atmospheric pressure. A dynamic

range of four or five orders of magnitude is obtainable in a single head [5].

Figure 6 also illustrates schematically the strain gauge type of diaphragm gauge.

In this case deformation of the diaphragm causes a proportional output from the

attached strain gauge. Sensitivities and dynamic range tend to be less than those

of the capacitance diaphragm gauge. However, the price of the strain gauge type

diaphragm gauge tends to be lower.

Both these gauges, with their high sensitivities, are prone to errors caused by

even small temperature changes. Temperature-controlled heads or correction

tables built into the electronics have been used to minimize this problem. Other

sources of error in all solid-wall gauges are hysteresis and metal fatigue.

3.1.3 Indirect Reading Gauges

265

3.1.3

INDIRECT READING GAUGES

Indirect reading gauges measure some property of the gas that changes with the

density of the gas.

3.1.3.1 Thermal Conductivity Gauges

Thermal conductivity gauges use the property of gases to exhibit reduced thermal

conductivity with decreasing density (pressure). The thermal conductivity de-

creases from a nearly constant value above ~

torr to essentially 0 at pressures

below

10 ""^

torr. The gauge controllers are designed to work with a specific sen-

sor tube and substitutions are limited to those that are truly functionally identical.

Figure 7 shows how the heat transfer regimes are related to the Knudsen number

(see Part 1 of this handbook) which can be related to the geometry of the sensor

and then to pressure.

PIRANI GAUGES

The Pirani gauge is perhaps the oldest indirect gauge that is still used today. In

operation a sensing filament carrying current and producing heat is surrounded

by the gas to be measured. As the pressure changes, the thermal conductivity

changes, thus varying the temperature of the sensing filament. The temperature

Fig.

(/)

• - • T 1 1—

RADIATION

1 1 -.1

, • .

1—

/^\

CONDI.

1 1

CONVECTION^

ICTION

1 J

_. 1 1

10^

10'

I0-'

Kn=X/d

10-3

Heat Transfer Regimes in a Thermal Conductivity Gauge. Reprinted with permission from

& Sons.

266

Chapter

3.1:

The

Measurement

Low Pressures

Rg.8.

GAUGE

TUBE

CDMPENSATIQN

TUBE

Basic Pirani Gauge Circuit. Reprinted with permission from A Users' Guide to Vacuum Tech-

change causes a change in the resistance of the sensing filament. The sensing

filament is usually one leg of a Wheatstone bridge. The bridge may be operated so

that the voltage is varied to keep the bridge balanced; i.e., the resistance of the

sensing filament is kept constant.

This method is called the constant temperature method and is deemed the

fastest, most sensitive, and

accurate.

To reduce the effect of changing ambient tem-

perature, an identical filament sealed off at very low pressure is placed in the leg

adjacent to the sensing filament as a balancing resistor. Because of its high ther-

mal resistance coefficient, the filament material is usually a thin tungsten wire; in

fact a 10-watt light bulb works quite well [6] (see Fig. 8).

A properly designed, compensated Pirani gauge with sensitive circuitry is ca-

pable of measuring to

""*

torr. However, the thermal conductivity of gases varies,

which causes a variation in gauge response. These variations can be as large as a

factor of 5 at the low-pressure end and as high as 10 at the high-pressure end of

the scale (see Figure 9). If the composition of the gas is known, use can be made

of the calibration curves supplied by the manufacturer. Operation in high partial

pressures of organic molecules—oils, for example—is discouraged.

THERMOCOUPLE GAUGES

The thermocouple is another example of an indirect reading thermal conductivity

gauge. The thermocouple gauge sensing element consists of a filament, usually of

resistance alloy, heated by the passage of a constant current (see Figure 10). A

thermocouple is welded directly to the midpoint of the filament or preferably to a

conduction bridge at the center of the heated filament. This provides a means of

directly measuring the temperature. With a constant current through the filament.

3.1.3 Indirea Reading Gauges

267

Rg.9.

10-

10^

2 io3

-J

o I0»

„

PRESSURE (Torr)

2 I0-' 1

( I'M I ' '

> 1

> < '

1 1 1

1 1

1 1 1

1 1

1 1 1 1 1

1 Ar

jFREON ^

1 ico^^;;;;^/

' / / f^

\ J / J /j»0

HpO //My'

Ar ^

^\////y^y^

Kr^s,^

^^^V"""^

/^^^v^He

y^^^X^C^^^^^

y/W/ ^"-C^FREON

z;^^^ 1 C02

^,...l.iL.ii ,

,.-L...

. t... Ill 1 1 i i

101 I02

•

/I III/

/ AIR /'-

^Ne"

-^ -

^^^

........,^^

I02

10'

1 §

I0-'

10-2

I0» 102 103

INDICATED PRESSURE (Pa)

10^

Calibration Curves for Pirani Gauge. Reprinted with permission from Leybold-Heraeus

G.M.b.H., Postfach 51 07 60,5000 Kohn, Germany.

the temperature increases as the pressure decreases. This happens because there

are fewer molecules surrounding the filament to carry the heat away. The thermo-

couple output voltage increases as a result of the increased temperature and is an

inverse indication of the pressure. The thermocouple gauge can be operated in the

constant temperature mode just like the Pirani gauge.

Fig. 10.

CONDUCTIVE

BRIDGE

THERMDCDUPLE

HEATER

Thermocouple Gauge

268

Chapter

3.1:

The Measurement of Low Pressures

Fig.

METER

READING

(microns)

600

400

200

200 400 600

800

1000

ACTUAL PRESSURE - McLEOD GAUGE (microns Hg.)

Calibration Curves for Thermocouple Gauge

The same considerations concerning gas composition stated for the Pirani

gauge apply to all thermal conductivity gauges. Figure 11 shows the calibration

curves for a typical thermocouple gauge. Like the Pirani, the thermocouple gauge

can be optimized for operation at various pressure ranges.

As with the Pirani gauge, operation in high partial pressures of organic mole-

cules—oils, for example—is to be avoided. One manufacturer preoxidizes the

thermocouple sensor for stability in "dirty" environments and for greater inter-

changeability.

THERMISTOR GAUGES

A device closely related to the Pirani gauge is the thermistor gauge. In this gauge,

a single thermistor is used as one leg of a bridge circuit. The inverse resistive

characteristics of the thermistor element unbalances the bridge as the pressure

changes causing a corresponding change in current. A sensitive micrometer cali-

brated in pressure units measures the current. The thermistor gauge covers about

the same pressure range as the thermocouple but may have a slight edge in with-

standing contamination by oils and other organic compounds. As is true for other

thermal conductivity gauges, the exact calibration depends on the nature of the

gas measured. In a well-designed bridge circuit the plot of current vs. pressure is

practically linear in the range of

"^ to

torr

[7].

Modem thermistor gauges use

constant temperature, as discussed earlier.

3.1.3 Indirect Reading Gauges

269

CONVECTION GAUGES

As noted, the high-pressure operating range of thermal conductivity gauges is

normally limited to

torr. This is due to the near-constant thermal conductivity at

pressures higher than 1 torr. However, there is a small contribution to heat trans-

fer caused by convection. Several manufacturers have developed gauges that use

convection to extend the usable range to atmospheric pressure and slightly above

[8,9,10,11,12].

Because this process uses convection, orientation is critical.

The ConvectronTM uses the basic structure of the Pirani with special features

to enhance convection cooling in the high-pressure region [13]. To use the gauge

above 1 torr (133 Pa), the sensor tube must be mounted with its major axis in a

horizontal position. If the only area of interest is below 1 torr, the tube may be

mounted in any position. As mentioned, the gauge controller

designed to be used

with a specific model sensor tube, and since extensive use is made of calibration

curves and lookup tables stored in the controller, no substitution is recommended.

The Televac convection gauge uses the basic structure of the thermocouple

gauge except that two thermocouples are used [14]. As in any thermocouple

gauge, the convection gauge measures the pressure by determining the heat loss

from a fine wire maintained at a constant temperature. The response of the sensor

depends on the gas type. A pair of thermocouples is mounted a fixed distance

from each other (see Figure 12). The one mounted lower is heated to a constant

temperature by a variable current power supply. Power is pulsed to this lower

Fig.

12.

! !

UNHEATED TC

HEATED TC

^^s^

Convection Gauge