Basic Principle of Pirani Gauge

Basic principle of Pirani gauge

A conducting 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. The conductivity of the surrounding media inturn depends on the densisty of the surrounding media (that is, lower pressure of the surrounding media, lower will be its density). If the density of the surrounding media is low, its conductivity conductivity also will be low causing the wire to  become hotter for a given giv en current flow, and vice versa. Description of Pirani gauge

The main parts of the arrangement are: 1. A pirani pirani gauge gauge chambe chamberr which enclose enclosess a platinum platinum filamen filament. t. . A comp compen ensa sati ting ng cell cell to mini minimi mi!e !e vari variat atio ion n caus caused ed due due to ambi ambien entt temp temper erat atur uree changes. ". The The piran piranii gauge gauge chamber chamber and the compen compensat sating ing cell cell is housed housed on a whea wheatt stone  bridge circuit as shown sho wn in diagram.

Operation of Pirani gauge

1. A constant current current is passed passed through through the filament filament in the pirani pirani gauge chamber chamber.. #ue to this current, the filament gets heated and assumes a resistance which is measured using the bridge. . $ow the the pressure pressure to be measured measured (applie (applied d pressure) pressure) is connecte connected d to the pirani gauge gauge chamber. #ue to the applied pressure the density of the surrounding of the pirani gauge filament changes. #ue to this change in density of the surrounding of the filament its conductivity changes causing the temperature of the filament to change. ". %hen %hen the the temp temper eratu ature re of the filam filament ent change changes, s, the resis resistan tance ce of the filam filament ent also changes. &. $ow the the change in resistance resistance of of the filament filament is determined determined using using the bridge.

'. This This change change in resis resista tance nce of the piran piranii gauge gauge filam filament ent becom becomes es a measu measure re of the the applied pressure when calibrated.

Note: Note: [high [higher er pres pressur suree – higher higher densi density ty – higher higher conduc conductiv tivity ity – reduc reduced ed filame filament nt temperature – less resistance of filament] and vice versa.

Applications of Pirani gauge

sed to measure low vacuum and ultra high vacuum pressures. Advantages of Pirani gauge

1. Th They ey are are rugg rugged ed and and ine inepen pensiv sivee . *ive *ive acc accur urat atee resu result ltss ". *ood *ood respo response nse to to pressu pressure re chan changes ges.. &. +elation +elation between between pressure pressure and and resistance resistance is is linear linear for the range range of use. '. +eading +eadingss can be taen taen from a dista distance. nce.

imitations of Pirani gauge

1. -irani -irani gauge gauge mus mustt be chec checed ed freue freuently ntly.. . -irani -irani gauge gauge must must be calibr calibrated ated from from diffe different rent gases gases.. ". /lectri /lectricc power power is is a must must for for its its operat operation. ion.

A !imple Pirani "auge reading to #.##$ mB

0ommercially available -irani gauges seem to  be unnecessarily epensive. The basic design consists of a simple coil of wire eposed to the vacuum to be measured and connected as an arm arm of a %heat heatst ston onee brid bridge ge,, with with a mete meter  r  reading the outofbalance voltage of the bridge. %ith %ith suitably suitably chosen chosen compone components, nts, the outof outof  balance voltage can vary var y with the pressure pres sure from fro m about 12m3 down to 2.221 millibars. Although commer commercial cial versions versions,, albeit albeit with with som somee etra etra soph sophis isti tica cati tion on,, can can cost cost over over 422 4222, 2, the the instrument described here can be constructed for  one hundredth that sum and prove as useful in the laboratory as its more epensive cousin.

Basic Principles of Operation

If a wire, surrounded by a gas, is heated electrically, heat is lost from it by three processes: radiation, radiation, conduction and convection. The first of these is independent independent of the pressure of the gas and cannot therefore contribute to its measurement although it gives rise to a constant loss of heat. 0onvection contributes significantly at high pressures, but the heat loss caused  by it is not proportional propor tional to the pressure and in the ideal case, is independent indep endent of it. 0onduction of heat by an ideal gas is proportional to the pressure over a range approimately lying  between 12' m3 and 12 m3. In practice, the variation of heat loss from a hot wire with  pressure can be eploited between 2.221 m3 and 12 m3. The instrument described here operates in this range. %he Pirani "auge &ead

The construction of the the -ira -irani ni gauge head is shown on the right. A glass enve nvelope ope of  diameter  5mm is  provided with two 1 mm  bore capillary inlets with wide openi penin ngs as shown. A filament structure comprising 2.6mm diameter stainless steel wire supports 7, welded to a stainless steel wire coil / ( 2.1mm diameter wire, '8cm in length, wound in an open heli diameter 5mm, resistance at 2 0 of  &.8 ohms) was supported by a short length of glass tube to which the 2.6mm wires were cemen cemented ted using using 0era 0erama mabon bond d '89 (Arem (Aremco co -rodu -roducts cts Inc). Inc). Th Thee 2.6mm 2.6mm  wire wiress were were cemented into the capillary tubes using Araldite standard /poy. ( eparate eperiments were conducted to eamine the uality of seal produced in this way. 3y applying vacuum, the resin could be drawn down the capillaries for about 2 mm prior to hardening. It was found that cured seals made in this way were sound at least to 12 ' m3 pressure. A high vacuum epoy is available which is capable of sustaining a vacuum of 129 m3, but this was considered un necessary in the present application.) After hardening the epoy, the 5mm tube was drawn down as shown and fitted with an inlet tube of 8mm ;#.

%he Pirani "auge 'ontrolle 'ontrollerr

There are three modes of  oper operat atio ion n for for a -ira -irani ni gaug gauge: e: (i)const (i)constant ant current current,, (ii)con (ii)constan stantt resi resist stan ance ce and and (iii (iii)c )con onst stan antt voltage. The first is termed the -iraniusted to give 2.88? 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 '22 degrees 0 at 2.221 m3, and the out of balance voltage indicated by the meter falls to about 2.26?. Operational Data

%ith %ith the values of componen components ts shown shown in the schematic, schematic, ;ut of balance balance voltages voltages were were measured against pressure using a ?acuum *enerators -irani gauge type -I+ 1A. The data are shown in the table:

1 22 2

If identic identical al values values of compone components nts are used and the specifications of  the -irani gauge head are carefully  percent log foll follow owed ed,, the the data data in the the abov abovee ?bridge 7#  pressure tabl tablee can can be used used to prod produc ucee a calibrated calibrated scale for the 1mA meter. 3elow is a drawing of such a scale In use the ad>ustable resistor +& is m? set set to give give the the mete meterr read readin ing g as 7# at 1 atm pres ressure sure bef before ore  pumping down the gauge. 8 82 1 22 "

6

' 62

58."8

2.5&'1 2.



& 22

82.82

2."212 2.

2 .&

" &2

'1.'1

2."969

2 .

 82

"9."9

2.8959

2 .1

 12

"1.5

1

2 . 29

1 9'

9.'&' 1.2&'

2 . 25

1 5'

5.2"

1.2989

2 . 28

1 8'

'

1.15

2 . 2' 9

1 '6

".655 1.91

2 . 2 6

115

16.56

2 . 2" 8

1 '

2 . 21 2

112

2 . 22 '

1 2'

2 . 22 "

92

2 . 22 1

62

-+/+/ m3

1.'858

0ompari 0omparison son with with the commer commercial cial *enerators -irani *auge 15.9" 1.&&" ?acuum *enerators and with a @ceod gauge showed 18.6  that that the the inst instru rume ment nt cons constr truc ucte ted d indicated a pressure within a factor  1'.9 ."212 of  of the pressures measured with the the other other instr instrum ument entss across across the 1".8"8 .' range. 12.828 "