Files_Tech.docs_High Vacuum Gauge Calibration System_Kreisman-1960

7/29/2019 Files_Tech.docs_High Vacuum Gauge Calibration System_Kreisman-1960

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NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONGoddard Space F l i g h t CenterWashington, D.C.

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I 1 ,17 November 1960

--RATION O F AMERICA_ c I -


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. VGCA T e c h n i c a l R e p o r t60- 2

A HIGH VACUUM GAUGE CALIBRATION SYSTEM

Wallace S . K r e i s m a n

C o n t r a c t N AS 5 -2 70

NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONGoddard S p a c e F l i g h t C e n t e r

Washington, D . C .

1 7 N o v e m b e r 1960

GEOPHYS I C s CORPORATION OF AMERICAB e d f o r d , M a s s a c h u s e t t s


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A HI GH VACUUM GAUGE CALI BRATI ON SYSTEMby

Wal l ace S. Kr ei smanSUMMARY

An ul t r a- hi gh vacuum t ype met al and gl ass sys t emhas been con-st r uct ed f or t he pur pose of cal i br at i ng vacuum gauges i n t he pr essur er egi on f r om 760 to 10bakeabl e at t emper at ures up t o 450 C.

-*-7 t or r . The hi gh vacuum por t i on of t he system i s0

A mer cury manomet er ser ves as a pr essur e st andar d i n t he r egi onf r o m 760 t o 20 t or r , and t hr ee speci al l y desi gned, bakeabl e McLeod gaugeswi t h over l appi ng pr essur e r anges ser ve as pr essur e st andards i n the regi onf r o m20 to r r to 1 x 10 t or r . Theor et i cal accur aci es of 1 percent or4bet t er ar e expect ed f or t he McLeod gauge r eadi ngs i n t hi s pr essur e r egi on.Pr essur es f r om 1 x 10 to r r to 10 t orr r egi on can be measur ed, but t he4 -7accur acy of t he readi ngs depends on adsor pt i on and out gassi ng ef f ect s.

Resul t s per t ai ni ng to t he r epr oduci bi l i t y of measur ement s andcompar i sons of var i ous gauge r eadi ngs ar e pr esent ed.measur ement t echni que i s used t o det er m ne how t he vacuum gauges and ot her

A pr essur e r i se

sys t emcomponent s ar e behavi ng.

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CONTENTS

PageSummary ............................................................ iINTRODUCTION ....................................................... 1SYSTEM CONSTRUCTION ................................................ 3PRESSURE STANDARDS ................................................. 10OPERATING PR0CEDS ............................................... 14SYSTEM AND CmPONENT TESTS ......................................... 16PRESSU RE R I S E MEASUREMENT TECHNIQUE ................................ 2 2ACI


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I NTRODUCTI ONI n connect i on wi t h t he desi gn and devel opment of hi gh vacuum

gauges and mass spect r omet er s, i t i s necessar y t o have some means of accu-r at el y cal i br at i ng t hese i nst r ument s f or pur e gases and m xt ur es. Aspeci al i zed syst emhas been const r uct ed to f ul f i l l t hi s requi r ement and,at t he same t i me, provi de a means f o r st udyi ng such pr ocesses as i oni zat i ongauge pumpi ng and out gassi ng, adsorpt i on, t emperat ur e ef f ect s, et c. I nt hi s way, t he syst em i s used not onl y t o cal i br at e gauges, but t o i mpr ovecal i br at i on pr ocedur es as wel l .

The pr obl ems of pr essur e gauge cal i br at i on have not r ecei ved ex-t ensi ve t r eat ment i n t he l i t er at ur e. Such a t r eat ment r equi r es i nt egr at i onof i nf ormat i on about pr essur e gauges, pumpi ng syst ems, gas f l ow, adsorpt i onand desor pt i on, et c. Dushman, f or exampl e, ( Ref erence I) t r eat s each oft he subj ect s ment i oned above rat her extensi vel y, but says l i t t l e or not hi ngabout cal i br at i on work. -\The recent book on pr essur e measur ement by Leck( Ref erence 2) does have a s hor t ( 6 page) chapt er on gauge cal i br at i on.Gut hr i e and Wakerl i ng ( Ref erence 3 ) al so ' devote some space to a di scussi onof what t he var i ous vacuumgauges act ual l y measure, Each of t he t hr eebooks j ust ment i oned pr ovi de ref er ences t o i ndi vi dual papers concerni ngvacuum gauges and vacuum measur ement .

Onl y a ver y f ew vacuum gauge cal i brat i on syst ems have been di s-cussed i n t he l i t er at ur e.cal i br at i on syst em i s gi ven i n a paper by L- Cl anc ( Ref er ence 4 ) . Hi s systemused waxed and greased connect i ons and was not i nt ended f or use at pressuresbel ow 10 Al per t ( Ref er ence 5) has devel oped a nul l - r eadi ng absol ut emanomet er t hat can be used wi t h a pr essur e r educt i on t echni que t o measure

A descr i pt i on o i a demount abl e t ype vacuum gauge

-5 t o r r .

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l ow pressur es i n a cl osed syst em Such an ar r angement has not at t ai ned wi deusage.

The hi gh vacuum gauge cal i br at i on syst em descr i bed i n thi s paperi s basi cal l y a gl ass- met al , ul t r a- hi gh vacuum system al l essent i al vacuumcomponent s of whi ch may be baked out at t emper at ur es up t o 450 C. Thi st ype of desi gn was chosen t o m ni m ze t he cont am nat i on of i nt r oduced gasesand t o f aci l i t at e t he st udi es of adsor pt i on, desor pt i on, pumpi ng, et c. ,previ ousl y ment i oned.

0

Vacuum gauge cal i br ati ons f r om pr essur es of 7 6 0 torr to pressurest or r can be per f or med wi t h t hi s apparat us. A mer cur y manomet er-5bel ow 10

and a set of t hree mer cur y McLeod gauges ar e t he pri mary pressur e st andar ds.These uni t s have been devel oped speci al l y f or t hi s pur pose.

Pre l i m nary tests of syst emcomponent s and s yst emoper at i on haveA descr i p-een made and some operat i ng pr ocedures have been est abl i shed.

t i on i s gi ven of a pr essur e r i se met hod of t est i ng vacuum gauges and l owpr essur e phenomena.

A gr eat deal of wor k r emai ns t o be done t o devel op cal i br at i onsyst em component s and t echni ques f or measur i ng l ow pressures.def i ni t e need f or new l ow pr essur e st andar ds and f or car ef ul eval uat i on ofexi st i ng st andar ds and t he methodol ogy of t hei r use. I t i s hoped t hat t hesyst em descr i bed her ei n can be used to make cont r i but i ons i n t hi s ar ea ofr esear ch.

There i s a

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SYSTEM CONSTRUCTION

The c a l i b r a t i o n s y st e m is shown s chema t i ca l l y i n t he b l ock d iag ramof F i gu re 1.o p e r a t i o n . F i g u r e 3 i s a close-up showing some of the de ta i l ed co ns t ru c-t i o n .f u l t o r e f e r t o t h e b l o ck d i ag ra m a n d t h e two ph ot og r ap hs t h a t f o l lo w .

F i g u r e 2 i s a n o v e r - a l l vi ew o f t h e c a l i b r a t i o n s y st em i n

I n c o nn e ct io n w i t h t h e d e s c r i p t i o n t h a t fo l l ows , i t w i l l prove he l p -

The e n t i r e s y st e m i s mounted on a t h r ee s ec t i on , modu la r - typee l ec t r on i c s cab i ne t . The ma j or po r t i ons o f t he u l t r a -h i gh vacuum s ys t em(which in cl ud es th e McLeod gauges) ar e mounted wi th in the Veeco motori zedl i f t , model FH h igh t empera ture oven. The oven, fe a t ur in g in te r io r dimen-s i on s of 23" x 18" x 20", i s a n i n t e g r a l p a r t o f t h e s ys te m .oven a re two p a r a l l e l s t a i n l e s s s t e e l mounting bars , 3" xa re l o c a t e d j u s t a bo ve t h e i n s u l a t e d o ven b as e a n d a re spaced about 9 i nchesa p a r t .t o t hes e moun ting ba r s , and a la rg e pa rt of the vacuum system i s connectedt o , and suppo rted by, th e va lv es . The McLeod gauges a r e s e a te d i n p l a s t e rmolds which r e s t on a moun ti ng p l a t e l oca t ed bet ween , and f a s t en ed t o , t h emount ing bars .by the same r i g i d s t u r c tu r e t o minimize r e l a t i v e movements due t o mec han ica land t he rma l s t resses .s t r e s s . The major s tresses are caused by t e m p e r a t ur e g r a d i e n t s t h a t b u i l dup du r i ng t he bakeou t pe r i od .

Wi th in thex 22%", which

G r a n v i l l e P h i l l i p s t yp e C u lt r a -h ig h vacuum (UHV) va lv es a re f a s t e n e d

I n t h i s way, the components wi th in t he oven a r e a l l s uppo r t ed

T i gh t en i ng t he UHV valves i s one such mechanica l

The pumping po rt io n of th e vacuum sys tem i s f a i r l y c o nv e nt io n al .I t c o n s i s t s o f a Welch Model 1400B 2 1 l i t e r / m in . mecha nica l fore-pump anda n H . S . Martin Company model M-40112 80 l i t e r / s e c . , t h r e e j e t , wat e r - coo l ed ,g l a s s , mercu ry d i f fu s i o n pump. The fore-pump i s mounted i n a t r a y t h a t

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B o t t l e

bakeoutc u t o f f - ---ca n t

ve n tbakeoutc u t o f f

IL a r g eReservotr.Vo lve '

cu t o f f

vent and leakr * t e s t vo l ve

D e ss i co n thermocoupleG o u g et F o r eMer cur y F o r e v o l v e

I . v o c u u m t *C o Id M e ch a n i ca l

Fo re Pumpi f f u s i o n V ac u u m T r a pP u m p R e s e r v o i r* I c b

0 RA NV IL L E P HIL L IP S V A L V E@ H O K E V A L V E

- ACUUM CONNECTIONS

S m a l lReservo i r

Figure 1. Block Diagram of Vacuum Gauge Calibration System

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Figur e 2. Overall V. i e w of t h e Calibration System5


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Figur e 3 . Close-up V i e w of Oven In te r i o r andMcLeod Gauge Mercury Reservoirs

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r e s t s e i t h e r on t h e c a b in e t c a s t e r b r a ck e ts o r e l s e i s s u pp o rt e d d i r e c t l yfrom the ; : loor by means of fou r corn er jac ksc rew s. When supported from thef l oo r , the mechanica l pump i s e f f e c t i v e l y i s o l a t e d f rom t h e c a b i n e t , b e in gconn ected t o th e vacuum system o n l y by a l eng t h o f rubbe r t ub i ng . The v i -b r a t i o n m i ni ii .i za ti on a f f o r d e d b y t h i s c o n s t r u c t i o n i s es s e n t i a l f o r optimumperf orm ance of th e mercury manometer and McLeod gaug es.

A fore-vacuum c o l d t r ap (d ry i c e ) , a va l ve , and a l a r g e g l a s sre s e rv o i r s ep a ra t e t he mechani ca l and d i f f u s i o n pumps. The r e s e rv o i r makesi t po ss ib le t o tu rn o f f the mechanica l pump comple te ly when req ui re d .

On the h igh vacuum s i de of the d i f fu s i on pump t he re i s the con-ve n t i on a l l i qu i d n i t rog en co l d t r a p . The main pumping l i n e runs from t h i st r ap s t r a i g h t up t hrough t he i n s u l a t e d oven bas e and connec ts t o t he LH Vpumping va lv e. The pumping va lv e connects to t he man ifold which, i n tur n,has connec t i ons t o two r eagen t g ra de , 1 l i t e r g as b o t t l e s a nd a l a r g e g a sc y l i n d e r ( t a n k g as i n Fi g ur e 1 ) . The l a t t e r c o n n e c ti o n ca n be e a s i l yb roken f o r t he pu rpose o f ven t i ng t he s ys t em. I t should be n o t e d t h a t e a c ho f t he t h re e connec t i ons j u s t ment ioned i s made through a p a i r o f v a l ve sr a t h e r t h a n a s i n g l e v a l v e . The t h r e e b a ke o ut c u t o f f v a l v e s a r e Hokemodel 411 diaphragm type va lv es .o f t he c a l i b ra t i on s y s tem be low t he i n s u l a t e d oven bas e .

They a r e l o c a t e d a t t h e l e f t hand s i d e

The pumping manifold i s connected t o th e measuring vo lume th rough

t he pumping man ifo l d co l d t r a p . Th i s t r a p ac t s t o keep condens ab le ma t e r i a l sor i g i n a t i n g i n t he pumping , ven t i ng and gas sample por t i ons o f t he s ys t em.ou t of th e measuring voluine.s p h e r i c a l f l a s k h a vi ng a Veeco RG-75 i on iz a t io n gauge tu be a t t a ch ed t o oneneck , a Hast in gs R ayd is t model D V - 1 7 , 0 t o 1000 m i cr o n p r e s s u r e r a n g e , g l a s s

The measuring vo1um.e i s a 1 l i t e r , t h ree neck

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env elop e thermocouple gauge at ta ch ed t o t he s econd neck, and t h e t h i rd neckav a i l a b l e fo r gauges t o be ca l i b r a t ed . The t he rmocoup le gauge i s used t og i v e c o n ti n uo u s q u a l i t a t i v e i n d i c a t i o n of p r e s s u r e w i t h i n i t s range .

The McLeod ga uges a r e mounted i n p la s t e r molds wi th in th e oven,and t h e i r pumping t ab u l a t i o ns are connect ed t og e t he r and l ead i n t o t heme as ur in g voltgne thr oug b t h e McLeod gauge va lv e and t h e McLeod gauge co ldt r a p , The mercury manometer al s o leads i n t o the McLeod gauge cold t r a pv ia the mercury manometer valve.

The UHV t e s t gauge va lv e can be used t o connect th e measur ingvolum e d i r e c t l y t o a ga ug e b e i n g t e s t e d . T h i s v a l v e h a s t h e g l a s s t u b u l a -t i on s ea l ed when it i s no t connec t ed t o a gauge, s o t h a t t h e v a l v e c an bel e f t o pe n d u r in g b a k eo ut .

A UHV gas expans i on va l ve j o i n s a small c a l i b r a t e d volum e t o t h eI n u s e , t h e c a l i b r a t e d volume i s f i l l e d w it h a gas a t a

The remainder of the syste m (having a known measured volume)measuring volume.known pressure.

i s th en pumped down t o a low background p r es s u re , a f t e r wh ich t h e gas i n t h ec a l i b r a t e d volume is al lowed t o expand in to th e remainder of the sys tem.By moni t o r i ng t he new p r e s s u r e i n t h e s ys te m , i t i s p o s s i b l e t o m ea su reads o rp t i o n and pe r form ca l i b r a t i o n work.

The mercury cu to f f v a lv es fo r t he low pr es su re and d ual mediumand hi gh pr es su re McLeod gauges are loca ted below t he oven enc los ure wherethey may be kep t c oo l wi th an a i r blower dur in g the bakeout p er iod .

An Ea li ng model 6165 ca t he t ome t e r i s u se d t o r e a d t h e p o s i t i o n sof t h e mercury columns i n t h e McLeod gauges and th e manometer. The ca th e-tometer I s mounted on a metal s t a n d t h a t i s e q u i p p e d w i t h f o u r c a s t e r s f o re a s y m o b i l i t y a nd four jackscrews for s u p p o r t i n g t h e s t a n d i n a f i x e d

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p o s i t i o n . The s t a n d may b e r i g i d l y j o in e d t o t h e c a l i b r a t i o n s y st e mca b i n e t by means o f a n ang l e i ro n b racke t . To a i d i n rea d ing th e McLeodgauges and manometer , i l lu mi na t i on boxes a re a p p r o p r i a t e l y p o s i t io n e d ont h e s y st e m a nd c o n t r o l l e d b y a s wi t ch a t t h e c a t h e t o m e t e r .emptying of th e McLeod gauges i s a l s o c o n t r o l l e d by a n e l e c t r i c a l s w it c ha t t h e c a t h e t o m e t e r .

F i l l i n g a nd

Mercury i s broug ht i n t o and take n ou t of th e McLeod gauges byr a i s i n g a n d l ow e ri ng v a c uu m - ti g ht , s t a i n l e s s s t e e l r e s e r v o i r s t h a t a recoup led t o t he McLeod gauge v ia f l e x i b l e m e t a l h o s e s .t r o d uc e d i n t o t h e s e r e s e r v o i r s t h ro ug h small v a l v e s l o c a t e d a t t h e t o p s o ft h e r e s e r v o i r s . J u s t be low t h e s e v a l v e s a r e l oca t ed c l o s ed -end mercu rymanomete rs t o i nd i c a t e t he s t a t e of vacuum ex i s t i n g above t he r e s e rv o i rmercury .

Mercury can be in -

A m o to ri ze d l i f t i s used t o r a i se and lower th e mercury reser-v o i r s . The l i f t i s a modi f i ed ve r s i on o f a c om m er ci al ly a v a i l a b l e b e l lja r l i f t .t h a t t u rn s a saginaw screw.two p a r a l l e l u p r i g h t s t e e l rods , r ides up and down on t h e screw.rods are c o n s t r a i n e d t o move v e r t i c a l l y by f i x e d - p o s i t i o n b a l l b u sh i ng s .A h o r i z o n t a l r e c t a n g u l a r b a r , w hic h may be a d j u s t e d i n p o s i t i o n , i s clampedt o t he s t e e l r o d s n e a r t h e i r u p p er e nd .a re s uspended f rom t he r ec t a ngu l a r ba r by T - s l o t b ra cke t a r r angemen ts s ot h a t the y may be e a s i l y removed. Brackets a re also prov i ded t o s u s pendt h e r e s e r v o i r h o l d e rs a t a f i x e d l e v e l b elow t h e m e rc u ry c u t o f f v a l v e s .Th i s i s done s o t h a t th e mercury of the McLeod gauges can be lowered belowt h e c u t o f f valves a n d t h u s i s o l a t e d from the vacuum sys tem dur ing bakeo ut .

It makes use of a 1/15 horsepower double redu ct i on g ea r motorA saginaw nut , which i s f i r m l y a t t a c h e d t o

The two

Two heavy a luminum reservo i r ho lders

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There a re f o u r c o n t r o l p a ne l s on t h e c a l i b r a t i o n sy st em .f o r co n t ro l o f t he mechan ica l and d i f fu s i on pumps; t he s econd i s a modi f iedV ee co c o n t r o l u n i t f o r the io n i za t io n gauge ( pro v i s ion has been made t or e du c e t h e i o n i z i n g e l e c t r o n c u r r e n t t o v e r y small va l u es and t o measu ret h e s e c u r r e n t s ) ; t h e t h i r d i s a s t a n da r d f i v e - p o s i t i o n H a s t in g s R a y d i stc o n t r o l u n i t f o r t h e t he rm oc ou pl e g a ug e s; a n d t h e f o u r t h p a n e l c o n t r o l st he s peed w i t h whi ch t he mercu ry r e s e rvo i r s are r a i s e d o r l o w e r e d .f o u r u n i t s a re i n a d d i t i o n t o t h e oven te m p e ra t ur e , f a n , a n d l i f t c o n t r o l st h a t a re on t h e oven i t s e l f .

One i s

These

PRESSURE STANDARDS

The mercury manometer i s t he fundamen t al pr i mary p re s s u re s t a n -d a rd o f t h e c a l i b r a t i o n s y st em .i s u s e d d i r e c t l y as a s t a n d a r d .the manometer i s u s e d t o c he ck t h e p r e s s u r e r e a d i n g s o f t h e h i g h p r e s s u r eMcLeod gauge. The h igh pr ess ure McLeod gauge range ov er la ps th a t o f th emedium pr e s s u r e McLeod gauge , so t h a t t h e l a t t e r ga ug e r e a di n g s may a l s obe refe re nc ed t o those of the manometer . F i na l l y , th e medium pr ess ur e McLeodgauge range o ver l aps p a r t o f the range of the low pr ess ur e McLeod gauge , s ot h a t t hes e r ead i ngs a l s o may be r e fe r r ed t o t hos e o f t he manomete r.

For p ressu res above 20 to r r , the manometerIn t he p r e s s u r e r e g i o n j u s t b elow 20 t o r r ,

The mercury manometer i s of t he conven t i ona l U-t ube va r i e t y , andi s c o n s t r u c t e d o f 3 / 4 i n c h I.D. p r e c i s i o n - b o r e g l a s s t u b i n g .the manometer i s connec t ed v i a a Hoke 440 bel lows type vacuum va lve t o t hemercury manometer valve with in the oven .d i s t i l l e d i n t o t h e manometer so t h a t no t r a c e of g a s b ub b le s a lo n g t h e g l a s sw a l l s i s ev i den t . The mnomet e r i s kept con t inu ous l y under h igh vacuum ex-cept when i t i s i n u s e .

Each le g of

Mercury w a s ca re fu l l y vacuum

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The manometer i s r ead w i t h a cathe tometer which i s equipped wi tha micrometer screw f o r a c c u r a t e l y me as ur in g d i s t a n c e s up t o 30 m i l l i m e t e r s .The smal les t d i v i s i o n on t h e m i cr om e te r s c a l e r e p r e s e n t s 5 microns , andes t imates t o th e ne ar es t micron may be made. For a c o n s t a n t p r e s s u r e i nt he s y s t em, a se r ies of measurements of th e mercury column he ig ht d i f fe re nc e(w i th t he columns be ing hand tapped between rea d ing s) y ie lde d s tan da rd de-v i a t i o n s o f a b ou t 1 0 t o 20 microns . Devia t ions l e s s t h an t h i s a re o b t a i n e di f t he columns a re no t hand tapped.

The th re e McLeod gauges were co nst ru cte d t o cover th e pr es su re-7range from 20 t o r r down t o t he 10 t o r r r eg i on , w i t h p re s s u r es bet ween

10 t o r r and 20 t o r r be ing measured wi th 1 p e r c e nt a c c u ra c y o r b e t t e r .-4

P a s t e x p e r i e n c e h a s i n d i c a t e d t h a t one c o ul d n o t hope t o s a t i s f y more s t r i n -gen t low pressure requ i rements a t t h e p re s e n t s t a t e - o f - t h e - a r t .

The McLeod gauges were des igned i n s uch a w ay t h a t t h e y c o u ld b emounted w it hi n t he oven encl os ur e and baked o ut a t h i gh t empera t u re w i t hth e o th er sys tem components . This requi rement l im i te d the gauge d imens ionsas i n d i c a t e d i n F ig u r e s 4 and 5 .combined i n a s i n g l e u n i t t o c o ns er ve space and permit one mercury reser-v o i r t o f i l l b o th ga ug es s i m u lt a ne o u sl y .

The medium and high pressure gauges were

The hi gh p re ss ur e McLeod gauge measure s pr es su re s from 20 t o r r-1t o 2.00 x 10 t o r r w i th a c a l c u l a t e d a c c u r a c y o f 1 per cen t , and l ower

p r e s s u r e s w i t h c o r r e s p o n d i n g l y l ess ac cu ra cy . The medium pr e ss u re McLeodgauge measures pressures from 4.00 x 10 t o r r t o 4.00 x t o r r w i th 1-1

per cen t accu racy , and l ower p re s s u res w i t h l ess accu racy .McLeod gauge measures pressures from 8.00 x 10 t o 8.00 x t o r r w i tha c a l c u l a t e d a c c u r a c y o f 1 perc en t , and l ower p re s s u res w i t h l e s s a c c u r a c y .

The low pressure-3

11


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1

A

I "Not Over 15,-

C

B F

Figure 4. Low Pressure McLeod Gauge12


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A

H

I

c-----W" ---

-

OVER

Figure 5. D u a l Medium and High Pressure McLeod Gauge13


18/31

The dime nsion s of t he l a t t e r gauge a re s uc h, t h a t i n sq u a re l a w o p e r a t i o n ,1 mm c o r r e s p o n d s t o a p r e s s u r e o f 2 x 10 t o r r . The d e l i b e r a t e o v e r-l app ing o f t h e 1 pe r cen t a ccur acy p r e ssur e r anges o f t he t h r e e gauges i sa fe at ur e which enab les gauge measurements t o be compared, thu s rev ea l i ngs y s t e m a t i c e r r o r s t ha t depend on gauge geometry.

-7

Mercury i s vacuum d i s t i l l e d in t o the two McLeod gauge s ta in l es ss t e e l r e s e r v o i r s and i s kep t under vacuum cont in uously t o minimize ou t -g a s s i n g d u r i n g o p e r a t i o n .and th e r e have neve r been any gas bubb le s v i s ib l e a t t h e mer cur y g l a s si n t e r f a c e .

The mercury surface i s a lways c l ean and b r i gh t ,

The he ig h ts of th e McLeod gauge mercury columns a re normal lymeasured t o 0.05 mm wi th the cathe tome te r . D i f f e r ences be tween the he i gh t sof th e columns t h a t do no t exceed 30 m can be measured t o a few micronswi th the micr omete r ve r n i e r ad jus tment o f t he ca the tome te r .

OPERATING PROCEDURESS t a r t i n g w i t h t h e c o m pl et e c a l i b r a t i o n s y s t em a t a tmospher ic

p r e s s u r e , the sys t em i s evacua ted i n the fol lowin g manner in o r d e r t o k ee pi t a s c l e a n as p o s s i b l e . F i r s t , d ry i ce i s placed on th e fore-vacuum col dt r a p dewar and t h e mechani cal pump i s t u r ned on. When the fo re pr es su rereaches a v a l u e o f t h e o r d e r of 10 micron s, b oth th e pumping valve and thef or e- vacuum i s o l a t io n va lve a re c los ed, th e dewar i s removed from the forevacuum t r ap , and the t ra p i s warmed up t o dr iv e of f the vapo rs th a t havecondensed th er e . When th e t r ap i s c lean , the dewar wi t h dry i c e i s a g a i np l ac ed i n p o s i t i o n , a nd t h e i s o l a t i o n v a lv e i s opened.sion pump i s t u r n e d on, and the di f fu s i on pump col d t r ap i s h a l f - f i l l e d

The mercury di f fu-

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wi t h l i qu i d n i t rog en . The pumping va l ve i s opened, and tho ov n po r t i ono f t he s y s t e m i s pumped down. When the gl a s s thermocou ple gau gl re ad se s s e n t i a l l y z e r o , t h e i o n i z a t i o n gauge i s turned on so that the ; :ys tempre s s u re can be mon it o red con t i nu ous l y .i s s u f f i c i e n t l y l o w , bakeout may be commenced.t h e d i f fu s i on pump co l d t r ap i s f i l l e d t o t h e t o p w i t h l i q u i d n i t r o g e n , andt h e s y s t e m i s r ea d y f o r u s e .

When the pr ess ure i n th e s y s t e mAf t e r bakeou t and coo l i ng ,

Af t e r t he bakeout pe r i od , t he s y s t e m i s us ua l ly a l lowed t o pump- Idown to a l i m i t i n g p r e s s u r e o f a b o ut 2 x 10 t o r r . A t t h i s p o i n t , l i q u i d

n i t r o g e n i s in t ro duce d in to th e dewars th a t sur round the pumping mani fo ldco ld t r a p and the McLeod gauge co l d t ra p . The l e ve l of t h e l i q u i d n i t r o g e ni n t h e s e d e wa rs s h o u ld be k e p t f a i r l y c o n s t a n t ,

With the system a t a low p res s u r e , t he gas b o t t l e s can be opened,and t he t ub i ng up t o t he now c l o s ed Gra nv i l l e Ph i l l i p s g as va l ves can bef i l l e d w it h g a s .o f g a s a r e i s o l a t e d b etw ee n t h e v a lv e s an d ca n b e us ed f o r c a l i b r a t i o nwork.

By c l o s i n g t h e g a s ba ke ou t c u t o f f v a l v e s , s m a l l amounts

The Gra nv i l l e Ph i l l i p s u l t r a - h i gh vacuum va l v es can be u s ed t oadmi t gas in to the sys tem from low leak r a t e s t o v e ry h ig h l e a k r a t e s .f o r t u n a t e l y , f i n e a d j us t m e nt of t h e l ea k r a t e w i t h t h e s e v a l v e s i s d i f f i -c u l t . S e t t i n g s co u ld be o n l y r o ug hl y r e pr od uc e d w i t h t h e a i d of a to rquewrench. N e w G r a n v i l l e P h i l l i p s s e r i e s 9100 v a r i a b l e l e a k s a r e b e in g u s ed

Un-

t o r e p l a c e t h e s e ga s v a l v e s .The normal ca l i b ra t i on procedu re con s i s t s of t h e f o l l o w i n g s t e p s :

The system i s f i r s t pumped down and baked o u t at a t e m p e r a t u r e of between

15


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300 and 45OoC fo r one hour o r more .pre ssu re i n the measur ing volume, as read by the i on iz a t io n gauge , i s a ta l e v e l t h a t i s w e l l below th e pres sure a t whi ch ca l i b ra t i ons a re t o bemade. A t t h i s t i m e , a c o nt i nu o us f lo w of t h e c a l i b r a t i n g g a s i s e s t a b -l i s h e d s o t h a t t h e l ow es t c a l i b r a t i o n p r e ss u r e e x i s t s i n t h e me as ur in gvo lume .

System pumping i s c on ti nu ed u n t i l t h e

A s e r i e s of p re s s u r e r ead i ngs i s t a k e n w i t h b o t h t h e a p p r o p r i a t epre ssu re s t a nda rd McLeod gauge and th e gauge be ing ca l i b r a t ed . The gasflow i s t h e n i n c r e a s e d s t e p by s t e p t o o b t a i n hi g he r p r e s su r e s i n t h emeasuring volume and th e measuring procedure i s r e p e a t e d a t e a c h s t e p . I ti s i mp or ta nt t h a t s u f f i c i e n t t i m e be a l lo w ed f o r t h e e n t i r e s y st e m t o r e a c hi t s new h ig her p r ess ure each t i m e t he gas flow i s i n c r e a s e d .

SYSTEM AND COMPONENT TESTS

Only a l i mi te d amount o f exper imenta l work has been ca r r i e d ou tt o d a te w i t h t h i s s y s tem. M ost of the work has been concerned wi th t e s t in gth e ope ra t ion of the var iou s sys tem components - - especia l ly the McLeodgauges , mercury manometer, t he contin uous flow of va r i o us gas es t h rough t hemeasuring volume a t d i f f e y e n t p r e s s u r e l e v e l s , a n d t h e p r oc e d ur e s f o r b ak in gth e sy stem and pumping down t o low background p r es su re s.

- 9Background pressures a s low as 5 x 10 t o r r i n t h e m e as ur in g

volume, as rea d by t he Veeco ion iz at io n gauge, have been ach ieve d by usi ngon l y a moderate bakeout a t 300 C fo r one hour , fo l lowed by over n igh t io ngauge pumping. The sys tem has a l s o been baked ove rn ig h t a few t i m e s a t450 C wi t h no d i f f i c u l t y . Background p res s u res of 2 x 10 t o r r , as r e a dby t h e i o n i z a t i o n ga u ge , a r e rou t i n e l y e s t ab l i s h ed i n t he measu r ing volume(w i th the McLeod gauges and th e i r mercury i n t he s y s t e m ) without any bakeout

16

0

0 -7


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Cont i nuous f l ows of pur e, r eagent gr ade ni t r ogen have been es--7t abl i shed i n t he syst em cor r espondi ng to pr essur es , i n the l ow 10 to r r

r egi on, as r ead by t he i on gauge. St eady f l ows of pr epur i f i ed ni t r ogenf r om t he hi gh pr essur e cyl i nder and pr essur e regul ator have been obt ai ned,cor r espondi ng t o syst em pr essur es as hi gh as 20 t or r , as r ead by t hemanomet er . The di f f usi on pump i s t ur ned of f , of cour se, dur i ng t hese r el a-t i vel y hi gh pressure measur ement s,

A ser i es of exper i ment s were per f ormed t o t est t he operat i on oft he McLeod gauges. The gauges wer e f i l l ed at var i ous speeds, and t he op-t i mum r at e of f i l l i ng, as det er m ned f r omobser vat i on of t he mer cur y mot i oni n t he capi l l ar i es, was est abl i shed. Experi ment s wer e made to deter m net he ef f ect of hand t appi ng and mechani cal l y vi br at i ng t he capi l l ar i es atl ow f r equenci es of t he or der of 60 cycl es per second.set t l ed yet , but i t appear s t hat the vi br at i on t echni que, whi l e yi el di ngmor e r epr oduci bl e r eadi ngs, t ends t o rai se t he mercur y t o nonequi l i br i umposi -t i ons, f r omwhi ch i t may not r ecover i n t he case of smal l bor e (1 mm di a.or l ess) capi l l ar i es .of r eadi ngs, t he onl y accept abl e met hod appear s t o be a cont r ol l ed f i l l i ngof t he capi l l ar i es under condi t i ons of m ni mum vi br at i on. For t he hi ghpr essur e gauge wi t h i t s 2 mm di a. bore capi l l ar y, t he equi l i br i umpos i t i onof t he mer cur y col umns i s r eached i n a mat t er of seconds. For t he medi umand l ow pr essur e gauges wi t h thei r smal l bor e capi l l ar i es, t he equi l i br i umposi t i on i s at t ai ned af t er sever al m nut es.

Thi s mat t er i s not

Si nce hand tappi ng gi ves r i se to poor r epr oduci bi l i t y

The i ni t i al compar i son of t he r eadi ngs of t he mer cur y manomet erwi t h t hose of t he hi gh pr essur e McLeod gauge was made by bl eedi ng l abor a-tory ai r i nt o the syst em t hrough a f i l t er , des i ccant , ext er nal l i qui d

17


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n i t r o g e n c o l d t r a p , a nd a Vac t ron i c Var i-Vac ad j u s t ab l e l eak . The r e s u l t sof th es e measurements a re given i n Table I . The i n c r e a s e i n t h e e r r o r f o rp r e s s u r e s of a b o u t 6 t o r r a n d h i g h e r a r e b el i e ve d t o b e d ue t o po or i l l u -min at io n of th e mercury menisc i in the manometer , when th e levels becomese par a te d by t h i s amount . This compar ison i s cons i de red on l y a p r e l i m i n a r yone.

The pre l imi nary compari son between the r ead ings of th e h igh andmedium pr e ss ur e McLeod g auge s w a s performed with a cont inuous f low of p r e-p u r i f i e d n i t r o g e n fr om t h e h i g h p r e s s u r e c y l i n d e r t h ro u g h a n e x t e r n a ll i q u i d n i t r o g e n c o l d t r a p a n d th e V a r i - V a c a d j u s t a b l e l e a k .t h i s c om p ar is on a re summarized i n Table 11. The medium pressure gaugegave con s i s t e n t l y l ower r ead i ngs t han t he h i gh p re s s u r e gauge by amountst h a t a re d e f i n i t e l y g r e a t e r t ha n the systematic a nd e x p e r i m e n t a l e r r o r s o fthe two gauges. I t i s p o s s i b l e t h a t a d s o r p ti o n a nd d e s o r p t i o n e f f e c t s a rer e s p o n s i b le f o r t h i s s y st e m at ic d i f fe r e n ce i n t h e r e a d in g s .

R e s u l t s o f

When medium and low p r e s s u r e McLeod gauge re a d in g s were t aken o ff o u r d i f f e r e n t p r e s s u re s o f p ur e n i t ro g e n i n a con t i nuous f l ow fa s h i on ,t h e s a m e s o r t of r e s u l t s were ob t a i ned as d i s cus s ed above . In t h i s case,t he low p re s s u re gauge y i e l de d r ead i ngs t h a t w e r e 8 t o 10 percen t be lowthos e ob ta ine d wi th th e medium pre ssur e gauge . These read i ngs are showni n T ab le 111.

There a r e a t l e a s t t h r ee p o s s i b le e x pl a na t io n s f o r t h e se r e s u l t s .The f i r s t i s t ha t w a t e r vapo r i s pre sen t i n t he McLeod gauges and i s be i ngcondensed ou t by t he low pre ssu re gauge .i s be i ng ads o rbed i n t he gauges -- t o a g r e a t e r e x t e n t i n t h e low p r e s s u r egauge.

The second i s t h a t t h e n i t r og e n

The t h i r d e x p l a n a t i o n i s th a t ou tga ss in g f rom th e mercury more


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COMPARISON O F MERCURY MANOMETER AND HIGH PRESSUR McLEOD GAUGEREADINGS FOR A CONTINUOUS FLO W O F DRlD LABORATORY A IR

( och pressure value below i s an overage o f f i ve read ings )

AVRAG PRESSURE VALUES

M

mm ffg

McLeodGougePressure

PHPGI . 4.20 mm ffg2 . 3 0 13.386

5.7799.721

1 5 . 7 5 6

- 0.21 %+0 . 0 4+0 . 0 3- 0.8/4-1 . 6 7-I- . 2 5

STANDARD DE Vl A TIONS OFAVERAG PRSSUR VALUS

Mer curyMonometerDevia ion

0.009 mm Hg0.037

0 . 0 1 90 . 0 1 60 . 0 / 60 .009

Mc LeodGouge

Dev i 0 ion

0 . 0 0 3 mm Hg0 . 0 0 3

0 .007

0 . 0150 .038

0 .010

/9


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COMPAR/SON O f H I G H PRESSURE AND MEDIUM PRSSUR McLOD GAUGtFRAD/NGS FOR A C O N TI N U O U S FLOW O F PRPUR/F/D N I T R O G E N

- .H i g h

Pr es u reGauge Deviation

( E a c h p r es s u r e v a l u e b e l o w i s a n a v e r o g e

M e d i u mP r e s s u r eGouge Deviat ion

o f f i v e r e a d i n g s 1

P r e s s u r eGauge Reading%PG

A Y E R A G PRSSUR V A L U E S

%#PG- ffPGHPG

H i g hP r e s ure

PHPG

0 . 001 m m f f g

0.0020. Of0.003

m m f f g

. 77. 2 9 4.363

O.OO/ m m f fg0 . 0 0 2o.oo/0.003

M e dium I E r r o r

0 . 0 855 m m f fg

0. ? 40.2900.350

- 2 . 8 9 %2.08f .503.58

i20


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COMPAR/SON OF AifD/UM PRS.SUR A ND LOW PRESSURE McLEOD GAUGf f A D / N G S FOR A CONTINUOUS FLOW O F PUR N/Tf fOGAf

3.5/3.764 . 5

1 I

3.2/ 8 . 5 53 .45 8.253 . 8 / 8.20

MD/UM PRESSURGAUGE READ/NGMPG

LOW PffSSURG A M R AD / N GLPG

/ . 4 6 mm Hg 1.32 mm Hg - 9 . 5 9 %

2/


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e f fe c t i v e l y i n c rea s es t he p re s s u re i n t he s ma l l e r oiediuzn p re s s u re gauge .

PRESSURE RISE MEASUREMENT TECHKIQL'E

I t may be of i n t e r e s t t o d e sc r ib e b r i e f l y a technique which w ea re c u r r e n t l y u si n g f o r d i a g n o s t i c p u rp o se s , t h a t i s , t o h e l p u s determinehow the gauges and other system components a r e behaving . In t h i s t e c h n i q u et he s y s t em i s f i r s t baked and t hen pumped down t o a low background pressure.With the pumping valve open, a s m a l l gas flow from a 1 l i t e r gas b o t t l e i se s t a b l i s h e d i n t h e pumping m a n i f ol d , and t h e e q u i l i b r i m p r e s s u r e i n t h e

measuring chanber i s n o t e d . A t a convenien t t ime, thP piimping valve i sc l o s e d a nd t h e p r e s s u r e i s al lowed to r i s e i n t h z s y st e m. I f t h e l e a k i smol ecu l a r , t he p re s s u re i n t he sy st en i w i l l r i s e I n a n e x p o n e n t i a l f a s h i o n ,t he i n i t i a l phase of which i s p r a c t i c a l l y l i n e a r . Dttring t h i s p e r i o d o fr i s i ng p re s s u r e , va r i o us measu rement s a r e trade w i t h t he d i f f e re n t gauges .

Measurements of t h e r i s i n g p r e ss u r e a s a a d e w i t h a n i o n i z a t i o n

gauge a r e shown in Fig ure 6 .i n t e rm i t t en t u s e o f t he i on i z a t i o n gaage . The gauge i s t u rned on fo r 15seconds every 10 mi nu t es .f i r s t 60 m in ut es . By way of c o n t r a s t , c u r v e B r e p r e s e n t s t h e r e a d in g so b t a i n e d w i t h t he i on i za t i o n gauge on con t i nuous l y . Although the ion iz inge l e c t r o n c u r r e nt w a s mai n t a i ned a t the low val ue of 100 microamperes i ne a ch c a s e , t h e e E f e c t of io n i za t io n gauge panping i s s t r i k i n g l y e v id e n t.

Curve A r e p r e s e n t s t h e r e a d i n g s o b t a i n e d v i a

The measured pressL.re r i s e i s l i n e a r d u r i ng t h e

When McLeod gauges ar e inc lud eJ i n t he cl os ed s y s t e m i n whi cht h e p r e s s u r e i s r i s i n g , one can measure the ev o lu t io n of condensab les fromt hes e gauges by t ak i ng i n t e rm i t t en t i on i za t i on gauge r ead i ngs w i t h and w i t h -ou t a l i qu id n i t ro ge n co ld t ra p sep ara t in g the McLeod .1nd io n i za t io n gauges

2 2


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N G U R 6MEASURSEMENT O F RISING PRSSUff /N ACLOSED SYSTM WITH AN /ON/ZAT/ON GAUGE

-USED E I ~ H E R o w I N u o u s u ORIN TERM rTENTLYA I INTERMITTENT READINGSBI CON r/wous READINGS

TIME ( m i n u t e s )0 ' 20 40 60 80 /oo 120 / 4 0 /6 0


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as s hown i n Fi gure 7. W t hout t he col d tr ap, t he i oni zat i on gauge measur est he par t i al pr essures of t he mer cur y vapor , wat er vapor , and ot her conden-sabl es as wel l as t he ni t r ogen pr essur e, as shown i n cur ve C. The rateof r i se of t hi s t ot al pr essur e, however , can be compared wi t h t hat of t hepur e ni t r ogen r at e of pr essur e r i se.

Usi ng t he pr essur e ri se t echni que, t he r eadi ngs of an i oni zat i ongauge can be compar ed wi t h t hose of a McLeod gauge.shown i n Fi gure 8 . No col d t r aps wer e pr esent i n t he syst em Thi s par -t i cul ar experi ment s howed t hat t he total r at e of pr essur e r i se of bot h con-densabl e and noncondensabl e gases was measur ed t o be t he same wi t h a McLeodgauge and an i nt erm t t ent l y oper at ed i oni zat i on gauge. I n t hi s case bot ht he i oni zat i on and McLeod gauges measur e t he pressur e of t he condensabl esas wel l as t he pur e ni t r ogen. The i oni zati on gauge al so measures t hemercury vapor pr essur e, and t hi s must be s ubt r act ed out i n order t o compar et he r eadi ngs. When t he evol ut i on of condensabl e gases i s smal l enough, acol d t r ap woul d be used t o keep mercury vapor out of t he i oni zat i on gaugedur i ng t hi s compar i son of gauge r eadi ngs.

Such a Compar i son i s

ACKNOWLEDG?l ENTSI woul d l i ke t o expr ess my t hanks to Dr . R. F. K. Herzog f or hi s

many cont r i but i ons t o t hi s pr oj ect and to A. Supr enar d and F. Robert f ort echni cal assi st ance i n t he const r uct i on and mai nt enance of t he syst emThi s wor k was per f or med under t he sponsor shi p of t he Nat i onal Aer onaut i csand Space Adm ni st r at i on.

24


29/31

28

26

2 422

20

l8

/6/ 4/2

/ O8

.64200 30 60 90 /20 /50 /


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504846444240

38363432302826242220/8I 6

/ 4/2

/O864200 30 60 90 /2 I50


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REFERENCES

1. Dus hmn, S. , Sci ent i f i c Foundat i ons of Vacuum Techni que ( . Tohn W l ey &Sons, I nc. , New Yor k, 1949) .

2 . Leck, J . H. , Pressur e Measur ement s I n Vacuum Syst ems ( The I nst i t ut e ofPhysi cs, London, 19571, pp. 132- 7.3 . Gut hr i e, A. , and Waker l i ng, R . K . , Vacuum Equi pment and Techni ques( M: , Gr awHi l l Book C o . , I nc. , New Yor k, 1949), pp. 141- 3,4. LeBl anc, .I . A. , "VacuumGauge Cal i br at i on Syst em" i n Nat i onal Symposi umon Vacuum Technol ogy Tr ansact i ons, ( Per gamon Pr ess , New Yor k, 1956) ,pp. 78- 81.5 . Al per t , D. , "New DeVel OFl nent S i n The Pr oduct i on and Measurement of

Ul t r a Hi gh Vacuum" , J . App1. Phys. 2 4 ( 7 ) : 869- 870, J ul y 1953.

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