Thermostat for small objects with temperature range of 100 to 400 

Bilger, H. R.

Nicolet, M.-A.

English

NASA Technical Reports Server

A FLEXIBLE, S IEPX 
H. R. Bilgerx  and M-A. Nicolet  
C a l i f o r n i a  I n s t i t u t e  of Technology, Pasadena, Ca l i fo rn ia  91109 
/-- - 
- - 
Summary 
0 A f l e x i b l e ,  inexpensive thermostat f o r  t h e  tenqerature range 100 K 
0 
t o  400 K i s  described.  Liquid n i t rogen i s  the  coolant and a  gas 
se rves  a s  t r a n s f e r  medium. The temperature can be s e t  t o  b e t t e r  
than 1 '~  and i s  held t h e r e  wi th in  l/2OK by an e lec t ron ic  con t ro l  
3 system. The r e l s t i v e l y  small  working volume of the  order  o f  lOcm 
al lows quick changes of  temperature, a  d e s i r a b l e  f e a t u r e  i n  t y p i c a l  
/ 
semiconductor experiments . 
*On leave  from 0,nahoma S t a t e  Universi ty,  S t i l l w a t e r ,  Oklahoma 74074 
M 
0 
IC) (ACCESSION NUMBER) 
5 Y -- Jp-- 
'3 1~ (PAGES) (CG3E) 
- 
& 
d 
Ci (i4ASA Ck OR TMX OR A 3  NUMBER) 
2 
33 
(CAI EGBRY) 
https://ntrs.nasa.gov/search.jsp?R=19700005799 2020-03-12T01:26:01+00:00Z
1. I n t r o d u c t i o n  
A number of t h e r m o s t a t s  h a v e  been d e s c r i b e d  i n  the  r e c e n t  l i t e r a t u r e ,  
Some of them c o n f i n e  t h e  t e m p e r a t u r e  f l u c t u a t i o n s  w i t h i n  t h e  t e s t  chamber 
-3 0 t o  less t h a n  o r  10 K by s o p h i s t i c a t e d  mechanical  d e s i g n s  o r  
advanced e l e c t r o n i c  c o n t r o l  s y s  tems, o r  bo th .  - 6, Others  a r e  d e s i g n e d  
0 t o  meet s p e c i f i c  purposes  (' - c o v e r i n g  t e m p e r a t u r e s ' r a n g i n g  from 4.2 K 
t o  h i g h  v a l u e s .  Those of r e f s .  10 and 11 o p e r a t e  w i t h  a  s t r e a m i n g  g a s  a s  
t e m p e r a t u r e  b a t h .  Compared t o  t h e  more c o n v e n t i o n a l  m c ~ h o d s  based on 
s t a t i c  t h e r m a l  e q u i l i b r i a  t h i s  p r i n c i p l e  o f f e r s  t h e  advan tages  o f  speed 
i n  r e s p o n s e ,  h i g h  f l e x i b i l i t y  i n  l a y o u t  and i n t i m a t e  t h e r m a l  c o n t a c t  w i t h  
t h e  o b j e c t  under  i n v e s t i g a t i o n  i n d e p e n d e n t l y  of i t s  mechan ica l  s u p p o r t .  
These  advan tages  a r e  s i g n i f i c a n t  p a r t i c u l a r l y  f o r  s m a l l  o b j e c t s  w i t h  
i n t e r n a l  h e a t  d i s s i p a t i o n ,  such  a s  a r e  encoun te red  f r e q u e n t l y  when s t u d y -  
i n g  e l e c t r i c a l  phenomena i n  s o l i d s .  
The t h e r m o s t a t  d e s c r i b e d  i n  t h i s  paper  combines t h e  f e a t u r e s  o f  t h e  
s t r e a m i n g  g a s  a s  t e m p e r a t u r e  b a t h  w i t h  t h o s e  o f  a n  e l e c t r o n i c  c o n t r o l  of 
moderate  s o p h i s t i c a t  ion .  The r e s u l t  i s  a g e n e r a l  purpose ,  inexpens  i v e  
sys tem.  To p e r m i t  i t s  c o n s t r u c t i o n  by s i m p l e  means, vacuum i s o l a t i o n  i s  
avo ided  and o n l y  commonly a v a i l a b l e  m a t e r i a l s  a r e  used.  The sys tem meets 
t h e  f o l l o w i n g  s p e c i f i c a t i o n s  : 
0 (i) a  t e m p e r a t u r e  r a n g e  e x t e n d i n g  approx imate ly  from 100 K t o  
- 
4 0 0 ° ~ ,  
(ii) a  s t a b i l i t y  and r e p r o d u c i b i l i t y  o f  about  ~ O K ,  
3 ( i i i )  a modest a c t i v e  volume of  t h e  ordel- o f  lOcm , 
( i v )  a  s i m p l e  and f l e x i b l e  d e s i g n  which can e a s i l y  be  adap ted  t o  
v a r y i n g  e x p e r i m e n t a l  c o n d i t i o n s  such  a s  a c c e s s  by l i g h t ,  s h o r t  e l e c t r i c a l  
c o n n e c t i o n s  f o r  East  e l e c t r o n i c  pulses, magnet ic  f i e l d s ,  etc. Xe f e e l  
t h a t  t h e s e  cor id i t ions  a r e  f r e q u e n t l y  encountered i n  o t h e r  l a b e r a t o r  i e s  and 
t h u s  r e p o r t  h e r e  on t h e  main f e a t u r e s  of our the rn ;os ta t .  I n  c o n t r a s t  t o  
r e f s .  1 0  and 11, a  s u i t a b l e  e l e c t r o n i c  r e g u l a t o r  i s  a l s o  d e s c r i b e d .  Th is  
c o n t r o l  sys tem a d j u s t s  t h e  t empera tu re  t o  any  p r e s e l e c t e d  v a l u e  w i t h i n  a  
few minutes  and a l s o  i n s u r e s  t h e  long term s t a b i l i t y .  
A complete  d e s c r i p t i o n  o f  t h e  whole sys tem,  w i t h  drawings ,  d iagrams ,  
p a r t s  l i s t  and s u p p l i e r s  i s  made a v a i l a b l e  e l sewhere  ( see  f i n a l  pa ragraph) .  
11. D e s c r i p t i o n  of t h e  System 
F i g u r e  1 shows a  p i c t u r e  o f  t h e  t h e r m o s t a t .  A 4 l i t e r  dewar w i t h  wide 
neck c o n t a i n s  l i q u i d  N2. P u r i f i e d  a i r  o r  n i t r o g e n  gas  from an e x t e r n a l  
supp ly  f lows th rough  s p i r a l s  o f  copper t u b i n g  immersed i n  t h e  l i q u i d .  The 
co ld  gas  i s  t r a n s f e r r e d  t o  t h e  t h e r m o s t a t  chamber through a  b r a s s  t u b e  
which i s  t h e r m a l l y  i n s u l a t e d  a g a i n s t  t h e  room by t h r e e  l a y e r s  of 1/2" t h i c k  
p o l y e t h y l e n e  foam and one o u t e r  l a y e r  o f  1/4" t h i c k  s h e e t  o f  t h e  same 
m a t e r i a l .  T h i s  t r a n s f e r  t u b e  i s  r i g i d l y  a t t a c h e d  t o  a  p h e n o l i c  p l a t e .  
The p l a t e  i t s e l f  i s  clamped on to  t h e  t o p  o f  t h e  dewar and s e r v e s  as  a  t i g h t  
l i d  t o  i t .  A h o l e  i n  t h e  l i d  s e r v e s  bo th  a s  a  v e n t  f o r  t h e  e v a p o r a t i n g  
n i t r o g e n  and f o r  r e f i l l i n g  l i q u i d  K2 d u r i n g  long  r u n s .  I n  F i g .  1 t h e  
t r a n s f e r  t u b e  h a s  t h e  shape o f  an  elbow s o  a s  t o  r e a c h  between t h e  p o l e s  
of  a magnet o r  over t h e  t o p  of a  bench. The whole u n i t  can be r a i s e d  o r  
lowered w i t h  a  l i t t l e  j a c k  and moved on four  c a r t w h e e l s  (below bottom of 
t h e  f i g u r e ,  n o t  v i s i b l e ) .  
The thermostat c'r~amber housing t h e  h e a t e r  arid t h e  a c t i v e  volume i s  
a t t a c h e d  t o  t h e  end of  t h e  t r a n s f e r  tube w i t h  a phenolic  f l ange .  Poly- 
ethylerze a l s o  s~crrou.nds t h i s  j o i n t  ( l o c a t e d  under t h e  thickened seg rne~ t  of 
polye thylene  i n  F ig .  1) and t h e  chamber asserr~bly (open-ended head s e c t i o n  
of po lye thylene  i n  F ig .  1 ) .  With t h i s  layout ,  d i f f e r e n t  head assemblies  
a r e  e a s i l y  in te rchanged t o  s u i t  d i f f e r e n t  needs. The o b j e c t  under s tudy  i s  
a t t a c h e d  t o  a f r o n t  cover  o f  phenol ic  which s e a l s  o f f  t h e  chamber wi th  an  
O-ring. The f r o n t  cover i s  the rma l ly  i n s u l a t e d  with polyethylene a s  w e l l .  
  his cover  i s  removed i n  F ig .  1 t o  show t h e  chamber opgning.) A s+ensing 
thermocouple i n  good thermal  c o n t a c t  wi th  t h e  o b j e c t  i s  a l s o  i n s e r t e d  i n t o  
t h e  chamber through t h e  cover,  as w e l l  as o t h e r  l e a d s  and feed- throughs  
necessary  i n  t h e  experiment.  The sens ing  thermosta t  con t ro l s  a n  e l e c t r o n i c  
r e g u l a t o r  which provides  e l e c t r i c a l  power t o  t h e  h e a t e r .  For f a s t  response 
t o  changes i n  t h e  due temperature,  t h e  w a l l  chambers a r e  made of t h i n  b ra s s  
t ub ing  and t h e  h e a t e r  c o n s i s t s  o f  p r i n t e d  c i r c u i t - t y p e  meanders of  a t h i n  
meta l  s t r i p  on a f l e x i b l e ,  h e a t  r e s i s t a n t  s u b s t r a t e  about 1" wide and 4.5" 
long .  This  s u b s t r a t e  i s  wound i n  a s p i r a l  a t  t h e  i n n e r  end o f  t h e  chamber. 
The co ld  gas  i s  fo rced  t o  c i r c u l a t e  w i t h i n  t h e  s p i r a l  before  e n t e r i n g  t h e  
chamber proper .  The e l e c t r i c a l  connect ions t o  t h e  hea t e r  and a supplementary 
thermocouple a r e  v i s i b l e  i n  F ig .  1 t o o .  
The gas  l e a v e s  t h e  chamber through ho le s  l o c a t e d  towards t h e  f r o n t  i n  
t h e  t h i n  b r a s s  w a l l .  
 he ho les  a r e  t o o  sma l l  t o  be seen i n  t h e  photograph.)  
The gas  t h e n  flows back along t h e  ou te r  s i d e  of  t h e  wa l l  before  escaping t o  
t he  o u t s i d e  through t h e  tube  p o i n t i n s  dox:.nk:ards i n  F ig .  1. Tnis reverse-f low 
arrangement improves t h e  unifor ini ty  of t h e  temperature wi th in  t h e  chamber 
and c u t s  doxn t h e  requiresiozts  on gas  consl~:iption. A s t r a i g h t  p a t t e r n  of flov: 
can a l s o  be adoptcd, a t  some l o s s  of  performance. Tinis elinliriaies t h e  doilbie 
w a l l  o f  t h e  chamber and s i m p l i f i e s  t h e  mechanical design s t i l l  furth?r. For 
ope ra t ion  above room terzperatures  t h e  det:ar i s  l e f t  empty. 
The e l e c t r o n i c  c o n t r o l  s y s t e a  con ta ins  a commercial component oven 
f o r  t h e  r e f e r e n c e  junc t ion  of  t h e  therniocou?le. A temperature compensa.ted 
Zener diode provides  a s t anda rd  from which a vo l t age  i s  Per ived  whose va1u.e 
equals  t h a t  expected from t h e  thermocouple a t  t h e  thermosta t  temperature 
d e s i r e d .  This vo l t age  i s  sub t r ac t ed  from t h e  a c t u a l  emf of t h e  thermo- 
6 
couple,  t h e  d i f f e r e n c e  i s  ampl i f ied  u p ' t o  about  1 0  t imes and'  f e d  back i n t o  
t h e  h e a t e r .  For convenience, t h e  poten t iometer  s e l e c t i n g  t h e  vo l t age  i s  
0 
re ferenced  t o  a v i r t u a l  e l e c t r i c a l  0 C po in t  and i s  equipped wi th  a d i g i t a l  
d i a l  c a l i b r a t e d  i n  m i l l i v o l t s .  Standard thermocouple t a b l e s  can then  be 
used d i r e c t l y  t o  s e t  t h e  temperature.  
111. Desc r ip t ion  o f  Performance 
The h i g h e s t  teniperature a c c e s s i b l e  i s  g iven  by t h e  so f t en ing  p o i n t  o f  
- 
t h e  polye thylene  foam (about 9 0 ' ~ ) .  Higher temperatures  can be reached 
e a s i l y  by i n s e r t i n g  a l a y e r  of  more h e a t  r e s i s t a n t  i n s u l a t o r  between t h e  
wa l l s  o f  t h e  thermosta t  chamber and t h e  foam. The lowest  temperature i s  
determined by t h e  accep tab le  consumption of  coo l ing  gas .  The cool ing  
c a p a c i t y  of  t h e  gas  decreases  t o  zero a t  7 7 ' ~  and t h e  ope ra t ion  a t  temperatures  
0 
of 100 K and below becomes i n c r e a s i n g l y  was t e fu l .  I n  t h a t  range, a co ld  
f i n g e r  arrangement i s  s u p e r i o r  t o  convect ive coo l ing  with g a s .  
Short- t ime f l u c t u a t i o n s  (minutes and below) measured on a p i ece  o f  
0 b r a s s  weighing a fetr granis a r e  t y p i c a l l y  confined t o  an  i n t e r v a l  of 1/2 K 
over  t h e  whole temperature range of  t h e  system. Lon%-term f l u c t u a t i o n s  
(hours and days) l i e  w i t h i n  the  same l i m i t s ,  The s e t t i n g  o f  the temperature 
6 has an abso lu te  e r r o r  o f  approx imate ly  1 K O  I n  opera t ion ,  t he  thermostat  
consumes t y p i c a l  l y  1 -$/h o f  l i q u i d  n i t r ogen ,  600 t o  700 u/h o f  ( n i t r ogen )  
gas and a maximum o f  70W a t  f u l l  hea te r  power. I t  takes about 5  minutes 
f o r  t h e  new steady s t a t e  t o  be reached when t he  temperature i s  increased by 
a 100 K, and about t w i c e  as long f o r  a  downward s tep  o f  the same magnitude. 
A complete d e s c r i p t i o n  o f  the  thermostat,  w i t h  drawings, diagrams, 
p a r t s  l i s t s  and supp l i e r s ,  i s  conta ined i n  ~70-18959,  We thank D r ,  J, 
Hopkins, NASA E l e c t r o n i c s  Research Center, who encouraged us i n  pursu ing  
t h i s  work. The f i n a n c i a l  support  under NGR 05-002-100 o f  the  NASA 
E l e c t r o n i c s  Research Center, Cambridge, Massachusetts, i s  a l s o  very  
g r a t e f u l l y  acknowledged. 
Figure Caption 
F ig .  1 General  view of t h e  thermosta t  Grithout t h e  e l e c t r o n i c  c o n t r o l  
system). The tiorking volume i s  l oca t ed  a t  t h e  l e f t ,  wi th  f r o n t  
cover removed, t o  r e v e a l  t h e  opening o f  t h e  chamber. 
R e f e r e n c e s  
1. A: van I t t e r b e e k ,  e t  a l . ,  Cryogenics  5, 8 (196Lc). 
2. N. I. Davidenko and I. G. Fakidov,  I n s t r .  Exp. Techn.  No. 5,  1239 (1967). 
3 .  J. B i g e l e i s e n  e t  a l . ,  Rev. S c i .  I n s t r .  - 39,  353 (1968). 
4 .  S .  A r a j s  and J .  W .  Conroy, J a p a n .  J .  Appl.  Phys.  - 8; 614 (1969).  
5. J. Hennephof e t  a l . ,  J. S c i .  I n s t r .  2, 96 (1969).  
6. C. P. P ickup  and W.  R. G. Kemp, Cryogenics  2, 90 (1969).  
7. D. McSweeney e t  a l . ,  Rev. S c i .  I n s t r .  2, 1324 (1965). 
8. B. Sharon  and D.  T r e v e s ,  Rev. S c i .  I n s t r .  - 37,  1252 (1966). 
9. C. R. Duda, Rev. S c i .  I n s t r .  39,  1484 (1968).  
10. H. K. K e s s l e r ,  Rev. S c i .  I n s t r .  - 37, 517 (1966).  
11. W. P. Knox and J. D. C u t h b e r t ,  Rev. S c i .  I n s t r .  - 39, 1181 (1968). 
Figure 1 


A flexible, simple thermostat for small objects and the temperature range of 100 deg K to 400 deg K

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