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Use of a thermopile to measure the supercooling of water
Williams, G. P.
USE
OF
A THERMOPILE TOMEASURE
THE
SUPERCOOLING OFWATER
G.
P
.
Williams/
C
U S E O F A THERMOPILE T O MEASURE THE SUPERCOOLING OF WATER
by
G. P. Williams
At the 1963 meeting of the International Association of Scientific Hydrology in Berkeley, the u s e of a thermopile t o detect the supercooling of s e a w a t e r under a n A r c t i c Ice Island was reported. In M a r c h 1964 s o m e r e s u l t s f r o m t h e s e t e s t s w e r e published (1). The principle
involved is simple: a thermopile c o i ~ s i s t i n g of thermocouples in s e r i e s is embedded in a flat plat; one side of the plate is coated with a non- i c e nucleating m a t e r i a l , the other side left uncoated. If the plate i s suspended in supercooled w a t e r , ice will nucleate and grow on the uncoated side while the coated side will r e m a i n ice f r e e .
The s u r f a c e t e m p e r a t u r e of the ice-covered s u r f a c e will be
0°C;
t h a t of the i c e - f r e e s u r f a c e will r e m a i n a t the t e m p e r a t u r e of thesupercooled w a t e r . The output voltage due t o the t e m p e r a t u r e difference a c r o s s the plate can be m e a s u r e d with a portable potentiometer. It
should be possible, t h e r e f o r e , t o m e a s u r e supercooling of a few hundredths of a d e g r e e with suitable thermopiles. In J a n u a r y and F e b r u a r y 1964 e x p e r i m e n t s w e r e undertaken t o t e s t the technique f o r m e a s u r i n g supercooling i n r i v e r water during f r a z i l ice production. The r e s u l t s of t h e s e t e s t s a r e r e c o r d e d in t h i s Building R e s e a r c h Note.
-- - *
- 2 -
DESCRIPTION
O F
APPARATUSF i g u r e 1 shows a photograph and a n end view of the apparatus which, f o r simplicity, i s t e r m e d a " F r a z i l Ice Indicator.
"
It i s composed of a Beckman and Whitley heat flow t r a n s d u c e r of dimensions 1 by 3 in. and about 1/ 16 in. thick, surrouhded by a bakelite guard plate of the samethickness with dimensions 6 by 3$ in. The bakelite plate and thermopile l e a d s a r e covered with a thin, protective lead covering. A w i r e i s
threaded through the indicator and a weight attached to the end of the w i r e . When the indicator is suspended in a swift flowing s t r e a m , the plate t u r n s like a wind vane, always pointing in a direction p a r a l l e l to that of the c u r r e n t .
CALIBRATION O F THE INDICATOR
Before the indicator could be used to m e a s u r e t e m p e r a t u r e , it was n e c e s s a r y t o calibrate it. A rectangular hole, just fitting the outside dimensions of the indicator, was c u t in the side of a plastic cylinder of 5-in. inner d i a m e t e r . The m e t e r was mounted in it and the edges sealed. The cylinder was filled with crushed ice and placed in a bath of anti-freeze the t e m p e r a t u r e of which could be controlled. A t h e r m o -
1
couple was installed in the a n t i - f r e e z e bath about in. f r o m the c e n t r e of the indicator and the t e m p e r a t u r e of the bath r e c o r d e d on a Leeds and Northrup Type H r e c o r d e r . The output f r o m the thermopile
embedded in the indicator was recorded on a Leeds and Northrup Type G r e c o r d e r .
P
Mechanical s t i r r e r s w e r e used t o keep the t e m p e r a t u r e of the anti- f r e e z e and the crushed ice bath a s steady a s possible during calibration t e s t s . It was found m o s t p r a c t i c a l gradually to cool the anti-freeze and m e a s u r e i t s t e m p e r a t u r e and the output of the thermopile sim~iltaneously. E a c h calibration r u n , cooling the bath f r o m about t 1 . O" C
t o-
1. O°C, took s e v e r a l hours. S e v e r a l p r e l i m i n a r y r u n s w e r e made using different types of r e c o r d e r s , manual potentiometers and different r a t e s of cooling.F i g u r e 3 shows four calibration c u r v e s oStained a f t e r the c a l i - bration technique was considered satisfactory. Calibration c u r v e No. 4 ( s m a l l plates) was obtained without the o u t e r , protective lead-covered bakelite guard plate.
It i s to be noted that the output of the thermopile i s about 30 t i m e s that of the thermocouple. This technique of measuring super cooling thus r e q u i r e s a potentiometer only 1/30th a s sensitive as that required if t e m p e r a t u r e i s m e a s u r e d by thermocoup2e.
Although the calibration was reasonably successful, s e v e r a l problems w e r e encountered. Most, such a s calibrating r e c o r d e r s o r controlling the t e m p e r a t u r e of the bath, w e r e readily overcome. One question concerning the technique remained unanswered. In a flowing r i v e r with the water supercooled a few hundredths of a d e g r e e , would the calibration ( t e m p e r a t u r e -heat fls-w relationship) be valid? In
conditions be valid f o r natural conditions where turbulence and, presumably, thickness of the boundary l a y e r over the plate would be different?
TEST O F THE INDICATOR
The indicator was f i r s t tested t o see whether it would detect s u p e r - cooling, the purpose f o r which it was originally designed. F r a z i l ice was produced in a bath in the laboratory using methods that have been described before (2). A thermocouple was inserted in the bath and the amount of supercooling recorded. The f r a z i l ice indicator was a l s o placed in the bath and the output of the thermopile measured during each run, one side of the indicator having been coated with a non-ice nucleating silicon g r e a s e .
In e v e r y t r i a l ice formed on the uncoated side but not on the
g r e a s e side, and the resulting output f r o m the indicator could be measured readily with a portable potentiometer. It was found, however, that it
was not possible to check the calibration curves because the readings f r o m the thermopile lagged behind the readings of the thermocouple owing t o the time required for the uncoated side 01 the plate t o become ice covered. Figure 4 shows the output of the thermopile and the output of the t h e r m o - couple f o r
a
typical f r a z i l ice run under laboratory conditions.- 5 -
The F r a z i l Ice Indicator was a l s o t e s t e d under field conditions. On 11 F e b r u a r y 1964, a f t e r the calibration r u n s w e r e completed in the laboratory, the indicator was suspended a t two locations in r a p i d s in the Ottawa River. Although f r a z i l ice was observed in the r i v e r , it was being produced a t locations that w e r e not a c c e s s i b l e , and it was not possible, t h e r e f o r e , t o t e s t the a p p a r a t u s in a section where the water was supercooled.
A check was m a d e , however, on the performance of the indicator by f r e e z i n g a thin l a y e r of ice on one side of the plate and suspending it in the rapids. The output of the thermopile was 0.043 MV and 0.052 MV a t two different locations; according t o the calibration c u r v e s , t h i s
indicated r i v e r w a t e r t e m p e r a t u r e s of i-0.032 and f 0. 036"C, respectively.
The output of the thermopile was quite steady and the readings w e r e reproducible, indicating that the technique might be s a t i s f a c t o r y for m e a s u r i n g the t e m p e r a t u r e of r i v e r water close t o 0°C.
DISCUSSION O F RESULTS
Preliminary. t r i a l s suggest that the a p p r a t u s will be s a t i s f a c t o r y f o r "indicating" the p r e s e n c e of super cooled w a t e r . F u r t h e r t r i a l s a r e n e c e s s a r y to discover the field conditions u ~ d e r which it can be used f o r t h i s parpose. The instrument a l s o h a s possibilities f o r m e a s u r i n g with a simple potentiometer the amount of supercooling of r i v e r water. I t s usefulness f o r this purpose w ~ l l have t o be checked by making m e a s u r e - m e n t s in r i v e r s where the amount of supercooling can be m e a s u r e d by
m o r e accurate means.
One problem related to testing the indicator i s that of locating the time and place in the rapids where water i s being supercooled. The location in a r i v e r where f r a z i l ice will be "manufactured" v a r i e s with the extent of open water, intensity of cooling and discharge of the r i v e r . In a l a r g e r i v e r it might require u s e of a cable-car to r e a c h inaccessible rapids where the supercooling occurs. Consideration should be given t o checking the F r a z i l Ice Indicator in s m a l l e r r i v e r s where f r a z i l - i c e - p r o - ducing rapids a r e readily accessible f r o m shore.
REFERENCES
(1) Utersteiner, N. and R. Sommerfeld. Supercooled Water and the Bottom Topography of Floating Ice. Journal of Geophysical Research, Vol.
69,
No. 6, March 1964, p 1057-1062.( 2 ) Williams, G. P. Some Observations on Supercooling and F r a z i l Ice Productio,z. Seminar on Ice P r o b l e m s in Hydraulic S t r u c t u r e s , Internatioaal A s sociation of Hydraulic Research, August 1959.
F I G U R E 1
F R A Z I L I C E I N D I C A T O R
( S I
D E V I E W )
Lon-g Lead Holding meter
Leads to Potentiometer
Guard
Heat Flow Meter
Covering
e
-
Embedded
in Plate
Covering
F I G U R E
2
C R O S S - S E C T I O N O F I N D I C A T O R
(End V i e w )
.1 - 2 . 3 .4 T H E R M O P l LE O U T P U T , M V
F I G U R E 3
C A L I B R A T I O N O F F R A Z I L I C E I N D I C A T O R - O U T P U T O F T H E R M O C O U P L E V E R S U S T H E R M O P I L E O U T P U T
