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Technical Note (National Research Council of Canada. Division of Building Research), 1965-08-01
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Preliminary evaluation of the Beckman Model 719 electrolytic
hygrometer
Hedlin, C. P.
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-DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
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PREPARED BY
C. P. Hedlin CHECKED BY G. O. H. APPROVED By NBH
DATE August 1965
PREPAREP FOR
SUBJECT
limited distribution
PRELIMINARY EVALUATION OF THE BECKMAN MODEL 719 ELECTROLYTIC HYGROMETER
PRINCIPLE OF OPERATION
The principle of operation of the electrolytic hygrometer makes it suitable for measuring low humidities such as are encountered in studies
of sealed double glazed windows. The moisture is removed from a
continuous stream of air by P205. and is then electroly?;ed. The amount
of current that is required give s a measure of the amount of moisture in the sampled air.
The Beckman hygrometer (Model 719) was designed to measure
the humidity of air in the range from approximately 1 to 1000 l'pm by
volume (-105°F to +5°F ､･ｷｾｯｩｮｴ temperature) with an air flow rate
that is equivalent in mass to 100 cm 3/min at 70°F and 29.92 in.Hg. A
vacuum pump is used to draw the air through the hygrometer. EVALUATION
To study the operation of this hygrometer. air was saturated at
a controlled dewpoint temperature by passing it through chipped ice in
a
pair of chambers, each 2 in. O. D. and 6 in. high. The saturator was
immersed in a stirred, thermostatted alcohol bath. Cooling was achieved
by circulating the alcohol through a heat exchanger immersed in a second bath containing a dry ice - alcohol mixture.
2
-Initially, a series of tests was made in the dewpoint temperature
range from _53°F to _98°F. In Figure I, the indicated humidity is
plotted against the value corresponding to the saturator temperature.
These observations were made over a period of about 10 hr. The
indicated moisture content was roughly 10 ppm higher than that
corre-sponding to the actual dewpoint temperature. To remove moisture
that might be on the walls of the tubing (l/8 in. stainless steel) leading to the instrument, a drying train containing magnesium perchlorate
was used in place of the saturator, and'it was allowed to run continuously
for several days. The indicated moisJure content ｾｲｯｰｰ･､ slowly,
reaching a minimum :value.iof about 1 j)pm after two days (Figure 2). When reconnected to the saturator, it still gave a value 4 to 5 ppm too high at a dewpoint temperature of _78°F.
It was thought that the problem might be caused by moisture being released by the tube wall or entering the system through a slow
leak. In either case, the small amount of moisture added to the system
would have a significant effect Or) the result at low dewpoint temperatures.
This behaviour is also' charactetistic of
Ｂｲ･｣ｯｭ｢ｩｮ｡ｴｩｯｾＢＬ
in which someof the hydrogen and oxygen is reunited in the cell after being electrolyzed,
and is then re -electrolyzed. As a result, too high a reading is obtained.
This occurs in cells having platinum electrodes but, according to the manufacturers, does not occur in this instrument as rhodium electrodes are used.
The rate at which moisture is released by the tube walls or enters by leakage is approximately independent of the air flow rate
after the system has been operated for several hours. It can therefore
be evaluated by employing two different flow rates. By plotting the
indicated moisture content against the flow rates, and extrapolating the line joining them to the zero flow rate, the error resulting from water originating spuriously, and from recombination, can be evaluated and
a correction applied. To obtain the results in Figure 3, the system was
allowed to operate for about 15 min at each flow rate. For this test, when
the indicated moisture content was 24. 9 ppm, the actual moisture content
was 20.5 ppm. Some results, based on the extrapolation method, are
shown in Figure 4. From these data it appears that this method gives
reliable results.
If the effect is due to water originating in the lines preceding
the cell, a change in the amount of bypass air ought to affect the instrument reading, and the correction described above would have to be done without
•
3-any bypass flow. In the limited number of observations that were carried
out, the bypass flow rate did not appear to affect the result. Consequently,
this may not be the source of difficulty.
The formula for pressure correction given on page 6 of the
instruction manual for the hygrometer is approximate in that the denomination should read "barometric pressure" rather than" 30 inches".
As an excessive amount of moisture will adversely affect the electrolytic cell, the unit was operated within the specified humidity limits
at all times. After roughly 300 hr of use, some difficulty was experienced
in getting the unit to function properly. After being out of service for a
period of a few hours, the cell appeared to be plugged. The resistance
to air flow was very high when it was started up, but would eventually
reach the proper value. Finally, it ceased to function altogether, and had
to be replaced. Very little information about the life of these cells is
available. According to one source of information, the life depends on
two factors: the level of humidity of the gas being tested, and the period of
usage. An estimate of 50,000 ppm hr was given. On this basis, if the
moisture content of the gas was 100 ppm, one would expect a life of 500 hr. It was emphasized that such an estimate was only approximate.
CONCLUSIONS
If extreme care is taken, humidities down to a few parts per
million can be measured with an accuracy of roughly 25 per cent. An
extrapolation technique must be used to obtain this accuracy. Consequently,
more than one measurement must be made and a period of a least half an
hour is required. The level of the humidity being measured must of course
remain constant during this time. Some experience and patience on the
part of the operator is required.
If the extrapolation method is not used, the instrument gives values
that are too high by an approximately constant value. At a concentration of
100 ppm, the error is roughly 10 per cent but at 10 ppm it rises to roughly 100 per cent.
The instrument requires a vacuum pump and some associated air
lines. While the instrument itself can be readily transported, the vacuum
pump is more difficult to move, and the system as a whole cannot be considered to be readily portable.
•
4
-The instrument was operated within the specified humidity limits
at all times. After roughly 300 hr of use, the cell ceased to function
properly and had to be replaced. A general conclusion about the life of
the cells cannot be drawn from such limited experience. Nevertheless,
it is indicative of what may be expected.
The method of producing the air sample has not been proven by an independent method to be reliable at the low dewpoint temperatures
used here. Good results, however, have been obtained with it at higher
temperatures, both at above freezing temperatures and with chipped ice at 10°F.
