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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 Model 990 Cambridge Thermoelastic
Dewpoint Hygrometer
Hedlin, C. P.
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https://nrc-publications.canada.ca/eng/view/object/?id=176f1d66-1858-4c5e-a6b2-cb7c39b8e7c3 https://publications-cnrc.canada.ca/fra/voir/objet/?id=176f1d66-1858-4c5e-a6b2-cb7c39b8e7c3
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
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1HIN II
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NOTlE
No.
448
L'M1TED DISTRIBUTION
PREPARED BYｃｾ P. Hedlin CHECKED BY GOH APPROVED By NBH
DATE
- August 1965
PREPARED FOR Limited distribution
SUBJECT PRELIMINAR Y EVALUATION OF THE MODEL 990 CAMBRIDGE
THERMOELECTRIC DEWPOINT HYGROMETER
The Cambridge thermoelectric dewpoint hygrometer has recently been placed on the market. It differs from some older dewpoint hygrometers in that it uses the Peltier effect to produce cooling of the mirror. The automatic con-trol removes the possible bias in making observations that may exist with non-automatic units, and the method of cooling, which requires little or no attention, may make it suitable for long-term recording of humidity.
Because of these featur es, it was decided to investigate its operating characteristics and to evaluate the accuracy attainable with it. It was calibrated using moist air provided by the atmosphere producer (Figure I) and subsequently used to measure atmospheric humidity simultaneously with two other techniques. In all cases, the ambient temperature of the instrument, hence the heat
rejection temperature, was approximately 70° F.
A small diaphragm pump was attached to the outlet side of the instru-ment and used to draw air through it at the prescribed rate (approximately 5 to 10 cc/min). Stainless steel tubing was used to convey the air to the hygrometer.
In the initial work with the atmosphere producer, the instrument indi-cated a dewpoint temperature approximately 5 F deg too high. After making the recommended checks, the mirror was cleaned with lens tissue. Following this, the instrument indication oscillated about the corr ect value with an amplitude of 1 or 2' F deg, and a period of roughly one min. The instruction book indicates that this may occur since cleaning removes the nuclei about which water
droplets ar e formed.
After about two weeks, with a total operating time of a day or two, stable readings were again observed. The values agreed quite closely with those of the atmosphere producer at dewpoint temperatures of about 70° F. The agreement was not as good when the dewpoint temperature was lowered. The difference
2
-reached about 3 F deg at -lO°F. This corresponds approximately to the accuracy limits claimed by the manufacturer. The lowest dewpoint temperature reached with it was about -250
F. This corresponds quite closely to the 100 F deg maxi-mum depression claimed for it.
The hygrometer was transferred to the Division's outdoor test station in Saskatoon and a stainless steel tube was placed so that air was drawn from the outside at about 1 ft from the building wall. A wet-bulb psychrometer and a Dewcel, located 30 ft from the building, provided a means of checking the values of the dewpoint unit.
During the early part of its use in the outdoor test station, it occa-sionally exhibited a slow oscillation, having a period of about 5 min at one time and up to 30 min at another. This was characterized by a sudden rise in tempera-tur e of 15 to 20 deg, followed by a gradual decline to about the corr ect value.
It was found that the balance setting was too low and, when correctly adjusted, the instrument again functioned properly. This setting was correct at the outset, and therefore it appears that it may need to be adjusted periodically.
In Figure 2, the values indicated by the Cambridge hygrometer are plotted against Dewcel data. These data were obtained over a period of about
six months. The Cambridge unit in general gave higher values than the Dewcel and wet-bulb instruments.
CONCLUSIONS
The accuracy of the hygrometer was tested in the laboratory over the dewpoint temperature range from 700
F to _250
F. At the higher limit, the accuracy was within about 1 F deg. The error increased as the dewpoint tem-perature was lowered. Most of the points fell within 3 F deg of the correct value.
After the mirror was cleaned, the instrument reading was unstable for a day or two but eventually regained stability.
In the initial operation, some difficulty was experienced in adjusting the llbalance" setting correctly, but once this was achieved it did not appear to need further attention.
3
-The instrument is not of the portable type. The components, however, consisting of the hygrometer. an air pump and associated tubing, weigh less than 40 pounds, and can quite readily be moved.
No special skill appear s to be needed to operate this unit. If there is a mal£unct ion it is usually obvious, since the instrument reading is unstable.
It was used to measure outdoor humidity for a period of about six months, and in that time no adjustments were found to be necessary. In a contaminated atmosphere, however. the mirror might require periodic attention.
