The new Foxboro dew-point recorder, “Dewcel,” which was built for air-conditioning control, has been adapted to meteorological use at Blue Hill Observatory and tested over a 21-month period. These tests indicate that a rugged recorder, having an accuracy within ± 3F° of that of an Assmann ventilated psychrometer in the natural wind (usually unstable conditions) is now available for station use. The moisture-sensitive element consists of two parallel electrical conductors with an A.C. potential of 25 volts between them, wound about a tube covered with wicking containing an excess of lithium chloride crystals. Dew point is indicated by the temperature of the cell as equilibrium between the ambient vapor pressure and lithium-chloride vapor pressure is maintained, rather than by the Diamond-Hinman principle of recording electrical resistance to maintain vapor-pressure equilibrium. A satisfactory shield for the sensitive element against wind, rain, snow and fog has been devised after considerable experiment. Other than re-doping the cell with a lithium-chloride solution every 90–100 days no servicing is required. Dry and wet thermocouple tests revealed rapid fluctuations of the dew point whenever the air is unstable; therefore, the lag of the instrument, 2 to 4 minutes to make 98% of the change to a sudden new value, is advantageous when dew points for synoptic purposes are required.

Of the three general types of dew-point recorders the Foxboro Dewcel appears to be best for routine synoptic weather reports.

The simpler mechanical type of thermal element and hair hygrometer linked has the disadvantages of several linkages and those inherent in the hair hygrometer. It is not readily adaptable to indicating or recording at a distance. Instruments maintaining a thermal element in a wetted condition fail during prolonged periods of freezing. The direct reading photoelectric type dew-point recorder operates over a wider range and with greater sensitivity although there is always an uncertainty between 15° and 32°F whether the dew or frost point is being measured. The photoelectric type requires special care and frequent attention during operation.

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* Expanded from paper presented at the American Meteorological Society meeting, at Cambridge, Mass., Dec. 1946. A report to the U. S. Weather Bureau under Contract cwb-8099, Feb. 18, 1949.