An Intelligent Chilled Mirror Humidity Instrument

Abstract

An intelligent, chilled mirror humidity instrument has been designed for use on buoys and ships. Our design goal is for the instrument to make high-quality dewpoint temperature measurements for a period of up to one year from an unattended platform, while consuming as little power as possible. The instrument uses a General Eastern Dew-10 chilled mirror sensor and is controlled by an onboard digital processor that is programmable in BASIC. Communications to an external logger are provided by an RS-232 compatible interface. The housing is made of PVC and is approximately 76 cm long ? 11.5 cm in diameter; the complete instrument weight is 5.8 kg. Energy consumption is typically 800 J per measurement; standby power consumption is 0.05 W. A series of dockside tests have been carried out to evaluate the long-term accuracy and reliability of the D10IQ. As a standard, we used an EG&G 200M Dewtrak chilled mirror instrument that was cleaned manually at frequent intervals. We found that the mean difference between the D10IQ and EG&E 200M was roughly 0.9°C, which is within the expected error of our calibration procedure, and that the standard deviation of the difference was about 0.8°C. The variance is contributed equally by a fast time scale random fluctuation and a 5- to 10-day period variation. For our purpose it is most significant that there was very little calibration drift of the D10IQ so long as the mirror reflectance stayed above a well-defined and readily monitored threshold value. Thus it appears that the D10IQ can provide a fairly reliable and accurate means for measuring humidity from unattended platforms.