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.
