Conference Contribution Details
P Xiao, RE Imhof, MEP de Jesus, Y Cui & the TEWL Calibration Consortium.
Contributed Talk, US-Regional ISBS Meeting, Orlando, October 2004.
The need for a reliable calibration method for TEWL instruments, where systematic and random errors are well understood and, above all, where the measured quantity can be related directly to established measurement standards, is widely appreciated. The method we have developed satisfies these criteria. It uses a calibrated microlitre syringe to dispense a small droplet of water, typically 1mg, into a calibration accessory coupled to the measurement chamber of the TEWL instrument under calibration. The instrument is then set to record a time-series of water vapour flux density readings, until the droplet has evaporated and the indicated flux density has decreased to zero. The calibration constant is calculated from the relationship between the mass of the water droplet and the area under the flux density time-series curve. Calibration accessories of different geometry and changes of temperature can be used to control the mean flux density during calibration. In this way, the linearity of response of the instrument can be measured.
The new Droplet Method can be used with all TEWL instruments capable of recording a time-series of water vapour flux density readings, which includes open-chamber, flowing air and condenser-based methods of measurement. We will present the theoretical basis of the technique together with example calibrations for the above three instrument types.
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