This is an end-of-the-day job, except for the wettest or most demanding applications.

Captured water vapour forms a layer of ice on the condenser. The thickness of this layer grows at the rate of ~26microns for every milligram (mg) of captured water vapour. To put this into context, with a typical volar forearm TEWL of 10g/(sq.m h), it would take over 2.5 hours of continuous measurement to accumulate 1mg of ice.

The main effect from ice build-up is a gradual change of calibration, attributable mainly to the reduction of measurement chamber length. This is illustrated by the measurements below, where ice was accumulated by repeatedly calibrating an AF200 instrument by the droplet method with 1mg droplets.

The blue correlation line indicates a 1% change of calibration for every ~2.5mg of accumulated ice. This translates into ~500 volar forearm TEWL measurements at 10g/(sq.m h).

In practice, you need to keep an eye on the Ice Monitors. You normally leave the instrument running all day and get rid of the ice when the job is done. Before going home, you switch the instrument to stand-by mode and allow the ice to melt and the water to evaporate into the ambient air overnight.

If you do a lot of high-flux measurements, then you may need to stop for ~10 minutes to get rid of the ice. Switch to stand-by mode for a few seconds while you dab off the melt-water on the condenser using one of the fibre-free cleanroom swabs supplied with the instrument. When done, switch back on, close the measurement orifice and wait for the baseline to settle.