FAQ - Answer 3

TEWL measurement speed depends on several factors, including instrumental characteristics, skin condition and the accuracy you want.

The AquaFlux determines TEWL from water vapour flux density measurements performed at a rate of ~2 per second. When you first place the measurement head onto the skin, the conditions in the measurement chamber are perturbed and it takes some time for the instrumental readings to settle. From then on, the software is in control, looking for the end-point that meets your accuracy requirements.

Normal TEWL Measurement
Normal TEWL measurements on acclimatised, healthy skin are quick, because there is no skin surface water (SSWL) to slow things down. According to the TEWL guidelines for open-chamber instruments [1, 2], you need to wait for the probe to recover after every TEWL measurement before starting the next measurement. This is not necessary with the AquaFlux. You can go from site to site without pause and get the job done with maximum efficiency.

This in-vivo site-hopping technique was first tested with a now obsolete Model AF102 AquaFlux in a series of measurements on 7 sites of a volar forearm [3]. The updated measurement times using a Model AF200 AquaFlux were found to be as follows:-

The main endpoint criterion used in the software is a user-specified Standard Deviation (StDev) over a user-specified number of the most recent flux density readings. This is indicated in the screenshots above by the cyan-coloured average bars, where the vertical length is ±1 StDev and the horizontal length indicates the data included in the average. The default settings for the AquaFlux are:-

StDev = 0.075 g/(sq.m h) over the 10 most recent flux density readings.

How this looks in practice is indicated in the uppermost screenshot. These settings were used for determining the measurement times in the table above.

If you want quicker measurements, then you can use a larger StDev setting. This is illustrated in the two lower screenshots with StDev settings of 0.3 and 0.5 g/(sq.m h). Clearly, the flux readings have not settled, but if you use the same settings throughout a study, then the relative TEWL values will be meaningful. These StDev values are typical for open-chamber instruments. The higher sensitivity of the AquaFlux makes them look inappropriate.