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FAQ - Answer 3
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| What is the TEWL measurement
speed of the AquaFlux ? |
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.
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Normal TEWL Measurement
Normal TEWL measurements on acclimatised,
healthy skin are quick, because there is no skin surface
water (SSWL) to slow things up. 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
site-hopping technique was tested in a series of measurements on 7
sites of a volar forearm. Click here to see the protocol used and main
results. The measurement times for a Model AF200 AquaFlux found from
this study were:-
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| Mean TEWL Measurement time |
41.5 ± 4.4 |
seconds |
| Probe Repositioning time |
6.1 ± 2 .0 |
seconds |
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Accuracy & Measurement Time
As indicated above, measurement time can
be traded for accuracy. If speed is important, then you can set less stringent
TEWL endpoint settings in the software. The effect of this is illustrated
in the three screenshots below.
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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 ±StDev and the horizontal length is 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.
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References
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| [1] |
J Pinnagoda,
RA Tupker, J Agner and J Serup. Guidelines
for transepidermal water loss (TEWL) measurement. A Report from the Standardization Group of
the European Society of Contact Dermatitis. Contact Dermatitis, 22,
164-78, 1990.
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| [2] |
V Rogiers and the EEMCO Group. EEMCO
Guidance for the Assessment of Transepidermal Water Loss in Cosmetic
Sciences. Skin Pharmacol Appl Skin Physiol, 14,
117-28, 2001. |
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