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| FAQ - Answer 1
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| How do AquaFlux TEWL measurements
compare with conventional open-chamber measurements ? |
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The
AquaFlux out-performs all its competitors in terms of accuracy, sensitivity,
repeatability, reproducibility and versatility.
Below, we present data on three aspects, namely
(A) how do they correlate, (B) how do the readings compare
and (C) where's the difference.
(A) How do they correlate?
In ideal conditions, open-chamber measurements correlate well
with condenser-chamber measurements. This was illustrated in
a recent side-by-side comparison using Tewameter TM300
and AquaFlux AF200 instruments. The aim of the study was to assess
skin damage and subsequent recovery caused by a 2% SLS solution
applied occlusively for 24 hours on mid-volar forearm skin
[1]. The experiments were performed on 22 healthy volunteers,
with measurements taken at baseline, 2.5 hours after patch removal
and repeated 24, 48, 72 and 96 hours thereafter. In all, over
650 same-site TEWL measurements, ranging in value from 4 to 83
g/(sq.m h) were performed with each instrument. The
raw data are shown below, where the correlation between their
readings was found to be r=0.98.
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(B) How do the readings compare?
Although the measurements correlate well, the the readings generally
differ. This is due to calibration differences, with no agreement
among manufacturers about best practice. The calibration disagreement
in the above study was found to be quite small, ~14%, but
disagreements by a factor 2 or more are not uncommon. Such
disagreements can be resolved by the traceable droplet calibration
method developed in collaboration with the UK National Physical
Laboratory.
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| (C) Where's
the difference?
The main difference between AquaFlux and open-chamber methods
is in the consistently high quality of AquaFlux measurements, especially
in less than ideal conditions. This is illustrated below, where
an AquaFlux signal is compared with those from two open-chamber
instruments under well-controlled laboratory conditions using
procedures recommended by the manufacturer, but without a shielding
box. Ambient air movements do not affect closed chamber AquaFlux
signals, but cause additional disturbance to open-chamber signals.
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Signal fluctuations cause
scatter in TEWL measurements. One comparative test, for example,
found AquaFlux TEWL repeatability to be more than ten times
better (CV=1.4%) than comparable Evaporimeter
repeatability (CV=16%). See IFSCC
Magazine, 5(4), 297-301, 2002 for details.
How this translates into practice is illustrated in the figure
below, where two skin sites of the same volunteer were repeatedly
tested in alternation.
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| The somewhat higher scatter in the
measurements on the palm of the hand may be due to skin heterogeneity
and imprecise probe placement. Note that no air-conditioning or
other ambient environment controls are needed for such measurements.
The above data were obtained at home, watching TV. |
References
[1] I Angelova-Fischer, TW Fischer & D Zillikens. Die
Kondensator-Kammer-Methode zur nicht-invasiven Beurteilung von
irritativen Hautschäden
und deren Regeneration: eine Pilotstudie. Dermatol Beruf Umwelt.
57(3):125 (2009). |
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