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AquaFlux Questions >  Answer 2

AquaFlux Answer 2

How do AquaFlux TEWL measurements compare with conventional open-chamber measurements ?

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 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.


AF200-TM300 Correlation

(b) How do the readings compare ?

Although the measurements correlate well, 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.

(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.


Sensitivity Comparisons

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%) [2].

How this translates into practice is illustrated in the figure below, where two skin sites of the same volunteer were repeatedly tested in alternation.

Repeatability

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). Click here to download the poster in pdf format.

[2]   RE Imhof, EP Berg, RP Chilcott, LI Ciortea and FC Pascut. New instrument for measuring water vapour flux density from arbitrary surfaces. IFSCC Magazine. 5(4): 297-301, 2002. Click here to download in pdf format.


 
     
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