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FAQ - Answer 2

How do AquaFlux TEWL measurements compare with Delfin VapoMeter measurements ?

The AquaFlux and the VapoMeter have one thing in common: they both use closed-chamber measurement methods. In every other respect, they are very different, right down to the measurement method itself. If it's shirt pocket convenience you want, then forget about the AquaFlux. If it's performance you need, then forget about the VapoMeter.

A comparison between an AquaFlux Model AF102 and a Delfin VapoMeter was published as a poster at the ISBS Meeting in Philadelphia in 2005 (see the Library section of this website, or click here for the pdf file). Here we show a similar comparison with updates to illustrate the performance of our latest AquaFlux Model AF200.

(A) In-vitro repeatability
An in-vitro test can give you an important insight into how well an instrument is performing. Are the readings constant when the flux is constant ?

Below are the results of an experiment consisting of 200 repeat measurements using an upside-down wet-cup source with each instrument. More...

These data are characterised by a coefficient of variation of CV=0.93% for the AquaFlux and CV=10.3% for the VapoMeter. This gives a measure of what each instrument contributes to the scatter in any experiment, in-vivo or in-vitro.

The bigger the instrumental scatter, the less reliance you can place on a single reading. According to Gaussian statistics, you would need >100 VapoMeter readings to match the standard error of a single AquaFlux reading.

(B) In-vivo repeatability
For in-vivo measurements, skin variability and instrumental scatter combine to produce an observed scatter that is larger than either one alone. The figure below illustrates how this works in Gaussian statistics.

Below is a table summarising the scatter of readings observed in an experiment consisting of 12 repeat measurements in rapid succession on 7 untreated sites of the volar forearm of an elderly volunteer. More...

The AquaFlux scatter shows a clear pattern across the 7 sites: lowest in the middle part of the forearm and rising towards either end. There is no such pattern in the VapoMeter CVs. There is also a significant difference in the magnitude of the observed scatter. The mean over all 84 measurements works out to CV=3.8% for the AquaFlux and CV=10.2% for the VapoMeter.

From the above it is clear that the much lower instrumental scatter of the AquaFlux enables it to resolve small differences of skin property, heterogeneity in this case. With the Vapometer, the measurements are dominated by instrumental scatter.

(C) TEWL Measurement speed
The two instruments work very differently.

The AquaFlux records ~2 flux readings per second. TEWL measurement speed depends on skin condition and software settings Flux readings settle quicker with dry, well acclimatised skin than with moist skin. Software settings determine how precisely the flux readings need to settle before a TEWL measurement terminates. The default criterion is a standard deviation of 0.075 g/(sq.m h) in a running average over the last 10 flux readings. You can adjust these values to trade off precision for speed. There is no waiting time between measurements - you can site-hop.

The VapoMeter measures vapour accumulation rate rather than flux. A typical contact time of ~10 seconds is required to produce a reading. This is followed by a recovery period of up to 90 seconds, where the measurement chamber needs to be voided of accumulated water vapour before the next measurement can begin.

AquaFlux and VapoMeter measurement times were compared in an experiment consisting of 12 repeat measurements in rapid succession on 7 untreated sites of the volar forearm of an elderly volunteer. Average repeat-times worked out to ~47 seconds for an AquaFlux Model AF200 and 38 seconds for a VapoMeter.

(D) Quality control
How do you know that you are measuring TEWL and not a momentary sweat gland emission or surface evaporation from a minute quantity of superficial moisture? The VapoMeter just gives you a number. The AquaFlux gives you detailed information in its recorded flux curves.

You've finished a study and are pondering the results. You spot something unexpected and are not sure how to interpret it. With the AquaFlux you can inspect the recorded flux curves and other supporting data. With the VapoMeter you're stuck. Which one gives you more insight into what went on? Which one may save you having to do a repeat-study?

(E) Versatility
The VapoMeter is limited to two spot readings per minute or less. It is therefore difficult to make measurements of changing properties, such as recovery after occlusion or the evaporation of formulation water. By contrast, the AquaFlux measures continuous flux curves, sampled about twice per second. This, together with its controlled microclimate makes the AquaFlux uniquely versatile, providing detailed information about sample property changes with time.

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