FAQ - Answer 2
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| 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.
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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 Publications 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.
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(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 on an inverted wet-cup source with each instrument. More...
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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.
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(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.
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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...
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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.
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(C) TEWL Measurement speed
The two instruments work very differently.
The AquaFlux records ~2 flux
readings per second. 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.
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(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?
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(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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