AquaFlux Answer 3
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
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 . 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 to average ~100 VapoMeter readings to match the
precision 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
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
data. 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
From the above it is clear that the much lower instrumental
scatter of the AquaFlux enables it to resolve small differences
in skin properties, 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?
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.
||RE Imhof, P Xiao, EP Berg & LI Ciortea. Rapid Measurement
of TEWL with a Condenser-chamber Instrument. Poster, ISBS World Congress on Non-Invasive Studies of the Skin, Philadelphia, September
2005. Click here to download in pdf format.