AquaFlux Answer 5
Are AquaFlux TEWL measurements affected by (a) probe angle, (b) contact pressure,
(c) atmospheric pressure, (d) probe temperature, (e) skin cooling, or (f) skin drying ?
(a) Probe angle
Angular dependence may be an issue with TEWL instruments, because
natural convection air movements arising out of the temperature difference between the skin and the
ambient air may disturb the measurement. However, different instruments using different measurement methods
are affected differently, as follows:-
According to published TEWL Guidelines [1, 2]
open-chamber instruments such as the C+K Tewameter can only be used
reliably on horizontal surfaces.
Unventilated-chamber instruments such as the Delfin VapoMeter have
also been found to be affected by probe angle , although the manufacturer claims orientation-independence.
For the condenser-chamber AquaFlux, detailed measurements show that the probe has an orientation-dependence
that is non-symmetrical, depending on whether the RH and T sensors in the wall of the measurement chamber
are above or below the chamber axis . For negative angles, where these sensors are below the chamber
axis, probe sensitivity was found to decrease by as much as ~6% relative to the calibrated value. For
positive angles, where these sensors are above the chamber axis, probe sensitivity was found to remain
within about ±1% of the calibrated value. The probe can therefore be used with all surface orientations
with little effect on sensitivity, as long as its inclination is confined to the positive semi-circle.
(b) Contact Pressure
Increases of contact pressure between the skin and a TEWL measurement
head may produce changes to the skin and changes to the measurement geometry. Changes to the skin include
compression of the underlying tissues and surface stretching, with the skin around the periphery of the
measurement chamber depressed and the skin within the measurement orifice raised into a convex spherical
shape. Changes to the measurement geometry include changes in the relative positions of the sensors and
a decreased separation between the sensors and the skin surface. Decreases of contact pressure may compromise
the seal between the skin and a measurement head and cause leakage and positional drift.
In practice, relatively large contact pressure effects have been reported for open-chamber instruments
[6, 7]. Guidelines [1, 2] recommend that the contact pressure of the probe onto the skin should be kept
low and constant, with measurements within a series preferably performed by the same operator.
For the unventilated-chamber VapoMeter, De Paepe et al
 observed no statistically significant change of TEWL readings (from 7±2 to 8±3[g/(m2h)])
when the contact pressure was increased.
The effect of contact pressure on condenser-chamber AquaFlux TEWL measurements
was assessed in an experiment using a spring balance to measure contact force. Measurements of mid-volar
forearm TEWL were repeated 18 times on the same site with contact forces progressively increased then
reduced in the range 0.1 to 2kg. The average TEWL over all 18 measurements worked out to 8.23[g/(m2h)] with
a CV of 1.7% . It appears, therefore, that the effect of contact pressure on TEWL measurement has
more to do with measurement head design than skin barrier property change.
(c) Atmospheric Pressure
|TEWL measurement methods all rely on evaporimetry, where TEWL is
inferred from the water evaporation flux in the air immediately adjacent to the skin surface. All the
commonly used methods involve the diffusion
water vapour through air, from the skin surface to the sensor(s) and beyond. The associated mass diffusion
coefficient, according to gas theory, changes with
temperature and pressure and this affects the calibration of TEWL instruments.
The effect on open-chamber TEWL measurements was first discussed by Nilsson
. He concluded that, at a given location, weather-related changes of atmospheric pressure could affect
TEWL readings by as much as ±6%. This was deemed to be too small for further consideration.
The effect on condenser-chamber AquaFlux and unventilated-chamber VapoMeter
instruments was investigated by Kramer er al . For the AquaFlux, measurements showed a clear trend
of increasing TEWL readings with increasing atmospheric pressure (gradient = [150±22]x10-4 [g/(m2h)]/hPa).
No trend with pressure was apparent in the VapoMeter measurements (gradient = [13±44]x10-4 [g/(m2h)]/hPa),
although a weak dependence cannot be ruled out, given the relatively large standard deviation of the
(e) Probe Temperature
|Probe temperature may be affected by hand heat as well as by ambient
temperature. The guidelines for open-chamber instruments [1, 2] recognise
probe temperature as an essential variable, with instrumental readings increasing with temperature until
the probe and skin temperatures are similar. Their recommendation is not to touch the probe
before and during measurements. Instead, they recommend to handle the probe with a burette clamp, its
cable, a coating or by wearing gloves.
Unventilated-chamber VapoMeter readings have also been found to be strongly
temperature dependent. By holding the VapoMeter between both hands, De Paepe et
al  caused its temperature to increase by ~6°C and observed an increase of volar forearm TEWL
readings from a baseline of 7±2[g/(m2h)] to 15±6[g/(m2h)], which works out to a temperature coefficient
of ~1.3±1.1[g/(m2h)] per °C temperature change.
By contrast, the condenser-chamber AquaFlux probe shows little effect upon
heating. A typical figure, calculated as the root-mean-square (RMS) average over 10 instruments, is ~0.05±0.06[g/(m2h)]
per °C temperature change .
(g) Skin cooling
|The heat flux caused by the low condenser temperature (conduction through the
air & radiation) is too small to cause the skin surface temperature to change significantly. We went
to some lengths to try to measure an effect, using highly sensitive thermocouples and superglue (PhD
thesis, Don O'Driscoll, London South Bank University, 2001). The main finding was that skin cooling was
dominated by conduction between the skin and the measurement head and any effect from the condenser was
masked by this. No effect on measured TEWL values was found. Don has made a full recovery from his superglue
(f) Skin drying
|There is a measurable effect from prolonged contact with a condenser-chamber
measurement head . The main
finding was that the TEWL decreased at a rate of ~0.1% per minute of contact. Given that a typical TEWL
measurement requires less than 1 minute of skin contact, the effect on accuracy is negligible. These
experiments used an AquaFlux Model AF100 instrument with a condenser temperature of -13.4°C. The
current Model AF200 AquaFlux uses a condenser temperature of -7.6°C, which has even less effect on
||J Pinnagoda, RA Tupker, J Agner & J Serup. Guidelines
for Transepidermal Water Loss (TEWL) Measurement. A Report from the Standardization Group of the European
Society of Contact Dermatitis. Contact Dermatitis 22: 164-78,
||V Rogiers & the EEMCO Group. EEMCO
Guidance for the Assessment of Transepidermal Water Loss in Cosmetic Sciences. Skin Pharmacol
Appl Skin Physiol 14: 117-28, 2001.
||JC Cohen, DG Hartman, MJ Garofalo, A Basehoar, B Raynor, E Ashbrenner
& FJ Akin. Comparison
of closed chamber and open chamber evaporimetry. Skin Res Tech 15:
||RE Imhof, MEP De Jesus, P Xiao, LI Ciortea & EP Berg. Closed-chamber
TEWL measurement:- microclimate, calibration and performance. Int J Cosmet
Sci 31: 97-118, 2009.
||K De Paepe, E Houben, R Adam, F Wiesemann & V Rogiers. Validation
of the VapoMeter, a closed unventilated chamber system to assess transepidermal water loss vs. the open
chamber Tewameter. Skin Res Tech. 11: 61-9, 2005.
||GE Nilsson. Measurement of Water Exchange
through Skin. Med Biol
Comput. 15: 209-18 1977.
||AO Barel, & P Clarys. Comparison
of methods for measurement of transepidermal water loss. Handbook of non-invasive methods and the skin. (J. Serup, G.B.E. Jemec,
eds), pp. 179-84. CRC Press Inc, Boca Raton 1995.
||G Kramer, P Xiao, J Crowther & RE Imhof. Multi-location
Clinical Trials: Do Tewl Readings Change with Altitude ? Poster Presentation, SCC Annual Scientific
Meeting & Technology
Showcase, New York 2015. Click
here to download in pdf format.
||LI Ciortea, E Fonseca, J Sarramagnan and RE Imhof. TEWL
and Stratum Corneum Hydration Changes Caused by Prolonged Contact with a new TEWL Measurement Head. The
Essential Stratum Corneum (R Marks, JL Lévêque & R Vögeli Eds) Martin Dunitz Ltd, London, UK, 2002,
here to download in pdf format.
Home Page Top
||© Biox Systems Ltd