Trans Epidermal Water Loss (Tewl); Principle; The Tewl X Probe - Cortexa Technology DermaLab Mini Instructions For Use Manual

13 Trans Epidermal Water Loss (TEWL)

13.1 Principle

Water loss as measured by the DermaLab
on Nilsson's Vapor Pressure Gradient method, which is
an open chamber method with minimal impact on the skin
being examined and, accordingly, very low bias to the
reading.
Ten temperature/humidity sensors are arranged in the
center of a cylindrical measurement chamber at 5 differ-
ent heights above the skin surface (5 layers). The meas-
urement chamber is open to allow the skin to "breathe"
freely, and the evaporation rate follows Fick's Law of Dif-
fusion when a steady state has been reached.
The TEWL probe measures the relative humidity and
temperature in 5 layers inside the chamber using two
sensor pairs in each layer and calculates the density
gradient of the water evaporation from the skin which
can be converted into a TEWL value in g/m
when the physical sensor arrangement and the geom-
etry of the measurement chamber are considered. As-
suming linearity of the vapor pressure gradient inside
the chamber, it is possible to estimate the absolute hu-
midity at the skin surface in g/m
pairs were used to calculate the TEWL in the previous
version of this product. By adding additional sensors, it is possible to reduce the measurement un-
certainty and get more information about the pressure gradient in the measurement chamber. It
also allows linearity of the pressure gradient (steady state) to be used as a stop criterion for the
measurement.
To obtain comparable and reproducible results when measuring trans epidermal water loss,
standardized measurement procedures are strongly recommended. Guidelines have been pub-
lished by The Standardization Group of the European Society of Contact Dermatitis:
Guidelines for trans epidermal water loss (TEWL) measurement

13.2 The TEWL X Probe

The probe is connected to the input connector on the rear
panel.
Place the probe on the skin as indicated by the arrow on
the front of the probe. The arrow indicates which side of the
measuring chamber shall point towards the skin. Do not
press the probe too hard against the skin - apply light to
moderate pressure.
®
Mini is based
2
/hour
3
. Only two sensor
J. Pinnagoda, R.A. Tupker, T. Agner, J. Serup
Contact Dermatitis 1990: 22: 164-178
CORTEX TECHNOLOGY ApS
47/66