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AquaFlux Questions >  Answer 1

AquaFlux Answer 1

What is TEWL and how do you measure it ?

(a) What is TEWL ?

  TEWL is the flux of (liquid) water diffusing through the Stratum Corneum (SC), from the Viable Epidermis (wet) to the surface (dry). The skin surface remains dry because the TEWL water evaporates into the adjacent air.


(b) How do you measure TEWL ?

  You cannot measure TEWL directly because that would imply measuring water flux inside the SC. Instead, TEWL is measured indirectly, by measuring the evaporation flux in the adjacent air. Of course, this only works as long as TEWL is the ONLY source of evaporation flux, ie skin surface must be dry and there must be no sweating.

Illustrated on the left are three instruments (evaporimeters) that are commonly used for measuring TEWL. They use three different measuring methods as described below.

     
 

(i) Open-chamber Method

  The measurement chamber comprises a hollow cylinder open at both ends, one end of which is placed into contact with the skin. The other end acts as an exhaust allowing the water vapour from the skin to escape into the ambient atmosphere. Under ideal conditions, the air inside the chamber is perfectly still and water evaporating from the skin surface diffuses through the chamber and into the ambient atmosphere. When water evaporates from the skin surface, the humidity of the air next to the skin increases above ambient humidity while the humidity at the exhaust opening remains close to that of ambient air. Sensors for relative humidity and temperature are positioned on-axis at two locations close to the skin surface, to measure the humidity gradient. According to Nilsson’s diffusion gradient measurement principle [1], this humidity gradient can be used to calculate the flux density (i.e. the amount of water diffusing through the SC per square metre of skin surface per hour).
     
 

(ii) Condenser-chamber Method

  The measurement chamber is in the form of a cylinder about the size of a thimble. One end of the chamber is closed by a condenser maintained at a controlled temperature several degrees below the freezing point of water. The other end of the chamber is open and acts as the measurement orifice that is placed into contact with the skin. The condenser continually removes water vapour originating from the skin, storing it as ice. This maintains a low humidity at the condenser, whereas the humidity at the skin surface increases with increasing water evaporation rate. The resulting humidity gradient is calculated from two humidity values measured at two spatially separated points. Relative humidity and temperature sensors mounted in the chamber wall provide one value. A second value comes from the condenser, where the humidity can be calculated from its temperature without the need for a second humidity sensor. This humidity gradient is used to calculate flux density using the same diffusion gradient measurement principle as the open-chamber.
   
 

(iii) Unventilated-chamber Method

  The measurement chamber is in the form of a cylinder about the size of a thimble. One end of the chamber is closed, the other end has a measurement orifice that can be placed in contact with the skin. The chamber is equipped with sensors for relative humidity and temperature. Water vapour from the skin surface collects in the chamber from which it cannot escape. This causes the humidity to rise with time, slowly at first but linearly thereafter. The flux density is calculated from the slope of the linearly rising part of the curve. After the measurement is complete, the chamber needs to be lifted from the skin to allow the accumulated water vapour to escape, otherwise the humidity would rise towards saturation level.
   

References

[1]   GE Nilsson. Measurement of Water Exchange through Skin. Med Biol Comput. 15: 209-18 1977.

[2]   RE Imhof, EP Berg, RP Chilcott, LI Ciortea and FC Pascut. New instrument for measuring water vapour flux density from arbitrary surfaces. IFSCC Magazine. 5(4): 297-301, 2002. Click here to download in pdf format.


 
     
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