MEASUREMENT OF TWEL (TRANS EPIDERMAL WATER LOSS)

Transepidermal water loss (TEWL or TWL) is the loss of water that passes from inside a body (animal or plant) through the epidermis to the surrounding atmosphere via diffusion and evaporation .
FACTORS AFFECTING TWEL.
• ENVIRONMENTAL FACTORS: humidity, temperature, seasonal variation.
• Sweat gland activity.
• Metabolic activity.
TEWL MEASUREMENT.
• TEWL is not measured directly, but inferred from measuring the change (or flux) in water vapor density at the skin surface compared with a point farther away from the skin .
• the units for TEWL are stated as grams of water per square meter per hour .
• TEWL can be measured using an.
1. open-chamber device, .
2. an unventilated-chamber device, .
3. or a condenser-chamber device.
• Because of the sensitivity and variability in measurement of TEWL, usually three or more readings are taken to calculate a mean value. .
TEWL DEVICES.
1.Open-chamber devices.
consist of a hollow cylinder that is placed in contact with the skin. Water vapor from the skin surface diffuses through the chamber and out into the ambient atmosphere. .
The humidity gradient is calculated from temperature and relative humidity readings from two sensors that are fixed at different distances from the skin surface.(fig A) .
ADVANTAGE: An advantage of open-chamber devices is that they do not occlude the skin and therefore leave the cutaneous microclimate relatively undisturbed. .
LIMITATION: they are vulnerable to environmental influences, such as disturbance from ambient air movements. .
2.Unventilated-chamber devices .
consist of a chamber with a closed upper end, which protects from ambient air movement disturbances. Water vapor from the skin surface collects in the chamber, causing the humidity to rise with time. Sensors in the chamber measure the rate of increase in relative humidity . .
This method requires the chamber to be lifted from the skin after every reading to allow the accumulated water vapor to escape. These devices therefore cannot be used for continuous TEWL measurement.(fig B) .
3.condenser-chamber device.
has become increasingly used, because it provides a dynamic reading of the trans cutaneous water loss. The upper end of the chamber is closed by a condenser that is cooled below the freezing point of water. .
The condenser removes water vapor from the chamber, enabling continuous measurements to be made without the need to interrupt the measurement to allow the water vapor to escape. .
The condenser also controls the microclimate within the chamber by protecting from ambient air movement and controlling the humidity. The water vapor density is measured in a similar way to open chamber devices by separately spaced sensors in the chamber . (figC) .