What is the operating principle of the VapoMeter?
The VapoMeter is equipped with a closed cylindrical chamber that contains sensors for relative humidity and temperature. When the VapoMeter is in contact with the skin or other measured surface, the relative humidity (RH%) in the chamber begins to increase. The TEWL value is calculated from the information the instrument acquires based on this increase.
The ventilation time of the chamber between measurements is automatically controlled. Relative humidity in the chamber has to decrease back to ambient RH level, before the VapoMeter is ready for next measurement. The ventilation period is longer after high values (after 10 g/m²h about 10 seconds and after 100 g/m²h about 60 seconds). The VapoMeter turns automatically off after the ventilation period indicating that it is ready for the start of the next measurement.
A measurement with the VapoMeter takes 7 to 16 seconds with the standard adapter and 9 to 34 seconds with the nail adapter. The measurement time is automatically controlled and shown on the display; the higher the TEWL, the shorter the measurement time. At the level of 10 g/m²h the measurement time is 10 seconds with the standard adapter and 20 seconds with the nail adapter. Under the level 1 g/m²h with the standard adapter and 2 g/m²h with the nail adapter the measurement time is 16 and 34 seconds respectively.
The measurement principle of the VapoMeter differs completely from the conventional devices in the market. It is a closed chamber system that calculates the evaporation rate from the increase of relative humidity in the measurement chamber. Compared with the open chamber technique, the measurement time is shorter, the device is smaller and portable and ambient air flows do not disturb the measurement. Continuous measurements are not possible, but since a single measurement period is short, up to 2 measurements per minute at low evaporation rates are possible.
The VapoMeter also differs significantly from the Aquaflux closed chamber device. The VapoMeter does not create any artificial micro-climate in the chamber and thus it does not change the skin's normal evporation rate. The VapoMeter does not need daily set-up or calibration procedures nor any de-icing process as do the other devices.
The VapoMeter is the only TEWL instrument in the market that is fully portable and that can be used in field conditions. Also, the VapoMeter is the only one with wireless data transfer.
The VapoMeter has been compared independently with the Tewameter (Courage & Khazaka, Germany).
We have compared the VapoMeter with Evaporimeter EP1 (Servomed, Sweden) and Dermalab (Cortex Technologies, Denmark). The VapoMeter can measure very high evaporation rates accurately.
Accuracy in measuring evaporation rate through a semipermeable membrane is ± 10% (TEWL > 5 g/m²h) or 0,5 g/m²h (TEWL< 5 g/m²h). This is tested against gravimetric evaporation rate (mass change of water samples covered with vapour permeable membrane). The coefficient variation (CV), when measured from water sample, is less than 6%. When measuring the skin, the CV is typically less than 10%.
The recommended measurement range is from 3 to 200 g/m²h, but the VapoMeter can measure evaporation rates beyond these values. With the nail adapter values up to 300 g/m²h can be measured. As the VapoMeter is based on the closed chamber principle, it is accurate in these high values. Conventional open chamber devices begin to underestimate TEWL values already at 50 g/m²h.
The VapoMeter is factory calibrated. Each calibration holds for up to 2 years unless the VapoMeter is used in very variable conditions or on a daily basis continuously. We recommend recalibration every other year or once a year for units in heavy use. Please contact your local representative if you need to recalibrate your VapoMeter.
The calibration service is performed in specific conditions at the manufacturing plant. Evaporation rates through semipermeable membranes are measured at 3 different levels and at different RH conditions. The water loss is calculated from the mass change of the water sample and your VapoMeter is programmed accordingly.
This ASTM based calibration method has been in use for many years and the achieved evaporation rates represent skin evaporation rates very well. This is not the case with so-called droplet calibration methods where water drop evaporation is speeded up during the calibration. These methods do not reflect the skin barrier's natural evaporation speed.
During the measurement the measured skin area has to be sealed by the adapter. Apart from this the VapoMeter is orientation independent and the angle of use does not affect the measurement values. This is due to the closed chamber and the measurement principle.
The standard adapter has on opening diameter of 11 mm and it is designed for use on normal skin sites. The standard adapter can be used for values up to 200 g/m²h. The nail adapter's opening diameter is 4,5 mm and it is fitted with a silicone ring. It can be used for hard surfaces like the nail or for other small difficult areas like the lip, eyelid, armpit, etc. It is also recommended to use the nail adapter when measuring very high evaporation rates, up to 300 g/m²h.
It is necessary to change the measurement mode of the VapoMeter when changing between the standard and the nail adapters. When using a standard adapter the VapoMeter display must show "SdA" upon startup and when using a nail adapter, the display must show "nlA" during startup.
The VapoMeter may be wiped clean with a soft and moist cloth, if necessary. Care should be taken not to touch the sensors in the chamber. The adapters that are in contact with skin can be sterilized separately by autoclave or ethanol.
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