Vaisala HMP240 - Manual (Page 9)

HMP240 series
M210300en User's Guide
7
2. ADVANTAGES OF A WARMED SENSOR HEAD
In some measurement applications, unwanted dew formation makes humidity
measurement difficult or even impossible. At a weather station, for example,
high humidity combined with rapidly changing outdoor temperature can
condense the water vapour in the air onto the sensor head. Until this dew
evaporates or dries, it is impossible to obtain a true reading. Dew formation is
also a problem in environmental chambers and in processes involving high
humidity, such as meat processing.
In some applications, rapid temperature changes are the source of difficulty.
Normally, relative humidity sensors must be at the same temperature as the
ambient air or the measurement is incorrect. At +20
°
C and 90 %RH a dif-
ference of +1
°
C causes an error of +6 %RH. This makes reliable humidity
measurements difficult in rapidly changing temperatures.
In other applications, the problem is a combination of both the dew formation
and rapid changes of temperature; a rapid rise in temperature can cause a
corresponding increase of water vapour in the ambient. In these conditions, the
temperature of the sensor head rises more slowly, and can remain below the
dewpoint of the ambient for a while. Dew then forms on the sensor head, and
it may take several hours or in the worst case, several days for the sensor to
recover from the condensation.
Vaisala's HMP240 dewpoint transmitter offers a reliable solution for humidity
measurement in all these demanding conditions. The humidity sensor head is
kept dry by warming it. As the measurement is fully temperature compensated,
changes in the process temperature do not delay the measurement. This means
that the response time of dewpoint measurement is proportional only to the
diffusion time of water molecules inside the sensor head.
The HMP240 has an excellent performance also at normal humidities and in
stable environments, but it will mostly be used in applications where very high
humidity can take place. In these kinds of applications, the use of traditional
transmitters has been restricted.