Manual

Chapter 3 ________________________________________ To Be Noted When Measuring Humidity
CHAPTER 3
TO BE NOTED WHEN MEASURING
HUMIDITY
This chapter describes issues that need to be noted in the measurement of
humidity.
In the measurement of relative humidity and especially in calibration, it
is essential that the temperature equilibrium is reached. Even a slight
difference in the temperature between the measured object and the sensor
causes an error. For example, at +20 °C (+ 68 °F) and 50 %RH, a
temperature difference of ±1 °C between the measured object and the
sensor causes an error of ±3 %RH. If relative humidity is 90 %RH, the
error is about ±5.4 %RH. A graph of the measurement error at 100 %RH
when the temperature difference between ambient air and the sensor is
1 °C is presented in Figure 1 on page 16.
The error is at its greatest when the temperature of the sensor differs from
that of the surroundings and the humidity is high. A difference of a few
degrees in temperature may cause water to condense on the sensor
surface. Efficient ventilation accelerates the evaporation of the condensed
water whereas in an unventilated space, it may take hours. The
HUMICAP
®
180R sensor returns to its normal functioning as soon as
water has evaporated. Contaminated water condensing on the sensor may
shorten its life span and alter the calibration.
VAISALA _______________________________________________________________________ 15