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User's manual for hytherograph LB-570P
15
temperature or in grams of water per kilogram of dry air), the relative humidity is a
function of temperature. For example, in typical room conditions (20 °C, 50% R.H.),
the temperature inside probe, which is more than 1 °C of ambient temperature, will
indicate decrease in the measured relative humidity of up to a value of about 47%.
Dependencies between different parameters, which are describing the climate, can be
traced back by using the "HUMIDITY CALCULATOR LAB-EL" (KALKULATOR
WILGOTNOŚCI LAB-EL) program.
The thermal inertia, which occurs between the structure of the sensor housing and air
probe in the probe head, is preventing against the alignment of the humidity sensor
temperature with the ambient temperature. The alignment of these temperatures occurs
by heat radiation and heat transfer through air. Thermal radiation encounters a large
thermal resistance of the air, which is surrounding the probe, and because of that it has
a negligible effect on the alignment of these temperatures. Air convection, which
results from the physical phenomenon of thermal expansion of air (lifting the warmer
air to the top) produces a relatively slow flow of air. The only effective way to
exchange the air, which is surrounding the humidity sensor, is forced air movement
around the probe.
This is the reason, why stabilization of the measurement results is obtained
after a while, depending on the value of changes in measurements conditions and,
above all, the intensity of the exchange of air around the measuring probe. For minor
variations (up to a few °C and little more than a dozen of % R.H.) and aeration of
about 0.2 ... 0.5 m/s, the indication stabilization with a negligible error in relation to
the accuracy will occur after about 1 minute. For large changes and a lack of aeration,
the time of result setting strongly increases and it may be about 15 ... 30 minutes.
Particularly disadvantageous phenomenon takes place when we displace the
instrument (for example during the winter) from a cold room to warm room, and when
in this warm room the dew point temperature is higher than the temperature of the cold
room. In such situation it takes place the condensation of water (retting) on the
surface of the cold humidity sensor with rapid increase of the humidity, which is
indicated by the meter. In this case, the results of the measurements will stabilize in
the proximity of the real humidity of the warm room not only after the equalization of
the sensor temperature and air temperature in the warm room, but after evaporation of
the water from the sensor surface , which will occur at approximately 60 minutes.
Therefore, in the winter, during the device transporting it is necessary to secure it (for
example by a cover) against excessive cooling down.
When during the measurements the probe is placed on objects, it should be provided
with the setting in ventilated area (For example on the stand) so air can freely
circulate around the probe. The probe should not touch its vents with objects, as the
temperature and humidity of these items will affect in an uncontrolled manner on the
measurement result. During measurements the probe should be kept as far as possible
LAB-EL Elektronika Laboratoryjna (electronics lab)
15
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