Hart Sceintific FLUKE 1620A “DewK” Erste Schritte Seite 26

1620A "DewK" Thermo-Hygrometer
Getting Started Guide
First, consider that the sensor actually measures its own temperature, not necessarily the tem-
perature of the air around it. Ideally, the sensor's temperature will be the same as the air, but
they might be different under less than ideal conditions.
One such condition is when there is a source of radiated heat that is "seen" by the sensor. Radi-
ated heat tends to heat up the sensor higher than the air around it (try shining a flashlight on
the sensor from some distance away). Some sources of radiated heat to avoid are incandescent
lamps, space heaters, and other high-temperature devices. If such objects cannot be eliminated,
consider using a heat shield between the heat source and the thermo-hygrometer sensor.
The sensor can also be heated by warm objects close by, perhaps a wall that is slightly warmer
than the air in the room or even another sensor placed next to it. For best results, allow adequate
distance between the sensor and any objects that might be at a different temperature than the air.
Other considerations relate to the self-heating of the sensor. Since the sensor contains electronic
circuits that release a small amount of heat, the sensor will naturally be slightly warmer than the
air around it. The calibration of the sensor takes this self-heating into account and compensates
for it. But, factors that alter the natural self-heating can cause errors in the measurement.
Self-heating does depend somewhat on the velocity of the air around the sensor. The sensor
is calibrated in nearly still air. Different air velocities can cause differences in the measured
temperature of the sensor: as much as ±0.06°C for velocities ranging from 0 to 10 cm/s. High
velocities can cause even greater errors that can be as large as 0.15°C below the temperature
measured at low velocities. Thus, it is recommended that the sensor be located where there is
minimal air currents.
The self-heating of the sensor could also be altered by nearby objects that insulate the sensor
from the air. Keep adequate air space around the sensor.
The self-heating of the sensor takes a few minutes to stabilize after the power to the thermo-
hygrometer is switched on or the sensor is attached. For best results, allow 15 minutes for the
sensor to settle after applying power.
The sensor takes some time to respond to large changes in temperature or humidity, such as,
for instance, when the sensor is brought from a cold or humid location to a warm or dry one.
Depending on the difference, the sensor can take several minutes to more than an hour to reach
full accuracy after conditions have changed.
Finally, moisture condensing within the sensor can cause erroneous or invalid measurements.
Condensation can occur if the sensor is brought from a warm, high-humidity environment to a
colder temperature. This can be avoided by first moving the sensor into low-humidity air at the
same temperature for about 30 minutes before it is placed at the lower temperature. If conden-
sation does occur, the sensor should recover once it dries out. This may take several hours.
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