4. Safety instructions
4.2 Special notes on use
•
If the device is brought into the work-room from a cold environment there is a risk that
condensation might form on the electronics. In measuring operations involving ther-
mocouples pronounced changes in temperature may cause substantial measuring er-
rors. You are advised therefore to wait until the device has adjusted to the ambient
temperature before starting to use it.
•
Before using the mains adapter make sure that the mains voltage is suitable.
•
Be sure to observe the maximum load capacity of the sensor power supply.
•
Sensors with their own integrated power supply are not electrically isolated from one an-
other
5. INTRODUCTION
5.1 Measuring humidity, basic principles
Humidity is always present in the atmosphere in the form of water vapor. The
percentage of water vapor per volume of air varies. Saturation vapor pressure
at any given air temperature is the maximum partial pressure of water vapor
(its gas phase) over an even surface of water (its liquid phase). This is temper-
ature-dependent; at each temperature there is a maximum quantity of water
vapor that can be contained in a defined quantity of air. Atmospheric humidity
can be specified either as
absolute humidity
or as
relative humidity
. In
these operating instructions humidity variables will be given first using the new
symbols defined in VDI/VDE 3514 and then in brackets using the old abbrevia-
tions seen in tables issued by the Deutscher Wetterdienst (German Meteoro-
logical Service).
Absolute humidity
d
v
(AH) is also referred to as water vapor content. This is
the weight of water vapor contained in one m3 of a mixture of water vapor and
air. Since 1 m3 will, depending on pressure and temperature, contain a differ-
ent air mass, it is usually easier to refer to absolute humidity per 1 kg of dry
air. This variable is known as the
mixture ratio r
(MH).
Relative humidity U
w
(RH) is the ratio of the
water vapor’s partial pressure
e´
(VP) to the saturation vapor pressure ew´ (SVP) in a mixture of water vapor
and air at
air temperature t
(TT). The temperature at which saturation occurs
(VP=SVP, RH=100%) is known as the
dewpoint temperature t
d
(DT). As
soon as the air falls below this dewpoint temperature the water vapor (gas
phase) condenses into droplets of water (liquid phase). Enthalpy h (En) is the
thermal content in the mixture of water vapor and air.
5.2 Psychrometer
Measuring principle
A psychrometer is a precision measurement instrument incorporating two
highly accurate temperature sensors; it is used to determine all the usual hu-
midity variables. One of these sensors is covered in a cotton sheath; this must
be kept permanently moist with water from a reservoir and cooled by means of
8 ALMEMO
®
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