9.
DEFINITION OF TERMS
RELATIVE HUMIDITY – is a measure of the amount of water vapour
in the air as a percentage of the maximum water vapour the same air
could carry. In scientific terms, it is the partial pressure of water vapour
divided by the saturation pressure of water vapour at that temperature.
DEWPOINT – is the temperature at which dew would form if the air
were to be cooled. As the air cools, so it can carry less water vapour;
the point at which dew forms is the saturation temperature at that pres-
sure.
SPECIFIC HUMIDITY – is the weight of water per weight of dry air and
is usually expressed in PPM (parts per million) which is milligrams per
kilogram
10.
CHANGING THE BATTERIES
When the BAT symbol appears the batteries should be changed im-
mediately. If this is not done the instrument will continue to work, but the
calibration of the unit can not be guaranteed.
To replace the batteries: Make sure the instrument is off, unscrew the
screw retaining the battery cover on the rear of the instrument using the
correct size flat-blade screwdriver. Remove the two cells and replace
with new alkaline cells to ensure long life. Be careful to observe the cor-
rect polarity. Take care not to over-tighten the screw when re-fitting the
cover, and not to lose the rubber sealing washer.
Note: NiCd or NiMh cells can be used but will result in reduced operat-
ing time. Do not attempt to recharge the cells in the instrument.
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