Vaisala HM34 - Manual (Page 4)

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2. OPERATION
The meter has four function pushbuttons and a display (Fig. 3).
1. On/off pushbutton (ON/OFF)
To switch the power on, press the ON/OFF pushbutton. This will
automatically start the measurement of relative humidity (%RH).
2. Temp measurement pushbutton (TEMP)
For temperature measurement, press the TEMP pushbutton.
TEMP measurement mode is indicated by a (+) or a (-) on the
display. Display calibration is either in degrees Celsius (°C) or
Fahrenheit (°F) as marked on the sticker on the backside of the
meter.
3. RH measurement pushbutton (%RH)
Relative humidity is always positive. If a colon (:) is indicated in
front of the displayed reading, the meter reads "negative" RH.
This condition may occur e.g. when the meter has not been
calibrated after sensor change.
4. Reading hold pushbutton (HOLD)
Pressing the HOLD pushbutton locks the reading in the display.
A (s) indicates the HOLD mode. The meter remains in HOLD
mode until measurement is resumed by pressing any of the
pushbuttons except ON/OFF.
5. Low battery indication
When battery voltage is low, a (B) is displayed. To ensure
troublefree operation, replace the battery after about 50 h of
operation.
Automatic power-off
The meter automatically switches off after about 3 minutes,
unless a pushbutton is pressed or the meter is set in HOLD mode.
Operating voltage
The meter uses a standard 9 V transistor battery (type IEC 6F22).
The battery is located under a protective lid on the backside of
the meter (Fig. 2).