User's Guide
Digital Psychrometer
Model RH390
MAX
MIN
°F•°C•WBT
Dew Point
Introduction
Congratulations on your purchase of the Extech RH390 Digital Psychrometer.
This device measures Humidity, Air Temperature, Dew Point temperature and
Wet Bulb temperature. Advanced features include Data Hold, Auto Power Off,
and MAX/MIN reading. Careful use of this meter will provide years of reliable
service.
Specifications
Function Range and Resolution Accuracy
±2%RH (20% to 90%)
Humidity
Temperature
Display Dual LCD
Response Time: <30 seconds (90% of final value in moving air)
Sensor Type Humidity: Precision capacitance sensor
Wet Bulb 0 to 80°C (32 to 176°F) (calculated from RH and Air
Dew Point -30 to 100°C (-22 to 199°F) (calculated from RH and Air
Operating Conditions 0 to 50
Storage Conditions -40 to 85oC (-40 to 185oF); <99% RH non-condensing
Power Supply 9 Volt battery
Battery Life Approx. 24 hours
Dimensions / Weight 200 x 45 x 33mm (7.8 x 1.7 x 1.3”); 200g (7 oz.)
0.0 to 100.0%RH
o
-20 to 70
(-4.0 to 158.0
-30.0 to -19.9
and 70.1 to 100.0
(-22.0 to – 3.9
and 158.1 to 199.0
C
o
o
F)
C
o
F
o
C
o
Temperature: Thermistor
temperature measurements)
temperature measurements)
o
C (32 to 122oF); < 80% RH non-condensing
±3%RH (10% to 20% and 90 to 95%)
±4%RH typical at 1% and 99%
o
C (±1.8oF)
±1
o
C (±2.7oF)
±1.5
F)
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Meter Description
1. Humidity & Temperature Sensor
2. LCD Display
3. Power ON/OFF button
4. °F/°C/WBT (wet bulb)/Dew Point button
5. MIN/MAX button
6. HOLD/Backlight button
Note: The battery compartment is located
on the rear of the instrument
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4
Display Description
1. Relative Humidity %
2. Dew Point
3. Wet Bulb
4. °C/°F
5. Temp/WB/DP Display
6. Maximum
7. Minimum
8. Low Battery
9. Hold
10. RH% Display
11. Auto Off
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MAX MIN
°C °F
Dewpoint
WetBulb
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3
Operation
Power
Press the power button( ) to turn power on or off.
Taking measurements
1. Hold the probe in the area to be
tested.
2. Allow adequate time for readings to
stabilize.
3. The Relative Humidity measurement
appears in the top display.
4. Pressing the °F/°C/WBT/Dew Point
button toggles the lower display
between Temperature (°F),
Temperature (°C), Dew Point (°C),
Dew Point (°F), Wet Bulb (°C) and Wet Bulb (°F).
Dew Point measurements
1. To select the Dew Point
measurement, press the
°F/°C/WBT/Dew Point button until
DEWPOINT (°F or °C) appears in the
lower right.
2. Dew Point temperature is now
displayed.
°C °F
°C °F
Dewpoint
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Wet Bulb measurements
1. To select the Wet Bulb measurement,
press the °°F/°C/WBT/Dew Point
button until WET BULB (°F or °C)
appears in the lower right.
2. Wet Bulb temperature is now
displayed.
Temperature measurements
1. To select the temperature
measurement, press the
°F/°C/WBT/Dew Point button until °C
or °F appears in the lower right.
2. Temperature is now displayed.
Data Hold
Press the hold button ( )
momentarily to freeze the displayed
readings.”HOLD” will appear on the upper
left-hand side of the display. Press the
hold button ( ) again to return to
normal operation.
°C °F
WetBulb
°C °F
WetBulb
MAX MIN
°C °F
WetBulb
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Minimum (MIN) Maximum (MAX) Function
The MIN/MAX mode allows the user to view
MAX MIN
only the lowest (MIN) or highest (MAX)
readings.
Press the MAX/MIN button once and “MAX”
appears on the display. The LCD is now
displaying the maximum humidity in the top
display and the maximum reading of the
selected parameter in the bottom display. The
°C °F
display will not change until a higher reading
is registered.
Press the MAX/MIN button again and “MIN” appears on the display. The LCD is
now displaying the minimum humidity and the minimum second reading in the
bottom display. The display will not change until a lower reading is registered.
To exit the MIN/MAX mode, press and hold the MIN/MAX button for 2 seconds
until the “MAX” or “MIN” icon disappears.
Note: Readings are automatically reset when switching from one display to the
next (MAX to MIN).
Backlight
Press and hold the light button ( ) until the backlight comes on. To turn the
backlight off press and hold the light button ( ) until to backlight turns off.
Automatic Power OFF
The meter has a default setting that automatically
MAX MIN
shuts the meter off after 10 minutes (indicated by
the symbol on the display. This battery saving
feature can be disabled. To disable the auto power
off function, simply press and hold the hold button
( ) while powering on. The symbol will not
appear when auto power off has been disabled.
°C °F
The meter will revert to the default auto-off mode
when powered off.
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Maintenance
Cleaning and storage
1. The meter should be cleaned with a damp cloth and mild detergent when
necessary. Do not use solvents or abrasives.
2. Store the meter in an area with moderate temperature and humidity (refer to
the operating and storage range in the specifications chart earlier in this
manual).
Battery Replacement
When the battery power falls low, the battery symbol will appear on the LCD.
Replace the 9 Volt battery by removing the Philips head screw holding the rear
battery compartment cover to access the battery compartment. Ensure that the
compartment cover is securely fastened when finished.
All rights reserved including the right of reproduction in whole or in part in any form.
You, as the end user, are legally bound (Battery ordinance) to return all used
batteries and accumulators; disposal in the household garbage is prohibited!
You can hand over your used batteries / accumulators at collection points in your
community or wherever batteries / accumulators are sold!
Disposal: Follow the valid legal stipulations in respect of the disposal of the
device at the end of its lifecycle
Copyright © 2011 Extech Instruments Corporation
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