Extech Instruments RH490-NIST User manual

User's Guide
Precision Hygro-Thermometer
Model RH490

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Introduction
Congratulations on your purchase of the Extech RH490 Precision Hygro-Thermometer. This device
measures Humidity, Air Temperature, Dew Point temperature, Wet Bulb temperature and Grains of
Water Vapor per Pound of Air. Advanced features include Data Hold, Auto Power Off, and MAX/MIN
reading. This meter is shipped fully tested and calibrated and with proper use will provide years of
reliable service.
Meter Description
1. Humidity & Temperature Sensor
2. LCD Display
3. Power ON/OFF button
4. °F/°C/WBT (wet bulb)/Dew Point button
5. Hold/MIN-MAX button
6. %RH, GPP, g/Kg, Backlight button
Note: The battery compartment is located
on the rear of the instrument
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
12. Grains per pound
1
2
3
6
5
4

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Operation
Power
Press the power button “ “ to turn power on or off.
%RH
1. To select %RH measurement, press the “%RH/GPP/g/Kg” button.
2. Hold the probe in the area to be tested.
3. Allow adequate time for readings to stabilize.
4. The Relative Humidity measurement appears in the top display.
5. 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).
6.
Mixing Ratio (Grains Per Pound or Grams Per Kilograms)
1. To select GPP or g/kg measurement, press the “%RH/GPP/g/Kg”
button.
2. Hold the probe in the area to be tested.
3. Allow adequate time for readings to stabilize.
4. The GPP or g/Kg measurement appears in the top display.
5. 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
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.
Wet Bulb
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.

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Temperature
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
1. Press the ”H MAX MIN” button momentarily to freeze the displayed
readings. ”HOLD” will appear on the upper left-hand side of the
display.
2. Press the ”H MAX MIN” button three more times to return to normal
operation. (Steps through MAX and MIN)
Minimum (MIN) Maximum (MAX) Function
The MIN/MAX mode allows the user to view only the lowest (MIN) or highest (MAX) readings.
1. Press the ”H MAX MIN” button twice and “MIN” appears on the display. The LCD is now
displaying the minimum value recorded in both the top and bottom display. The display will not
change until a lower reading is registered.
2. Press the ”H MAX MIN” button again and “MAX” appears on the
display. The LCD is now displaying the maximum value recorded
in both the top and bottom display. The display will not change
until a higher reading is registered.
3. To exit the MIN/MAX mode, press the ”H MAX MIN” button again.
The “MAX” icon will disappear.
Note: Max and Min values are recorded into memory while the meter is
on. The values are erased when power is turned off.
Backlight
Press and hold the light button “ “ until the backlight turns on. To turn the backlight off, press and
hold the light button “ until the backlight turns off.
Automatic Power OFF
The meter has a default setting that automatically 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, press and hold the ”H MAX MIN” hold button while powering
on. The symbol will not appear when auto power off has been disabled.
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 becomes weak 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.
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
Specifications
Function Range and Resolution Accuracy
0.0 to 10.0%RH ±3%RH
10.0 to 90.0%RH ±2%RH
Humidity
90.0 to 100.0%RH ±3%RH
-30.0 to -20.0oC
-22.0 to – 4.0oF
±1.5oC
±2.7oF
-19.9 to 70.0oC
-3.9 to 158.0oF
±1oC
±1.8oF
Temperature
70.1 to 100.0oC
158.1 to 199.0oF
±1.5oC
±2.7oF
Wet Bulb 0 to 80°C (32 to 176°F)
Dew Point -30 to 100°C (-22 to 199°F)
Mixing Ratio 0 to 1120 GPP (grains per pound)
0 to 160 g/kg (grams per kilogram)
Calculated from RH and air
temperature measurements
Display Dual LCD
Response Time: <30 seconds (90% of final value in moving air)
Operating Conditions 32 to 122oF (0 to 50oC); < 80% RH non-condensing
Storage Conditions -40 to 185oF (-40 to 85oC); <99% RH non-condensing
Power Supply 9 Volt battery
Battery Life Approx. 24 hours
Dimensions / Weight 7.8 x 1.7 x 1.3” (200 x 45 x 33mm); 7 oz. (200g)
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