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ENGLISH
Instructions
Compact Sling Psychrometer
Catalog No. B61503-0000 & B61503-0100
Principles
The Sling Psychrometer determines %Relative Humidity by measuring
the evaporation of water into the surrounding air.
Two thermometers are placed in flowing air, one thermometer bulb being covered by a wet wick.
Evaporation from the wetted bulb causes its temperature
to be depressed relative to that of the dry bulb.
In dry air evaporation will be rapid and the depression will be greater,
giving a low %RH reading.
In humid air there will be little evaporation and the depression will be small, giving a high %RH reading.
If the air is completely saturated no evaporation can take place. Both the wet and dry bulb thermometers
will give the same reading. This equates to 100%RH.
The %RH can be read off the Slide rule calculator integrated into
the psychrometer. Standard Humidity Tables can also be used.
Taking a reading
1. Open the instrument by withdrawing the inner frame from the case
2. Thoroughly wet the wick by placing the exposed end under cold running water or immersing it in water
for about 30 seconds. This will wet both the exposed wick and the wick coiled in the wick container. Hot
water must not be used as it may damage the thermometers. The wick will remain moist for several
hours. Always check that the wick is wetted before taking any readings
and ensure no moisture remains on the dry bulb.
3. To take a reading, set the psychrometer at right angles and holding
the case, rotate the frame for 30 to 60 seconds at between
2 and 3 revolutions per second.
Pull to open
Slide rule calculator
Wet bulb (covered by wick)
Dry bulb
Wick container
4. Stop the revolving instrument and note the Wet and Dry Bulb temperatures.
5. Close the instrument and use the slide rule calculator to determine the %RH Using the Slide rule
calculator
1. Locate the Wet bulb temperature on the relevant scale.
2. By sliding the inner frame out of the case, align the Dry bulb
temperature with that of the Wet bulb.
3. Read the %Relative Humidity from the centre scale at the location of the arrow
When the psychrometer is closed the slide rule can be used to calculate the %Relative Humidity
directly from the Wet and Dry Bulb temperatures.
The calculator has two scales; the upper scale should be used for dry bulb temperatures up to 70°F
(or 20°C on the Celsius reading model).
On higher temperatures the lower scale should be used.
To calculate the %RH reading:
In the picture above, the Wet Bulb temperature of 50°F is lined up with the Dry bulb temperature of 60°F.
This gives a reading of 49% Relative Humidity.
Maintenance and spare parts
If the wick becomes worn or dirty it can be cut off and replaced with a wick from the wick container. The
wick container can be removed with a twisting action and a new length of wick withdrawn.
Broken thermometers can be replaced by removing the screw
at the swived end of the frame.
A Spare Kit comprising two replacement thermometers and a spare container full of wick is available for
the compact psychrometer. When ordering the spare kit please state whether the thermometers are °C or °
F.
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