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HU 186 Room and Duct Hygrostats
Use of
Psychrometric
Chart
Installation
Room Type
Duct Type
Maintenance
Siemens Industry, Inc.
A sling psychrometer is probably the most readily available humidity measuring device.
Plotting the wet and dry bulb temperature readings from it on a psychrometric chart
determines the relative humidity.
As an example, assume a dry bulb temperature of 70°F, and a wet bulb temperature of
53°F, measured with a sling psychrometer. Using the psychrometric chart illustrated in
Figure 7, follow the 70° dry bulb temperature line vertically until it intersects the 53°F wet
bulb temperature line. This determines the relative humidity point. In this case, the 30%
relative humidity line goes through the point; therefore, the relative humidity is 30%.
See Technical Bulletin 155-210P25 (TB-145) for instructions covering terminal
roughing-in through the wall plate stage. HU 186 plugs into the terminal connector
through two holes in the wall plate. When facing the wall, the supply connection will be on
the left for a horizontal installation and on the bottom for a vertical installation. Two
captive mounting screws fasten the hygrostat body assembly to the wall plate. The cover
is held in place by two button head cover screws which are tightened with the calibration
tool. Wall Plate Kit No. 180-443 is included with the room hygrostat.
Cut an opening in the air duct. See Figure 10 for dimensions. Fasten the duct box flange
to the duct using four No. 8 sheet metal screws.
Item numbers in parentheses are called out in Figure 8 and Table 2. Clean the
membrane element (13) with a soft, dry brush when dirt or dust accumulates. The
hygrostat should not be totally disassembled in the field. The only parts which should be
replaced are items (5) through (7), (10) through (13), and (16) in Table 2 and Figure 8.
As an initial check, make certain that there is 15 to 25 psig of clean, dry supply air. Use
a pressure gauge and rubber tubing to measure control pressure at the test screw (15).
Figure 7. Psychrometric Chart.
Technical Instructions
Document Number 155-027P25
February 14, 2005
Page 5
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