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HU 186 Room and Duct Hygrostats
Operation
Reverse Acting
Direct Acting
Calibration
Siemens Industry, Inc.
The action consists of a combination of force-balance and flappernozzle characteristics with
internal feedback.
A decrease in humidity causes the membrane sensing element to shrink. The contracting
element, which is attached to the element lever, exerts greater force on the nozzle lever
through the sensitivity adjustment assembly. The nozzle lever closes off the nozzle, building
up pressure above the relay diaphragm. Movement of the diaphragm closes the exhaust
valve and opens the supply valve ball. Supply pressure passes into the control pressure
chamber below the diaphragm. The control pressure increases until its upward force on the
diaphragm is balanced by the pressure in the chamber above the diaphragm plus the bias
spring. The valve spring then closes the supply valve and a new balance point is reached
until a humidity change upsets it. Because of the bias spring, the minimum control pressure
is about 1/2 psig.
Here the basic sensing and lever movement is the same as described above. See Figure 4.
However, the nozzle is in a different location; it's on the opposite side of the nozzle lever.
Thus a decrease in humidity results in the flapper pulling away from the nozzle and
decreasing the control pressure.
The element and nozzle levers pivot on flexure pivots to minimize hysteresis. A bimetal link
on the element lever compensates for ambient temperature changes.
The hygrostat is factory calibrated to pass a control pressure of 9 + 4 psig at approximately
40% rh. The factory sensitivity setting is centered to give approximately 2 psig per 1% rh.
Item numbers shown in parentheses are called out in Figure 5.
Figure 4. Hygrostat Operation.
Technical Instructions
Document Number 155-027P25
February 14, 2005
Page 3
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