Siemens POWERS HU 186 Technical Instructions page 4

Technical Instructions
Document Number 155-027P25
February 14, 2005
Calibration,
continued
Room Type
Duct Type
Figure 5. Hygrostat Assembly.
Sensitivity
Adjustment
Page 4
1. Remove the cover using the calibration tool.
2. Loosen the test screw (2) 1/2 turn. Slide the rubber tubing of a pressure gauge over
the test port to measure the control pressure.
3. Measure the relative humidity near the hygrostat. See the Use of Psychrometric
Chart section.
4. If the room humidity is higher than desired, use the calibration tool to turn the
adjustment screw (1) counterclockwise to lower the humidity. A 1/4 turn of the
adjustment screw will change the set point about 10% rh
5. Cam post (4) is a coarse adjustment and should only be used if the desired set point
cannot be obtained by the adjustment in Step 4. It is primarily a factory adjustment to
obtain a parallel relationship between the nozzle lever (7) and the element lever (6).
1. Connect pressure gauge to control connection on duct box.
2. Measure relative humidity in duct.
3. Remove cap plug on duct cover and insert calibration tool to make adjustments given
in Step 4 above. See Figure 6.
4. Replace cap plug.
Item numbers shown in parentheses are called out in Figure 5.
1. Carefully loosen sensitivity adjustment (8) using a 3/16-inch wrench.
2. Slide the sensitivity adjustment (8) to desired position on the element lever (6).
Sensitivity will be 4 psi per 1% rh at the end nearest the element, down to 1/4 psi per 1%
rh at the end attached to the flexure pivot. See Figure 4. An overtravel spring on the
sensitivity adjustment assembly prevents damage to the nozzle on a reverse acting
hygrostat.
NOTE:
When the sensitivity is changed, the element must be recalibrated for set
point.
HU 186 Room and Duct Hygrostats
Figure 6. Hygrostat Calibration.
Siemens Industry, Inc.