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TH 188 Unit Mounted Thermostat
Calibration,
continued
Single Action
Dual Action
Reverse Acting
Stage
Siemens Industry, Inc.
The direct acting or reverse acting thermostat may be calibrated at any pressure. It is
suggested that 7-1/2 psi (52 kPa) be used.
On the HC thermostat, calibration should be at 7-1/2 psi (52 kPa) for both DA and RA. This
is due to the use of one thermal system and a reversing relay to give the action desired. If
calibrated at 7-1/2 psi (52 kPa) for winter, the summer calibration will be 7-1/2 psi (52 kPa).
If 9 psi (62.1 kPa) is used for winter, 6 psi (41.4 kPa) will be summer's calibrated pressure.
Figure 3 shows the relationship of the DA and RA calibration points. If the same pressure is
desired for both seasons, the RA section of the thermostat needs to be recalibrated (follow
the RA calibration procedures).
See Table 1 for pressures and changeover pressures.
The reverse acting stage is factory adjusted and does not require field adjustment. If the
adjustment is disturbed, re-adjust as follows:
1. With the main air supplies set for 25 psi (172 kPa), connect a PRV with gauge to the
supply side of thermostat and set at 25 psi (172 kPa).
2. Remove knob and cover from thermostat, as shown in Figure 4.
3. Open bleed screw on test port and connect gauge.
4. Rotate adjustment post to give an output within 1/2 psi (3.4 kPa) of calibration pressure.
5. Adjust PRV for 18 psi (124 kPa) air supply; observe the control pressure. If it is not
within 1/2 psi (3.4 kPa) of calibration pressure, adjust the reverse acting adjustment until
it is. Apply thread locker to the screw setting.
6. Remove test port gauge; close bleed screw.
7. Replace cover and knob.
8. Lock knob in place with pointer at bulb temperature.
9. Set knob at desired temperature setting.
Figure 3. Dual Action Calibration Points.
Technical Instructions
Document Number 155-064P25
April 1, 2005
Page 5
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