This is the pressure read on the gauge with the leak port completely clear to atmosphere. The T400 pilot orifice may be set without the gauge. Screw a gauge, with adaptor, into the branch line test port at the top of the thermostat. Lift the lid assembly, opening the leak port fully to atmosphere. The branch test gauge should read zero PSIG. Let the lid assembly rotate to the leak port, putting slight pressure on it to seal off the leak port. There should be a slight time hesitation before the branch gauge starts to raise its pressure, climbing steadily to the main air pressure. If this is not observed, remove the pilot orifice cap-screw from the thermostat. Adjust the pilot screw until you can barely hear a slight hissing sound from the air. Plug the cap-screw opening and rotate the lid assembly lifting the lid assembly, clearing the leak port completely to atmosphere. The Branch pressure should read zero PSIG. Let the lid assembly rotate to the leak port, putting slight pressure on it to seal off the leak port. There should be a slight time hesitation before the branch gauge starts to raise its pressure, climbing steadily to the main air pressure. Repeat adjustment on the pilot screw until you witness a slight hesitation before the branch pressure starts to rise steadily to the main air pressure. While we are on this obsolete thermostat, lets address the pivot adjustment. There are two pivots for the lid assembly, which varies the air flow from the leak port and two pivots on the bimetal assembly. You can make a spring hook with a paperclip to temporarily remove the small spring from the bimetal assembly. Both assemblies then may freely rotate on their pivots. Adjust the pivots allowing the assemblies to rotate freely with no lateral move at all. (Attempt to wiggle laterally with your fingers.) The leak port is only .0012”; therefore, flopping of either assembly will cause major calibration issues. (This pivot adjustment applies to all Johnson Control instruments with pivots.) Re-install the spring. Calibrate the thermostat as per page six. The calibration screw is at the bottom of the thermostat under the dial. Contaminants in the main air supply can be devastating to a pneumatic control system. (see report on Compressed Air Care starting at page 9-160) If the pilot orifice becomes plugged, the branch signal drops to zero PSIG. If the leak port (flapper nozzle) becomes plugged, the branch signal is locked at the main air pressure. Do not assume that factory calibration stated on the shipping box is accurate. I once checked three hundred new day/night thermostats, from a major manufacturer, for day set point, night set point and change-over point on one of our calibration boards. I did not find one (not one) that was correct for all three functions of the three hundred (300) thermostats. Other issues found were dead bimetals, wrong action bimetals, thermostats non-responsive to the dial and thermostats that would not build satisfactorily to maximum branch pressure. If a thermostat that is factory calibrated correctly for a control point of say six PSIG and your application requires a control point of 13.5 PSIG, it is still out of calibration by three Fahrenheit degrees, based on the common sensitivity of 2.5 PSIG per Fahrenheit degrees. Factory calibration being correct is based on luck, not good instrumentation. DAY/NIGHT HEATING THERMOSTATS Day/night heating thermostats control the room at a lower temperature during unoccupied periods than during occupied periods. The means of altering from unoccupied set point to occupied set point is a change in main air pressure. There are several different combinations achieving the change-over, from manufacturer to manufacturer and even sometimes within the same manufacturer. Some common arrangements have 3.24
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