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Setting the maximum pressure to 5.5 psi:
The 1" brass cap on the outflow valve controls this. The brass cap is under the lid to the left of the gears. There is a set screw that needs to be loosened to turn the brass cap. Clockwise should decrease the max pressure, counterclockwise should increase it. In theory, it should not affect the relationship between the pressure controller and the pressure head.
Calibrating the controller to the outflow valve:
Matching the controller to the pressure head requires realigning the gears to the right of the brass cap. I would fly it to altitude, set the controller to my desired altitude (with the rate knob at max rate). Wait 5 minutes, then mark the position of the right most gear (on the potentiometer) where it is in contact with the small center gear (on the motor). I would then change the controller until I got the desired cabin altitude + 700 feet, and mark the position of the gear on the left (which controls the bellows within the dukes pressure head) where it meets the center gear. I would then land and reposition the gears so that they were both facing the center gear at the marked position at the same time. You may want to watch the rotation of the gears as they change during flight to see if there is more than one rotation required to reach the set point for both gears. May take a few attempts to get right.
Note that the controller has no knowledge of the cabin pressure or actual altitude. All it does is control a motor in the pressure head based on a potentiometer position sensor. It assumes the pressure head is set up to provide the proper response to the position set by the motor.
Eliminating the takeoff overpressure ear pop:
Pin 4 opens a solenoid in the pressure head that connects the vacuum port directly to the bellows. This allows the vacuum to pull the bellows fully open (dump mode). So, if you have vacuum, you want pin 4 grounded when on the ground and taking off (to prevent the pop you get when going to full power). I added a small electric vacuum pump to mine to do this that goes on whenever pin 4 is grounded. Here is the source for it:
https://www.sparkfun.com/ $14.95 - ROB-10398 - Vacuum Pump - 12V
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