The rate knob sets how fast the cabin pressure will change after moving the dial.  If you are in test mode, turn the rate fully clockwise so the rate will change more noticeably.

 

The dukes should be set such that it pressurizes the cabin to about 700 feet above the field elevation on the dial, until it reaches 5.5 psi, at which point the cabin altitude will rise in lock step with increasing airplane altitude.

 

I fly out of a sea level airport.  I typically put the dial at 3000 feet airport elevation.  This gives me a 3700 foot minimum cabin pressure.  I do this so that, when on a precision approach, which typically start at about 3000 feet, my pulling the power to capture glideslope will not pop the ears of the plane occupants due to a loss of residual pressurization.  If I am flying to a high altitude airport, about half way through the flight I will reset the dial to the destination altitude plus 3000 feet.  If I know for sure I am not going to be doing a precision approach and can descend to pattern altitude with at least 50% power, then I will only add 1000 feet.

 

I set the rate knob at the 500 feet/minute rate and leave it there unless I am doing testing.  You can determine the proper position on your unit during testing by changing the dial and timing how long it takes to change the cabin altitude 500 feet, then loosening and repositioning the know so that, in the 500 foot position, the work RATE is lined up horizontally.

 

If I am cruising in the flight levels where 5.5 psi will not maintain a 3700 foot cabin, I will set the dial at 2000 feet BELOW the flight level I will be cruising at.  This setting gives me a 5.5 psi cabin every time.

 

I always do my descents at 65-70% power so as to have enough boost to maintain the 5.5 psi differential.

 

Note these comments assume the brass plunger on the dukes has been adjusted to the correct maximum pressure differential (mine is set to about 5.6-5.8 psi – this can be done while flying high – note it is very sensitive) and your dial and pressure head are properly matched (you can correct this yourself, but it takes a few flights as it can only be changed on the ground), as the connection between the two is not based on actual values but rather pre-set positions.

 

Pete