Pressure Pickup Points

[Ravoll]

Can the positioning of the pickup points have an effect on readings?
Problem is starting at stall conditions around 6" depression the PTS software/ Manometer is picking up pressure differences across the orifice,and increases with an increase in depression. By the time I'm stalled to around 16",the software's showing 38 cfm.
I double checked with a hand held manometer and the pressure difference is definitely there.If I open a valve on the test head mounted,the cfm readings climb on from the readings at 38cfm@16" stalled.

I've been all over this thing with smoke and soapy water to see if I have a leak somewhere and find nothing.


I had been running my motors at full tilt with an adjustable "air door" downstream of the orifice.I thought maybe the incoming air stream was causing static with the backside of the orifice,and have since changed to a Speed controller for the motors.
It made no difference ,accept it runs quieter and draws less current.

I played around with the air door and noticed that I can close it all the way,stall the motors up to 16" depression,and have 38 cfm stalled.Open the air door ,and as the depression drops, the cfm start going up.

Kinda at a loss here and have started thinking that the pickup positions are causing this.

Any other ideas out there?

[Brucepts]

In order for you to be seeing a CFM with no flow you would have to have a leak across the orifice plate either the plate itself or the part the plate is held by.

Another possibility would be; How high do you have the averaging set to? If the averaging is set high the buffer will not clear out due to no new readings coming in (stalled airflow) and it will sit at the averaged number it has from the buffer numbers.

[Ravoll]

I found it.It was the air door/bleeder causing problems.Not sealing properly I guess.I took it off and tightened the seals,and it just stays closed now.I do all the airflow trimming with the motor controllers now.Stalled to 16" depression shows a steady 0 cfm.

I didn't think about it during design,but evidently the air moving through the door when bleeding is having a siphon ,or capillary effect on the low side pickup.
I tested both differential pickups @16" stalled, against atmosphere.The one before the orifice is steady,and shows the expected drop in pressure when I crack open the air door, but the pickup after the orifice goes crazy,with pressure readings buffeting up and down.Since it sits more or less directly across from the air door, I reckon the incoming air is effecting it.

I thought putting the air bleeds after the orifice would be OK,but it seems otherwise.So no more air bleeding on the test chamber.
Gonna put bleeders on each end of the motor box.