Airspeed to PSI Conversion

There are many times when we have Pitot/Static leaks in lines or fittings that have to be located and replaced.

The usual practice is to isolate the line and apply pressure to it. We then use leak detector to find the problem area (presence of bubbles).

Using this method, there is always a concern of over-pressurizing the lines and splitting one open. I've used more, but ≈ 5 PSI usually is enough pressure to locate leaks.

A low pressure regulator with nitrogen can be used, but we usually just use the Pitot/Static Test Box. Using the pitot output only..... 400 Knots is a common setting.

The question arises as to what 400 Knots is equivalent to in PSI. We use a Barfield DPS1000 . The display can be changed to show different units of measure.

Knots to inches of mercury (inHg).....
  
     
     
     
  
The tester shows two values for inHg, Pt and Qc. A little digging was required to find the difference.....

Impact Pressure (Qc) is the pressure a moving stream of air produces against a surface that brings part of the moving stream to rest. It is the difference between the total pressure (Pt) and the static pressure (Ps). These pressure properties are related by the formula: Qc = Pt - Ps  (From Mensor.com )


The link above is worth a look. Needless to say, we want to reference Qc. Using a online conversion, we found the value 8.38 inHg to be a little over 4 PSI.
  
     
  
A quick PDF might help explain all this.