TECHNICAL EXPLANATION OF MEASURED STATIC AIR PRESSURES

[WAYNE3298]

One reason static pressure measurements in an HVAC system are unreliable is the air is not static. The measured pressures are dynamic and Pascal's law does not apply. Pascal's law that states the pressure is the same in all directions is only true for fluids (air) at rest. In a system where the air is pressurized but is not moving the air molecules bang into each other and fill the entire space until all pressures are the same. That happens almost instantaneously. In a duct since air is compressible the molecules don't reach steady state position because they are moving down the duct and their position changes with the effects of duct friction, elbows and etc. and the air is being compressed and decompresses constantly.
More later.

[lucaspenalva]

Because your readings are not purely static pressure - there will be also velocity pressure due to turbulence and/or incorrect use of instrument.

[bradluke0]

I find the most reliable static pressure readings are obtained by using a pitot tube and pulling it up against the inside of the duct or unit casing . In my opinion the most reliable discharge static is obtained by taking a reading on each side of the duct ( 4 readings ) and averaging them.

[WAYNE3298]

The term static pressure in a dynamic system in my opinion is a contradiction in terms. I have always taken static pressures at the equipment because it is required but I don't have much trust in them. If you can get a good run of duct near the fan inlet that is your most reliable reading.

[Tom Thermo]

If you traverse the duct measuring both dynamic pressure (velocity) and static pressure then you will find variations of both across the duct. Near the wall there may be a thick boundary layer downstream of a turn where the velocity may even be going backwards or "upstream" if measured downstream of a bad corner.

If the pressure tap on the business end of your manometer probe happens to have airflow directed towards the measurement point then the "static" measurement will read high because the kinetic energy contained in the velocity of the flow will be converted into static pressure when it comes to rest at the location of the pressure tap. If there happens to be a steady vortex from an upstream bend whipping right by the measurement point and if it causes an "above average" velocity in the vicinity of the pressure tap then the total pressure, or total energy, might be the same but the static pressure will read lower than expected.

What we really want to measure is the total pressure which includes static pressure plus dynamic pressure. I don't understand why our standard hvac probes are these weird looking static probes instead of a total/static pitot probe that gives both static pressure and velocity of the air. If they just added another hole on the front and another tap on the back we could get both or either.

Do they even sell total/static probes in the hvac industry? But without averaging measurements across the area of the duct (traversing), even the extra data of velocity head plus static head does not reduce the primary uncertainty of the calculation.

[Tom Thermo]

I just measured the ESP of my home unit and got 0.06 wc pressure supply side and - 0.56 return side for a total of 0.62"wc TESP

I don't believe the supply has only 0.06 wc pressure drop but it is remotely possible. I carefully chose each measurement point so that (in my mind) it wouldn't be in a bad part of the duct. Gotta find a way to traverse but I don't know an easy way yet. I just want a good TESP measurement but am not sure if I can ever get one without making it a science project.

I can just drill more holes ... my probes are only 6" long but I need foot long versions.

[WAYNE3298]

I have measured supply side SP'S that readout negative. I have also got readings in AHU'S where the SP after a coil was higher than entering the coil.
If you have a pitot tube disconnect the low side and use it to measure the SP'S. You can use it for SP traverse. I doubt you will like the result but worth a try.

[Tom Thermo]

Thanks Wayne I'm going to put that in my brain and let my three brain cells grind on it for a while. I think I'll move downstream and estimate losses for the intermediate transition. OH YEAH !!! (just killed a mosquito)

[WAYNE3298]

Thanks for all the input. I have just re-read all the posts and all you guys made my day. I've said it before but it's worth saying again "THIS SITE IS GREAT".
I started a post on water where I have a problem job but not much interest so far. It may be the holidays or not much interest in water or maybe not enough information there. If not interested in water say so. That isn't a problem.