Static pressure measurement 1

[rickg]

I need to take static pressure readings in a duct system and was wondering if the static pressure varies along the duct cross section. I know the velocity profile varies along the cross section due to wall friction and therefore the total pressure will be different, but is the static pressure going to be different also?

I started wondering this when looking at the different static pressure probes on the market. Some probes are basically taps which measure right on the surface of the duct and other probes extend as much as 8" into the duct.

Responses will help determine what type would better fit the application.

 

[91479184]

Dear Sir
As far as I know, the static pressure is, by definition, independent of the
velocity. Therefore, it is constant across an horizontal section of straight
tube. It may change with height (in a vertical traverse) or also with the
location in a curve, because of acceleration.
The static pressure probe may however not be perfectly built, and the
measured pressure may therefore include a part of the dynamic pressure. It
is for example the case for a simple open tube, even when its opening plane
is parallel to the main flow. Turbulence around the probe end may result in
an air velocity component perpendicular to the opening, A way to check this
is to measure the pressure differential between the static probe and the
atmosphere out of the duct, at various places or at various velocities.
The problem is then to build the probe and locate it in such a way that the
component of the velocity perpendicular to the probe opening is close to
zero. To get the static pressure with a good accuracy, I would prefer to
measure it close to the duct walls, where the velocity is smallest, and with
a probe having a small opening in a tube perpendicular to the flow, like the
static pressure probe in Prandtl tubes. A very common way in air handling
units is to fasten a small tube perpendicular to the duct wall.
Kind regards
Claude-A. Roulet Tel: + 41 (21) 693 45 57
LESO-PB, EPFL Fax: + 41 (21) 693 27 22
CH 1015 Lausanne email: claude.roulet@epfl.ch
Switzerland

http://leso

http://www.epfl.ch

 

[fanair]

AMCA recomends at least four pressure taps 90 degrees apart. This gives and average static pressure reading around the duct. The static pressure taps are all connected to a peziometer ring which in turn is connected to a manometer. Static pressure taps should also be in straight portion of the duct if possible.

 

[gently]

rickg
I tend to agree with the response of Jul 1, but would like to add another perspective. If you look at the way a Pitot tube works, it has one orifice which is faced into the air-stream and four orifices (orifi ?) perpendicular to the air-stream, each at 90 degrees to each other. This device measures velocity by subtracting the static pressure, being the average of the four perpendicular orifices from the total pressure being the single orifice facing the air-stream.
If the static pressure across the duct was not constant, this device would not work. However, in reference to your question, it is worth noting that the static pressure is measured by four, not one orifice, the intention being to cancel out the effects of turbulence.
Hope this is of some assistance

regards
gently

 

[kyong]

There are two causes of static pressure difference arising from velocity.

One is friction loss. If velocity is zero, static pressure difference by friction loss can't be existing. In the event of the question by rickg, we normally can say there is no velocity component of perpendicular direction of flow. Effect of turbulence to perpendicular velocity component is negligible because it makes only very short movement.

The other is tranform of energy between pressure energy and velocity energy. This should be applied, however, to a streamline, a bunch of streamlines or a flow. This can't be applied across the perpendicular section where no flow exists.

My conclusion is that velocity difference acorss the section dosen't make static pressure difference.

 

I would say that the simplest answer is that if the streamlines are parallel, then the pressure gradient must, by deffinition, be zero across the duct. So yea, Static pressure is indeed constant

 

[Islander]

I do test and balance.(thousands of sp readings)
a probe tap at 4" or 8" should not matter.
...as long as turbulence is not a factor.

turbulence causes me problems at times in high velocity areas of duct like right at the discharge plenum at ahu.
..so I'll move to a better area and reread.