Duct Static Pressure Mesurement

[tsccgy19]

Hello Group,
I am interested in measuring Duct Static pressure usint a Setra 0-5" WC 4-20mA pressure transmitter to control a VFD. How should I terminate my high pressure hose into the duct. I found at one of our suppliers what they call a Pitot tube which is about 4 inches long by and has a 2" bend at the end which they tell me should face the airflow. I thought a Pitot tube has two tubes. My instinct is to just place a piece of tubing into the duct perpendicular to the airflow. The pitot tube they supplied is closed on the end facing the airflow and had a few small holes on the side...
Thanks,
Steve

 

[KenRad]

tsccgy19,

What the supplier is calling a pitot tube is actually a static pressure probe. When you direct the closed tip of the probe into the airflow, the holes are perpendicular to the flow, and therefore they are sensing static pressure.

As long as your duct penetration is well sealed and you choose a location with a good velocity profile, a tube in the duct that is held perpendicular to flow should give a good static reading also.

---KenRad

 

[VEEKRISH]

tsccgy19,
I agree with Kenrad, a simple tube will do. Just ensure that the sealing is perfect.

 

[DRWeig]

Hi Steve,

Please don't be insulted if you already know this -- but my company sells many thousands of low-pressure transmitters (including the Setra model you mention) each year for duct pressure control through VFD's. You would be very surprised how many folks leave out the actual controller in this scheme. The pressure transmitter gives 4-20 mA out, and the VFD takes 4-20 mA in, so they just hook them together and then call us when they don't get good control (or worse yet, the VFD is direct-acting and thus speeds up on a rise in signal).

Just wanted to be sure you've got a decent PI or PID control module in the loop. If you don't, you'll wind up with simple proportional control over the tranmitter's entire span and you won't be happy. Worst case, you might blow some seams apart in your ductwork....

Oh, and the other answers to your original question are on the mark in my opinion too.

Good luck getting everything working!

Old Dave

 

[tsccgy19]

Thanks for the info. The installers had the holes in the tube parallel to flow so I switched it around and have a stable reading. We are using a Building Automation System to control the VFD and the control seems great so far. In my experimentation, I drilled a hole in the duct beside the pitot tube (static pressure probe) and inserted magnehelics on the new hole and the probe. They both read identical except the connection to the pitot tube is much more stable with less cavitation. I took the extra step to install a high pressure cutout (4.5 "WC)in the duct to be on the safe side and sleep at nights in case the VFD is switched to manual mode or otherwise loses control. Thanks for the help one and all...
Steve