Duct Static Pressure Mesurement
Duct Static Pressure Mesurement
(OP)
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
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





RE: Duct Static Pressure Mesurement
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
RE: Duct Static Pressure Mesurement
I agree with Kenrad, a simple tube will do. Just ensure that the sealing is perfect.
RE: Duct Static Pressure Mesurement
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
RE: Duct Static Pressure Mesurement
Steve