My main issues on outdoor pickup locations/devices is ice, spiders and wind.
If the pickup is well placed for avoiding ice and wind seems like the spiders think it can be a good place to live and start a family.
Chuckle, true enough.
Originally Posted by sysint
But do you know of any solutions besides checking time to time?
Its about like a conversation I had with an in-house maintenance guy. He had several VAV's with air flow sensors not working right. Well, actually, he didn't know what the real problem was. Just knew they weren't working right.
I disconnected tubing to pickup tubes and blew 25 psi air through em, gotten from nearby control line. In most cases, fixed his broken, except one when tubing itself was cracked and another where pitot inside duct was broken.
Told him, "Yeah, they never mention this sort of things to guys, but a good idea to blow out or otherwise clean out such things. Over time little holes get clogged with dirt/dust, bugs, pieces of insulation, or whatever."
Osiyo - great points and I do set them to the lowest setting 0.1" wc, but most 'specs' I've come across seem to favor 0.5. I like the bi- directional setting for trending purposes. I agree once your below 0.0" wc it doesn't matter much if your sensor is sensing correctly. Sometimes the sensor is off just a bit or being affected adversely so I like to test slightly positive and slightly negative according to the sensor aka some simple cracked door and paper tests just to see if air is going in or out then. Another thing I've recently found it useful for is alarming. > -0.025 at one building the gals have a hard time opening doors. So alarm at < -0.02" wc if > 5 minutes. Long story but building under construction yet and too many holes and during certain construction activities we can't bring in any outside air. Let's just say my trends were extremely useful in proving out a design flaw and a construction exhaust problem.
On another job, my first real 'learner' on wind effects we were able to find the correlation between consistent wind speeds above a certain value and the building going negative. That's the building I found the center of the flat roof to be the ideal sensing location. Ever since if I can get my outdoor reference to such a location I will.
Do you need to increase the size of the outside pressure tubing if you are going to be sharing that reading with multiple sensors? I plan on using one outside pickup port for two sensors, and in some cases 4 would be ideal. if the run is less than 100" 1/4" should be fine even if i am splitting in the control cabinet to 4 difference sensors right?
I confess that I'm not a big fan of building space static pressure control but in the right application it can be made to work. A light commercial building such as a medical professional facility with a couple of air handlers is a reasonable application. Larger buildings with multiple air handlers probably deserve air flow tracking. (Yes, it's expensive and requires careful setup)
There's a lot of good information in the thread:
Filtering Inputs: Critical to good performance. The signal to noise ratio when measuring space pressure is poor. Smoothing the input will improve both the perceived performance and the actual performance. If your DDC product doesn't provide for filtering inputs you can use a rolling average of the measured input.
Transmitter/Sensor selection: Given that you're trying to control to a small differential I strongly recommend using a bi-directional transmitter. If you were trying to control to a discharge temperature of 55 degrees you wouldn't select a temperature sensor that couldn't indicate less than 54 degrees. The output of your PID statement is going to be largely a function of deviation from setpoint. In order to accurately measure the deviation if the building goes momentarily negative a bidirectional transmitter will be necessary. When you select a bi-directional sensor read the spec sheet carefully. It seems that some manufacturers will note a transmitter as being "0-.1 iwc / bidirectional". In actual practice the sensor span may be -.1 to +.1 for a span of .2".
Control Sequence: Hopefully your building envelope is tight and you're bringing in the required minimum amount of air for ventilation purposes. If so, controlling the speed of the return fan should suffice.
Setpoint: .05" is a good starting point. Go lower if you can. You know that if the outside doors don't close properly you need to reduce the setpoint.