# Air pressure difference between floor & ceiling

1. Mar 22, 2013

### tinkeringone

Can't figure out how to get my digital manometers to read the pressure difference between the air near the floor vs. at the 8' high ceiling. It reads 0 IWC no matter how I do it. One of my instruments is a micromanometer (reads as low as .001 IWC pressure differential), and it reads 0 IWC there too.
I'm zeroing the meter at floor level, and have no hose on the "reference" port. And I gave a 9 foot manometer hose on the "signal" port with the meter at floor level and the open end of the hose a few feet from the ceiling.
I'm thinking that there be about a 0.108 IWC PD there with that 8' difference in height. Why am I getting 0? It isn't an air tight room, and no fans are on.

2. Mar 22, 2013

### Staff: Mentor

Are you accounting for the air pressure in the manometer tube...?

3. Mar 22, 2013

### tinkeringone

I guess not, if the tube you're referring to is the hose going from the digital manometer to the ceiling. I assumed that pressure in the hose was the same as the air pressure at the ceiling where the open end of that hose is.
Never had to do that just measuring static pressure, i.e. in an air duct. But I guess that was at about the same height as the meter.
How would I account for the pressure in that hose?

4. Mar 22, 2013

### AlephZero

No. The air pressure in a vertical length of hose will vary with height, exactly the same way as the air pressure in the room.

If you position the hose vertically, seal the end, and then bring it down to floor level you should see a difference in reading. But correcting the reading to give you an accurate measurement would be complcated, especially if the hose is flexible.

The straightforward way to measure this is with an instrument that measures absolute pressure, not relative, and is sensitive enough to show the small pressure difference.

5. Mar 23, 2013

### Yuri B.

Thank you, AZ, as I too was unsuccessfully "measuring" height air dP in my room with a diff. manometer. Now I see why - so simple. I have tried just now, and exactly, as you imply, it is not possible with a flexible hose as a slightest deformation of its walls causes huge pressure fluctuations.