Ask the Expert Question-and-Answer Archive

by Ron Joseph

Manometer Readings

Q. I just installed an inclined Dwyer manometer on my A/H unit. I zeroed the guage before turning on the fan and I have a new filter installed. The guage went to a negative reading (off scale) when the fan started. Seems as though my supply duct may be too small? Is this correct?

A. It seems that you hooked up the tubes "back to front", so to say. Swap the tube connections, start up the fan again and you should get a positive reading. The tube connected near the bottom of the inclined manometer should be attached in front of the filters, in the working section of the spray booth. The tube connected to the top end of the manometer should be connected behind the filters in the exhaust duct. When the fan is on, the exhaust section is under a slight vacuum and this causes the liquid in the manometer tube to rise.

Good luck,

Ron Joseph