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What function does the booth manometer perform and how do I understand the reading?

A frequently asked question when we are on site discussing spray booths. The manometer is a mechanical gauge to indicate when the exhaust filters require changing and will be inspected during any OSHA inspection.

 A manometer has two airports to compare the exhaust fan high pressure in front of the exhaust filters with the low-pressure area on the backside of the exhaust filters. The pressure difference is measured in inches of water column.


Temporarily mount the manometer on an outside vertical booth panel near the exhaust filter frame. Locate your high and low panel penetrations and install the bulkhead fittings at eye level 9” to 12” on each side of the exhaust filter. Try to have approximately the same length tubing from the high and low bulkhead fittings back to the manometer. However, same length tubing is not a requirement. The high-pressure manometer port is on the top left, and the low-pressure port to the right.

Remove the manometer from the booth panel, and level the manometer up-right on a table and turn the zero set knob counterclockwise until it stops, then turn clockwise 3 full turns. This puts zero in approximately the middle of the knob adjustment in either direction. Remove the fill plug and fill with gage fluid until the fluid reaches zero on the scale. Minor zero adjustments can be made after remounting, then replace the fill plug. Verify you have sufficient fluid by turning the knob fully clockwise so fluid can travel past the 0.5 in wc reading on the scale, then turn the knob counter-clockwise back to zero. If the gage is overfilled use a pipe cleaner or a straw to wick out excess fluid. Mount and level the manometer on the booth panel.


With new exhaust filters in place start up the exhaust fan. Indicate the starting pressure difference by placing the green self-adhesive arrow at this point. If your Finishing Department is in an Air Quality District which enforces the 0.25 inch water column (in wc) rule the red arrow is placed at 0.25 in wc to indicate filter replacement. If not finishing in an Air Quality District, then set the red arrow at 0.5 in wc on the scale

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