Using manometer for duct smoke detector test

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How do you hook up the magnehelic gauge hoses when the filters are in place?

The detector housing usually calls for it to be placed in an airstream of at least 400 feet per minute. This produces a velocity pressure between the two tubes of around 0.01 in. w.c. You can't measure this difference with a typical U-tube manometer just because this height difference is invisible to the eye. A 0-0.1 in. w.c. inclined manometer would do it, but your best bet would be a digital manometer that reads at least down to 0.001 in. w.c.

The part about filters on the probe tubes probably just relates to dampening oscillations from turbulent or erratic flow in the duct.

-CB