I use a Magnehelic differential pressure gauge for this purpose. http://www.dwyer-inst.com/htdocs/pressure/98-6p.html More expensive than a manometer, but a bit more forgiving. /Rudi > You can still get a mercury manometer from jc whitney or any > mail order > house, including mercury for 40 bucks. Except now mercury > is 260 bucks a > pound, or 12.95 for a little vial, instead of nearly free. >