Editing Check valve (section)

===Diaphragm check valve===
A diaphragm check valve uses a flexing rubber diaphragm positioned to create a normally-closed valve. Pressure on the upstream side must be greater than the pressure on the downstream side by a certain amount, known as the pressure differential, for the check valve to open allowing flow. Once positive pressure stops, the diaphragm automatically flexes back to its original closed position.<ref>{{cite web |url=http://www.northvalekorting.co.uk/prod-norval-perf.asp |title=Norval valve performance |publisher=Northvale Korting |last=Wright |first=Stephen |access-date=2009-05-19 |url-status=dead |archive-url=https://web.archive.org/web/20090427215539/http://northvalekorting.co.uk/prod-norval-perf.asp |archive-date=2009-04-27 }}</ref> This type  is used in [[respirator]]s (face masks) with an [[Mechanical filter (respirator)#Exhalation valves|exhalation valve]].