These Breather valves maintain the pressure within a range between the piped inlet and outlet pressure:
standard exhaust starting pressure +25mm Water (+245Pa)
standard intake starting pressure -25mm water (-245Pa)

Other pressures are available on request,the range available is:
exhaust pressure +20 to +100mm of water (+196 to +980Pa)
intake pressure -20 to -100mm of water (-196 to -980Pa)

Sizes available:
1.5" to 6"