I decided to test the bypass valve opening pressure of a FRAM Ultra XG3593A (original wire-backed) after I cut it open. It has the typical FRAM leaf spring bypass. I was curious if the bypass pressure rating was for initial, average, or fully-open pressure.
I measured the diameter of the valve, subtracted the area of the plastic supports, and used a kitchen scale to see how much force was required to open it.
Bypass valve spec: 12 psi
Initial opening pressure: 14 psi
~25% open: 16.5 psi
100% open: 22 psi (extrapolated, since my scale maxed out)
It seems that when FRAM gives the spec as a single value, it's for the initial opening pressure. It measured a bit higher than its rating, but some of that could be measurement error.
I measured the diameter of the valve, subtracted the area of the plastic supports, and used a kitchen scale to see how much force was required to open it.
Bypass valve spec: 12 psi
Initial opening pressure: 14 psi
~25% open: 16.5 psi
100% open: 22 psi (extrapolated, since my scale maxed out)
It seems that when FRAM gives the spec as a single value, it's for the initial opening pressure. It measured a bit higher than its rating, but some of that could be measurement error.