J valve pressure drop

[EHowe]

I was diving a J valve for the 1st time today. Prior to the dive when checking my gear I noticed that with the valve in the up non reserve position with 2500 psi in the tank, my SPG would drop a few hundred psi when reg is cycled. I moved valve to down reserve position and guage holds steady when cycled. Does'nt seem right to me. Did dive in reserve position w/o any problems. Is this right? Or am I looking at a possible weak spring?

[Herman]

Thats normal for a J valve.
When the air on both sides of the J valve are roughly equal and in "dive" mode, the reserve spring closes a valve, basically shutting off the valve- it is acting much like a modern second stage. When you draw down the air pressure on the reg side of the valve to tank pressure minus the amount of pressure the reserve holds back (300 to 500psi)the tank pressure forces the seat open supplying you air and you see that difference as pressure swing on your SPG. When you change to reserve mode, the lever removes all/most of the spring force on the reserve seat so air passes without being interupted. This is one time you want your spg to swing.

My J valve scared the dickens out of a buddy of mine the first time he saw one do that.
When I DM for OW classes, we show them effects of an almost closed vavle which looks exactly like a J valve in dive mode. I usually take one of my J valve tanks to dive in class to show them that they still exist and how they operate. Few instructors expose their students to J valves and capillary gauges, my classes see both.

[EHowe]

Thanks Herman. Thats why I love this forum. There is a wealth of Knowlage & experiance here from a relitivly small number of people, compaired to one of the big forums where you dont know who or what anyone knows, or good info from bad.