Pressure Changes

[Jim Katen]

I see this occasionally, but I've never heard a satisfactory explanation for it. Any ideas? 

 

[Chad Fabry]

 It looks like water is trapped on the discharge side of the valve. As you turn the screw clockwise it increases the volume on the discharge side so the pressure decreases- as the screw turns ccw, it reduces the volume there by increasing pressure    

[Jim Katen]

Perhaps. But why doesn't it happen with all of the bibs of this style? I test the water pressure at every house. This particular phenomenon only happens about once every year or so. 

[Marc]

Perhaps not every valve of that type is built the same way.

[Bill Kibbel]

I'd ask Dan Bernoulli,  but can't find his email address.

[Jim Katen]

11 hours ago, Bill Kibbel said:

I'd ask Dan Bernoulli,  but can't find his email address.

That's cause it keeps changing, depending on how pressured you are to find it. 

[Jim Katen]

11 hours ago, Marc said:

Perhaps not every valve of that type is built the same way.

Probably. I suspect that the valve seat only allows water to flow one way. To find the true pressure, you'd screw on the pressure gauge and just barely crack open the valve and wait for the pressure to build. 

[Marc]

Ya know, that gets me thinking...could it be that the gauge, that particular type of gauge, has a tendency to behave that way if tested with air instead of water?

[Jim Katen]

No, it works fine with air - I use it on frost proof bibs several times each week and they always have a slug of air at the beginning.