Vacuum pump oddity

I picked up a freecycled vacuum pump and motor a few years back for a round-2-it project for a vacuum bagging clamp to apply veneer to wood.
I'm starting to put the system together for its first job.
The pump is an Edwards Speedivac ED75 run by a 1/3 hp motor at about 500 rpm. Fresh Edwards oil filled just above the minimum mark.
Using about 600mm of 6mm od nylon pipe the output of the pump goes via a non return valve to a 3.9Kg ex calor gas bottle (empty!) with its original valve - volume about 10 litres. This line has a gauge scaled 0-760mm Hg connected via a tee piece.
All seems to work well. There is a minor leak somewhere in the push fit piping but I can live with that.
What I observe and do not understand is the following. With pump running and the valve to the gas cylinder open, the gauge reads 750mm Hg but when I close the valve it drops to 730mm Hg? This is repeatable under steady conditions after the pump has been running for 15 min or so and the body of the pump off cold. I've returned everything to atmospheric pressure and pumped down again with the same result.
The gas ballast valve is closed for all the readings. I can appreciate the gauge might not be accurate but why should the readings be different. I ask from curiosity alone as the level of vacuum I'm achieving will be more than enough for the job.
Can anyone explain please?
TIA
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 25 Jun 2017 17:14:58 +0100, Bob Minchin

A leak around the valve spindle, that seals when the valve is open, but opens when it's shut? Perhaps the packing needs attention.
--

Chris

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Chris Hogg wrote:

Mmm! a possibility I suppose. I'll investigate.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bob Minchin wrote:

Just to close this one off. The gland was just fine. The problem turned out to be the effect of the pulses from the vacuum pump affecting the gauge. When I fitted the gauge on the far end of 30m of tube, the pulses were damped out and the effect disappeared.
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.