Penning Measuring gauge configuration

[Ryyder]

We use, among others, Penning pressure gauges for determining pressures (aprox. 1E-7 to 1 mbar (about the same in Torr... for the Americans here ;-) ).
The Penning is placed just outside the recipient, behind a valve to make sure its never in pressures of 1mbar or more.

When using a smaller valve, the measured end pressure is higher (entrapped particles probably?). But the question now is, how to pre-determine how much higher the measured pressure will be due to this effect, without having to measure for each application.

My feeling is that a smaller valve and therefore smaller pipes in front of the gauge, result in relatively more particles being entrapped in the volume of the gauge. But I can't find a good way of approximating the effect.

More general:
* What is the effect a decreased diameter in front of the measurement?

Any thoughts are welcome!
Ryyder

 

[Ryyder]

No ideas at all? I was thinking it might be due to free path length being a lot more then the length and diameter of the pipe in front of the Penning.

If the inlet to this pipe is made smaller, molecules might get trapped inside the pipe?

 

[Astronuc]

Basically one is measuring the pressure of a vacuum?

Perhaps there is no one here who works with vacuum systems on a regular basis.


How small is small? Or what are typical dimensions?

 

[Ryyder]

Yes, measuring the pressure of the vacuum. And yes, maybe not many people have experience or direct knowledge, could be.

The dimensions are as follows, for two cases (in mm):
* Penning is typically DN40
* Pipe from penning to valve is DN40 or DN16, ~75 mm long
* Valve is DN40 or DN16
* Pipe from valve to recipient (the vacuum) is DN40, ~20 mm long

For the DN16 valve we see a clear increase in end pressure that is measured for the vacuum, even though we know the vacuum itself has the same end pressure (or at least will not change in the order of decades).

 

[andy taylor]

hi ryder..
the penning gauge and the size of the whole are directly linked for certain..ive worked with edwards and leybold gauges many years , no one has ever to my mid actually produced a document to corrulte any results..its the nature of the gauge I am afraid..ill put link to our gauges section if you ley me know ill could possibly geta pdf on sucha unit for you http://www.absolute-vacuum.com/product_gauges.php [Broken]