MS vacuum questions

[jkloepping]

I have a newbie question about a HP 5971 I am trying to make operational after long storage. First, if I cap the transfer line input to the MS, and simply run my rough pump (Alcatel 2012A) with the DP heater off, I get down to around 25 mtorr according to Chemstation. Is that a good number for a healthy rough pump and an MS with few or no leaks?

If I isolate the MS with an Edwards vacuum valve after pump down, and then let the unit sit overnight, when I return in the morning Chemstation reports near 3000 mtorr (there is still a vacuum, but no doubt outside the sensor's range). I've only run the unit (without the heated transfer line installed) for a total of maybe five hours or so, and this after a thorough cleaning including the DP pump with new Santovac 5. Should this setup hold a deep vacuum overnight?

Another newbie question. I also have the Hp vacuum tube gauge setup for measuring deep vacuum, and (for example) at 47 mTorr, the gauge reads "1.5 - 5" which I assume is 1.5x10^-5 Torr (a bit deeper than the rough pump should pull with the oil I am using). If I am correct, then the electronic gauge is showing a vacuum of .015 mTorr against the Chemstation reading of 47 mTorr. Is Chemstation really reporting uTorr? What silly mistake am I making?

[James_Ball]

25mTorr is good vacuum, or at least good enough to run it. Depending on the column flow I have seen them run about 70mTorr while flowing about 1ml/min through the column.

As for the vacuum being higher overnight, if you have yet to heat up the diffusion pump then it could be out-gassing from the oil that raises the pressure in the system or simply a tiny leak in the gasket sealing the top plate. You can never get them completely leak free and it doesn't take much to raise the pressure when you are not pumping on them.

If you had the diffusion pump on then 1.5x-5 would be rather high vacuum for a system that is capped off, I have had one down into the x-7 range before with diffusion pump hot and interface capped off. Typical readings while the instrument is running with column flow of helium at 1ml/min is about 5x-5, if using hydrogen it will be much higher, since the gauge is actually calibrated for nitrogen.

If you haven't yet, I would go ahead and pump it down with diffusion pump heated up and let it set overnight then try an autotune. If you have the source switch set to EI then it will fault out if the vacuum is too high as it checks the load on the filaments and shuts down if not enough vacuum is present.