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Diff pumps help keep the water OUT!

Posted: Fri Sep 18, 2015 7:00 pm
by Richard Hull
One reason I like the diff pump is that water in the system can and does wind up in the diff pump and its oil and is pumped out of it as the boiler reaches its operating range between 140-180 degees C. Once a first pumping and long run occurs, the water and volitiles are pretty much out of my system, unless I leave it un-attended for a month or more. (common after HEAS)

The common water signature and assurance that it is gone is as folllows:

Turn on the forepump and open the basllast valve fully and evacuate the fore line to about 25 microns or, hopefully, better. Open the valve to the diff pump and pump it down to a foreline indication of about 25 microns or better. Open the fusor valve and this raises the foreline pressure to almost a full torr, but instantly plunges back to about 25 microns and stabilizes there. Now turn on the diff pump boiler. As the boiler temp reaches about 80 deg C. the foreline pressure rises, (yes, I have a PID controller on the boiler to read the temp and control the fan)

This pressure continues to rise to about 40-50 microns as the boiler temp goes above 120 deg C. (volitiles and water being removed from the diff pump and its oil) The pressure in the fusor doesn't seem to rise more than about 2 microns during all this as the jets in diff pump go active and the mechanical pump ditches the water. Quickly, within seconds, the fusor chamber pressure plunges to 0.5 microns or (5X10e-4 torr). At this point I let the boiler continue to heat to about 160 deg C and by this time the foreline is back to about 20 microns and I shut the forepump ballast off.
As the ballast is closed I'll usually see about another 3 or 4 micron drop in the foreline to 15 microns or so. Ultimately, the diff pump drags the system down to the 7X10e-5 range and fusor operation with flowing D2 commences.

Fusion will heat the fusor to about 100 deg C and water molecules are continuously boiled out of the fusor walls. This is, inturn, pumped out of the system. Thus, a three day group of separate runs will have the bulk of the water out of my system, and, hopefully yours.

Note: I never use S.I. pressure units and also refuse to use torr in my discussion unless well below 1 micron. The fusor, in operation, and in most pump down scenarios, is always in the "micron range" It is a micron device. I religiously go to scientific notation torr for diff pump bottoming figures. However the fusor system never, ever dwells there long as fusor operation shoots back to the multi-micron level of real fusion operations due to flowing deuterium.

Richard Hull

Re: Diff pumps help keep the water OUT!

Posted: Fri Dec 04, 2015 6:13 pm
by Alex Cordesman
So I've seen issues of water in the oil discussed several places here and it didn't occur to me until I saw the shutdown procedures you guys recommend written out. The diffusion vacuum systems at my lab are always stored at the vacuum limit of the mechanical pump. The only time they see atmospheric pressure is when they are being taken apart for service. This also applies to transport btw. If we are moving a system between facilities, it is moved while vacuumed down. The only time this isn't done is for components that are so large that the must be disassembled to move them. Since we have spare systems ready to slide into place when the primary fails, I can tell you than holding vacuum for 6 months plus shouldn't be an issue for a well sealed unit. Stored like this they will fire up and drop to 10^-5 Torr or better in a pretty normal cycle time.


Re: Diff pumps help keep the water OUT!

Posted: Fri Nov 03, 2017 5:15 am
by Richard Hull
Thanks, Alex, for your input. (This reply is a bit late...Sorry) Your suggestions are those followed by working vacuumist professionals where the job is the thing and where money is made or research is done, daily. Fouled vacuum systems hold up the money train from leaving the station on time. Fouled vacuum systems to a researcher is a failure on the part of the team responsible for a small, but vital part of the research at hand.

For the amateur fusioneer, the vaccum system is but one of a number of new and often strange technologies tangling their feet. Quite often, costly vlaving is just not there as a needless expense. Likewise, a system, once it has performed, will be left unattended for months and leak down as it was never really "sealed" in the critical vacuum sense. Finally, when the amateur is working his vacuum system, he has the time to piddle and standby while his slightly leaky vacuum system struggles to reach a number of vacuum benchmarks needed to proceed to the goal of fusion.

In short, 95% of all vacuum systems in use here are somewhat lacking in the professional vacuumist touch. Why?... This is due to the fact that the amateur fusioneer must become the R&D design department, the high voltage electrical department, the vacuum systems team, the electronic measurement department, the fusion gas delivery systems team, the machinist shop, the welding shop and the data collection and reduction center. The fusioneer is a very specialized Renaissance man, who has taken on the mantle of being capable of many things, all inter-related. The successful fusioneer is a one man team capable of doing nuclear fusion.