I found info on Vacum Pump terminology and a catalog of numerous
Vac Pumps in the Grainger catalog. They sell mostly the
industrial types similar to the Welch unit. There is a Dayton
(house brand) listed which is direct drive and just under $300.
For speed of pull down it is important to use the largest piping
or hose connections and to use vacum rated hoses. ( some hoses
can collapse internally) During pull down there is a plateau at
about 1500 microns/millitorrs due to the removal of water vapor.
At the pump it may read lower but if valved off the reading will
slowly rise to 1500-2000 microns until throughly dehydrated. Doing
some calculations: 1500 microns = 29.9032 in merc & 2000 microns
= 29.8972 in merc. It is my opinion that most Direct drive vacum
pumps with alarge enough CFM (Cubic Feet per Minute) for the
volume of the chamber, should have no problems getting down to
29.9 inches in 10 minutes. On units with a gas ballast I
reccomend leaving the pump run for 15+ minutes valved off and
the gas ballast opened, following usage to help clear the pump of
moisture. This helps to prolong the pump life and makes for
speedier pump downs. Also the size of the tubing (orifice)
connecting the pump has a major effect on the speed of pump down.
I think a Welch or equivilant is great but a jewelers type (also
sold as a Refrigeration type) costs less and with proper usage
can be good.
http://www.grainger.com for the web site. over 350 branches,
wholesale tools,equipment,and supplies
Dan Wellman In Indianapolis