mmm@cup.portal.com (Mark Robert Thorson) (01/16/91)
I just acquired a vacuum oven and two old (but working) vacuum pumps, and I've got a few questions about using the equipment. First, I changed the old oil because I was afraid it might be crap. I replaced it with 30 weight single-viscosity non-detergent motor oil. It seems to work fine, but I was wondering whether this was a good idea (as opposed to buying oil specifically made for vacuum pumps). I'm not shooting for very high vacua -- I'd be happy achieving 10 um or even somewhat worse. I'm mostly concerned with preserving the life of the equipment, and I plan to do frequent oil changes hence I want something cheap and readily available. Both pumps are Duo Seal brand from W. M. Welch. They look to me like sliding vane type pumps, probably single stage but possibly two. Can anyone familiar with this brand tell me more about it? I want to do freeze-drying and perhaps make some aerogels. I'm concerned that overloading my vacuum chamber (8" x 8" x 12") might put out too much vapor for the pump to handle. Does anyone have tips on how to avoid this situation? My pumps have a sight glass on both sides of the oil reservoir, so I think I might be able to monitor the amount of water in the oil if I knew what to look for. The sight glass has two lines, indicating an upper and lower level. Is this like the maximum and minimum on a car engine dipstick? Or is it like the hot and cold marks on an automatic transmission dipstick? Which line should I fill to, and do I do it after the machine has been running or after it has been idle (running raises the oil level)? Does anyone remember what year and/or month the Scientific American article on aerogels appeared? In case you're interested, I paid about $200 buying the vacuum chamber (a Napco 5831) at auction, and $75 per pump from a local company that was disposing of extra pumps from a large lot they had bought at a different auction.
whit@milton.u.washington.edu (John Whitmore) (01/16/91)
In article <38073@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes: >I just acquired a vacuum oven and two old (but working) vacuum pumps, >and I've got a few questions about using the equipment. > >First, I changed the old oil because I was afraid it might be crap. >I replaced it with 30 weight single-viscosity non-detergent motor oil. The oil in an oil pump is not a lubricant; it is a sealant instead. Its main specification is for vapor pressure, and it is likely that motor oil will not do nearly as well as pump oil. >I want to do freeze-drying and perhaps make some aerogels. I'm >concerned that overloading my vacuum chamber (8" x 8" x 12") might >put out too much vapor for the pump to handle. Does anyone have >tips on how to avoid this situation? Contamination of the oil is the major reason for changing it (we usually go by odor); if a lot of water vapor gets into the oil, the stuff turns into a mayonnaise-like sludge (and pump performance vanishes). To prevent this, use a moisture-absorbing filter to protect the pump (a tube packed with dessicant might suffice). I'm not sure about making aerogels, but the techniques I've heard of (patents by Nicolaon and Teichner) use an autoclave (high pressure heated vessel) rather than freeze-drying. Since the liquids one removes from aerogels include alcohols, a LOT of attention to fire hazard and gasket compatibility is called for. One large aerogel-producing facility burned to the ground a couple or three years back because of some such problems. If freezing changes the density of the liquid much, it will break the solid part of the aerogel before the liquid is removed, so the resulting aerogel will be of inferior quality. >Does anyone remember what year and/or month the Scientific American >article on aerogels appeared? No, but other articles are Nicolaon, G. A. and Teichner, S. J. Bulletin de la Societe Chimique de France, 1968, V 5, p. 1900 Teichner, S. J. and Nicolaon, G. A., US Patent #3,672,833 (1968) John Whitmore
ghg@en.ecn.purdue.edu (George Goble) (01/16/91)
In article <38073@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes: >I just acquired a vacuum oven and two old (but working) vacuum pumps, >and I've got a few questions about using the equipment. > >First, I changed the old oil because I was afraid it might be crap. >I replaced it with 30 weight single-viscosity non-detergent motor oil. >It seems to work fine, but I was wondering whether this was a good >idea (as opposed to buying oil specifically made for vacuum pumps). >I'm not shooting for very high vacua -- I'd be happy achieving 10 um >or even somewhat worse. I'm mostly concerned with preserving the life >of the equipment, and I plan to do frequent oil changes hence I want >something cheap and readily available. I have two Vac pumps (2 stage, vane). One is a "Robinair", the other is a "J/B" (for refrigeration pumpdowns and dehydrating). Each holds approx 16 oz of oil. Motor oil (and compressor oil) "boils" at 250 microns or higher. Vac pump oil boils at less than 5 or 10 microns. Refrigeration pumps (two stage), can regularly achieve 40-50 micron (0 microns is "perfect"), if the oil is fresh. I change the oil every use (or more).. Running a pump, even for a few seconds, will contaminate the oil enough to degrade it. If one still has moisture in a system, the "blank off" (pump valved off) vac will be around 1500 microns (one needs a micron gauge for this). The Welch pumps in the dept here get their oil changed every 6 months or so, but they just hold deep vac chambers for months on end, and dont ingest moisture, etc, like refrigeration pumps do. If you go to a refrigation wholesaler, vac pump oil is around $10/gal. --ghg
mcdonald@aries.scs.uiuc.edu (Doug McDonald) (01/16/91)
I have used motor oil (30 weight single-grade, the very cheapest, the brand name is "Saphire" , for years, in vacuum pumps. I get a pressure of about 180 microns at first, dropping in a day or so to about 50 microns. We did this because we had to change oil every week or so - we were running fluorine through the pump, which makes a big mess. Doug McDonald