Diaphragm Pump Maintenance. (Welch Dryfast)

I am wondering how y’all take steps to maintain your diaphragm pumps. Currently I am using mine along with my roto evap to recover ethanol. I am attempting to maximize my vacuum seal and am looking for any advice. Additionally, do yall use cold traps with dry ice in tandem with the roto to recover some solvent that was in the vacuum tube headed into the pump? I am worried that the -56 degree cold trap is much colder than my condenser/chiller and I wont recover EtOH from the roto as efficiently. Most EtOH is now being recovered in the cold trap flask, is that ok?

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Rotary evaporators do not require extreme vacuum. In fact, I would draw less vacuum if a majority of the vapor was passing my condenser. Or run your condenser colder, or less heat in the water bath. Possibly all of the above…b


I have rebuilt the Welch 2042 and 2052 more times than I care to count.
Cold traps will save the pump and maybe give you 2x the time between rebuilds depending on throughput and of course how much vapor you are sucking through.

The nice part is they don’t blow up. When you notice a decline in vac performance do a rebuild and wa-la good as new.

Often, the price point of more rebuilds vs getting dry ice + trap / chiller pencils out to favor the rebuilding.

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could you briefly explain how to do the rebuild? It will be my first time and I am worried about errors. I have the manual with me but am a bit confused. I have a model 2014 from LS.

Luckily, the 2014 is easier to break down than it’s bigger brother 2042.
There used to be videos on youtube but not anymore (womp womp)

It sounds like you already have the manual (note sec 2.18 talks about traps). I couldn’t explain it better than the 7 steps in section 4. Let me know what part you are confused about. the hardest part is the last step, align the fan and DONT tighten past 20lbs or the teflon will deform and someone (or yourself) will be mad at you.

Diaphragm Removal

  1. Remove the four 4M x 45 screws and washers from the cover plate. Remove the cover plate and head from the plastic head base.
  2. Make sure to wear gloves before removing the diaphragm. Grab the diaphragm with both hands and turn counterclockwise to loosen from connecting rod.
  3. Remove the diaphragm retaining shims.

Installation of New Diaphragm

  1. Apply a minute amount of locktite to threaded diaphragm stud and fasten to connecting rod with shims previously removed.
  2. Tighten diaphragm with both hands. Tighten eccentric set screw down on to the flat of the motor shaft.
  3. Place the head and chamber assembly on top, rotate fan so that the peripheral lip of the diaphragm aligns with the groove on the underside of the chamber and base plate.
  4. Place the cover plate over the head and body using four 4M x 45 screws and washers. Tighten the bolts (no more than 20 in. lb.) or Teflon will deform

thank you so much!

Take pics as you take it apart, let me know if you have any issues.


Why would you want to draw a harder vacuum if your solvent is ending up in your cold trap? Ethanol is easy to strip at room temperature with a water aspirator, and recovery percentage will increase with the weak vacuum. Can someone explain why you would use a Welch Dryfast over a recirculating aspirator here? Is it just to speed the solvent recovery rate? Seems troulesome… Recirculating aspirator lets me skip the cold trap… weak vacuum means slow enough evap it doesn’t beat my condenser.


I’m undecided on the welch wobl the water aspirator type myself. I just killed my 4yr old elcheapo robinair filtering ethanol today

Just scored a wob-l for 300 shipped. There’s another one for 199 on ebay. Wish I seen that before spending 100 more

Hey @sidco! Thanks for your help with my diaphragm pump, really appreciate it.
I now have an inquiry about our Welch CRV pro4 pump if you wouldn’t mind. Recently we completed a routine oil change of the pump using 50/50 kerosene/pump oil to flush it out. The oil came out clean after the second flush. Unfortunately the pump now won’t get below a vacuum depth of 1200 micron. Before sending it in to be rebuilt I was wondering if there was any advice you could give about possible fixes. Could our LS vac gauge be giving us false readings? It appears to be clean inside. We tested the depth directly from the pump.

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could there be kerosine residuals off gassing?


That could be something I could look into- should I just flush the pump again with vac oil only?

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try another flush, verify it with another sensor if you can. make sure everyting is well sealed and tight.


Couldn’t you just run it for a while with the gas ballast open to quickly remove any residual kerosene?


thats another option.

Thinking about picking up a Wob-l 2585. It seems this is one of the larger one of these pumps. Im thinking about using it for filtering. But i just wanna make sure it will work for my operation.

I was really hoping i could use this without a cold trap but it seems like it would be safer to use a cold trap. Is this correct?

This would be for strictly winterizing and dewaxing. I typically dewax just below -65c. Will this be possible with one of these pump? I have god know how many different vacuum pumps here. Im just soo tired i filling cold traps…

Currently ive been using a regular cheapie rotary pump behind a huge stainless cold trap which i converted out of a old stainless recovery tank im no longer using. When im done in my trap there is a pretty significant amount of ethanol which makes me wonder if the diaphragm pump will have the same without a trap.

Let me know if im just wasting my money and keep running the cheapie. My cheapie is a 5 cfm $50 ebay pump that has lasted me over 4 years. Its still gets down to 100 micron or so. I constantly keep new oil in her and she only gets vac19 or vac20 oil depending what i have in the house. Its still buries the gauges on my extractors and thats the only reason i bought it.

I own 2 of them. Used just for the rotovap and buchner funnel bac assist. They work awesome. Ii scored them used at ebay. I didnt use a cold trap.


You said roto? I am just about to pick up a 5l solventvap and I have been looking for a good inexpensive pump to run it for evaping etho.

Would this work good on a 5L?


I use it on my 5l rotovap