Help with rotovap

Have a bit of an issue with a used rotovap I just picked up.

I have a temp probe inside boiling flask and another probe in water bath.

First I heat the water bath to 50c (double checked with second temp probe)

This gives my ethanol in the flask a temp of 47c

When I turn my vacuum pump on, as soon as distillation begins ~(28inhg) and the condenser is dripping the temp in the boiling flask begins to drop, as soon as the temp drops to ~32-33c distillation stops.

After distillation stops I bleed air back into the system and the temp rises.

Any input is appreciated

Pull vacuum first then kick on your heating bath until your tincture starts to evap

I tried that first and it won’t get hot enough, maybe the heating element is bad?

I believe what you are dealing with is evaporative cooling and your heater not being able to keep up with the strength of the vac?


Can you give us specs on the roto?

How many kw of heat are you putting in?

What roto are u using and how are u getting a temp probe in the boiling flask?


Not a traditional rotovap, but basically the same principle.

There is a 4500w heating element & a 1200w drum heater.

I believe the stainless steel is acting like an insulator once vacuum is achieved.

Today I will try to run the temps higher before pulling vacuum, if that doesn’t work I think I will have to re-think my heat source.

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i dont think you have adequate heating. pull your vacuum first then crank up your heating to max until you start seeing etoh dropping out.

If it wasn’t acting like an insulator before, why would it when vacuum is pulled?

You don’t show what you are using to control your heating elements (assuming its a PID of some sort), but if you have access to a temp probe that is inside your evaporation flask why not use that signal to drive your heating element instead of the temp of the liquid inside your drum? Especially without the fluid movement afforded by a rotating evaporation flask moving your thermal fluid around in a rotovap, the drum temp probe is going to lag significantly behind the temperature of whats inside your evaporation keg.

I found the issue, my heating element was wired incorrectly.

The PID is controlled by the probe inside the boiling flask, also there is an agitator inside the boiling flask and a circulating pump in the water bath.

The water bath runs ~20c hotter than the boiling flask under full vac.

Right now I am maintaining

BF 38c
WB 62c
Vacuum 600mmhg

I loaded 5 gallons in the still, I’m curious to see the recovery rates.

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keep us updated. i was thinking of building something similar

I don’t know the exact recovery time, ran 15 gallons through it in about 3-4 hours. All of it comes out 190+ proof. I will wash more biomass Monday and record some accurate data then.

I think once it’s dialed it should compete/surpass a 50l Roto.

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15gal in 3-4 hrs?

Very impressive. My puny 5L gets about .75g/hr

Show us some pics of that motor and passthrough

Also how do u collect your crude after?

Looks more like a pot still.

are you making a thin film on the evaporation surface? that gets you “thin film evaporator”, of which a rotovap is one flavor.

are you achieving that thin film by rotating anything?

if no to either, then it’s not a rotovap or particularly conceptually related to one (based on already agreed upon terminology)

do you have an agitator in there? (Edit: yep🤔)

Do you understand that you can have your water bath at 90C and your boiling ethanol we still be at whatever boiling point (37C?) you’ve set with your vac pump?

@greenbuggy is correct. you are controlling at the wrong spot

Edit: beat me to it! :thinking: congrats!! I really should know to read the whole thread before posting by now…


Shouldn’t you preheat your water bath/mantle prior to all this so that way after you gain vacuum you can just turn on the heat.