Roto Vap Pressure

Just getting started with new 10L Roto Vap Equip. Having trouble getting pressure up high enough I am told it is all about grease and fittings. How about the gaskets between the motor and the main Dragon Ball Flask? Any suggestions or tips welcome.

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Wait what?

Are you getting leaks or something?

You should be operating under negative pressure (vac) … Do you have a guage? What kind of vac pump? How full is your evaporation flask? How many rpm u spinnin that dragon ball at? Temp? What kinda solvent is in the solution?

Chances are you’re having problems getting the pressure low enough.

if you’re doing it right, you’re pulling a vacuum.

the deeper the vacuum, the lower the pressure. you will occasionally hear a deep vacuum (low absolute pressure) referred to as high vacuum. what we call deep vacuum (0.75 to 20 microns), the physicists do not. they like to play in the 10-9 to 10-12 micron range.

first question: with nothing but your gauge connected to you pump, how deep a vacuum can you achieve? this is your blank off value.

now how about with your empty rotovap?

if you can’t trivially find your leak, you might try using helium.

use the search function to see what others have posted as appropriate vacuum levels for solvent recovery in a rotovap. eg Search results for 'rotovap' - Future4200


I think I dont have the gaskets between the motor and the main flask seated right? I am only able to get pressure up to .02, about one-quarter of the guage.
I think I dont have leaks any where else though. I was spinning the ball at 120RPM. but I took out the content because I was not getting enough pressure. Trying to re-seat it now and test run with an empty flask.

can you tell us about your vac pump? manufacturer and model?

gauge? what units are you using. 0.02 doesn’t tell us a whole lot. especially if you insist that you’re having trouble getting the pressure high (rather than low).

It’s ok not to know the terminology when you get here, but continuing to misuse it won’t help us help you. so help me out here.


Make sure you’re putting hose on the barb of the vacuum that’s sucking.


I have pressure at about -.02 MPa. (mis que earlier when I forgot to say it was negative .02 Mpa. That might be the right pressure? if so, at what temperature? 80 celsius?

For equipment:
The Vac Pump is a SHB Series Model SHZ-95 made by Lab First Scientific.
The Chiller is a DLSB 3030, Lab First Scientific
The Rotory Evaporator is 10L RE-510 Lab First Scientific.
Yes I am definitely trying to get use to the language and equipment.
So thank you for any help you offer in clarifying, I am obviously new to this
And I have pressure at -.02MPa; which is in the middle of the green colored range on the gauge.
My questions is- is that enough pressure. If so, what is the temperature I should be aiming for?

What temp is your chiller reading at ? What temp are u set to on your heating bath?

-.2 doesnt sound very deep. Id check for leaks. I usually just listen for hissing noises. Some people use a soapy water spray to identify leaks, if your vac pump is on it will pull the soapy water through the leak and u will see bubbles. Mild contamination imo so i avoid the spray whenever possible.

Get used to identifying and fixing leaks. Buy good hose, I use the braided vinyl hose from the hardware store. Other people use tygon and other fancy stuff lol. Hose clamps! Clean/ non warped gaskets (some gaskets come defective if u buy cheap stuff). If you have Stainless steel or brass etc. , learn how to use thread tape (threading towards u, clockwise wrap) also get yourself a vice grip and adjustable wrench.

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thanks for those tips.
the temp chiller I have down to 5 degrees c.
the temp on the heating bath went up to 78c.
-.02 was the pressure reading.
I think I need to get the water bath up hotter?

Depends on what youre evaporating! If youre that hot u will probably rip solvent through your vac and not know it other than that buzzed feeling from huffing fumes! Youre using back pressure (hot water bath) along with negative pressure (vac) to move solvent up just high enough to leave your oil and be recondensed by your chiller (without flying past your chiller)

My chiller is set to -10 c and operates at 3 c without dry ice and -3 with dry ice (see posts above about inline dry ice coil)

My water bath starts out at 23 c when im dealing with ethanol and once my flask is very thick with oil i work my way up to as high as 60 c.

My vac gets all the way down to -750 mm hg or -29 in hg or -1 kpa.

My rotation is a little sketch when flask is heavy so I keep an eye on everything frequently.

Make sure u have a valve tee’d into your vac hose so u can go in and out of vac manually if needed.


Please correct me of I’m wrong. I’ll be firing up my 5l rotovap probably friday for then1st time.
I thought I read vac was best about-13 to -15hg. Reason why I bought those welch diaphram pumps

Sorry but we’re in the same boat hopefully making our first run Thursday

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youll be fine, a low vaccum source is all that is needed or wanted with a rotovap. mine usually sit around -25 hg.

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I’ve got to admit I have no idea what you’re talking about. Can you please send a picture of your gauge reading. If your pressure is .02 mpa absolute pressure, then that is 200 mbar, which is usable but not great. On the other hand, -.02 mpa gauge pressure would be something like 800 mbar absolute pressure, which is completely dismal and barely a vacuum at all.

I will send a picture of the setup tomorrow.
thanks for the questions and feedback.

I hope that these photos help:
the gauge:

and the general set up:

I am attempting to evaporate ethanol at about -.02 pressure. temperature is rising the water bath right now up to 70celsius and climbing. temperature in the chiller is at 2celsius.

Can you post a pic how you have the vac hoses attached to that vacuum pump?

Two Pictures show the unit top and the five way tubing coming out of left side
Last (or looks like the first) shows the three way behind the gauge.
The five into one hose from the vacuum pump runs into the bottom of the three way.
With this connection pattern, I am getting only .02MPA of pressure.
I tried many other combinations without other success.

Lastly, I disconnected the vacuum pump altogether and connected a rough pump to the last barb of the three way behind the gauge. Immediately pressure improved to .04MPA.

I tried unsuccesfully to load a video showing the whole thing.

Based on my test with the rough pump it seems to me I have the circulating vacuum pump connected improperly or it is not running properly.

thanks for any suggestions.
I will try another video.


Having used the water bath vacuum pumps you will be so much better off throwing it in the trash and replacing it with a cold trap and a Welch or Edwards mechanical vacuum pump.

You may have some leaks too but in my experience the warmer the water bath gets the worse vacuum it can pull and they are loud so it’s harder to hear any obvious hissing from a leaky fitting