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Vacuum Purging and Processing Tips

Hot damn, ah gots mah vacuum chamber and bodaceously big pump, so where do ah go fum heah?

Goooood question, so lets hit on a few key points.  There is more to vacuum purging and finishing than just sticking the oil in the chamber, and turning on the pump.

Tip # 1 Selecting vacuum levels and pumps.  Don't waste money and cannabinoids purchasing a 2 stage vacuum pump.

First lets look at what is going on when you place oil in a vacuum chamber and start raising the temperature.

The boiling points of compounds are given at a specific elevation, normally sea level.  That is because the weight of the atmosphere at sea level is about 14.7 psi, 760, 000 microns or millibars, or 29.92" Hg.  The weight of that atmosphere suppresses the boiling point, and if you remove that weight, the compounds boil at lower temperatures.

That is a good thing in that it allows us to cold boil away solvents with little heat, but it has its limitations, and one is that the same vacuum also changes the boiling point of the components suspended in the alcohol, that you are hoping to end up with when the alcohol is gone.

For instance, here is a chart, complements of Skyhighler, demonstrating what happens when you pull a vacuum on THC.  Note that at sea level, Delta 9 THC boils around 157C/315F.

Now look at what happens at -29.9196" Hg, where Delta 9 THC is boiling at  only minus -5C or 23F.

Whoaaaaa!  That sort of flies in the face of the common perception that nothing but a 2 stage vacuum pump will suffice, when most single stages will pull to below 100 microns, which is about -29.916" Hg.

THC boiling point of thc under vacuum

Tip # 2  Install a ball valve between the vacuum chamber and the vacuum pump, so when the pump is turned off, the vacuum line can be closed off, so as to not suck vacuum oil from the pump crank case, into the vacuum chamber.                       

Better yet, install a cross, with the pump ball valve in the straight through port and a vacuum gauge in one of the side ports, leaving the last port for another ball valve, which will allow backfilling directly from the atmosphere.

We install a filter on this port, so as to not vent dust into a fresh extraction.

Tip#3  The brains of the system is the compound pressure gauge, so get a quality unit and calibrate it at the start of the process, as well as periodically check it to see that it is returning to zero.

Bourdon gauges have flat partial circle shaped thin walled tube inside, that opens and closes its diameter under vacuum, and they take a little time to nestle in when new.

We like a Just Better M2-250B Refrigerant Compound Gauge ourselves, which has removable front cover, and a calibration screw that allows zero adjustment.

Never try to turn the adjustment screw under pressure, or you will most likely damage the adjustment screw.

Tip#4  Check your pump oil for color, clarity, and level before starting the process and regularly during use, most especially during cold boiling under vacuum.

We prefer a clear, colorless pump oil, so we can see if it is contaminated by looking at it through the oil level view window.  You can also note the level, as most have an upper and lower limit indicated.

All slide vane vacuum pumps puke oil and oil vapors out the exhaust and are noisy, so it will make a mess where you set it, unless you add a larger filter on the exhaust.

I just saw a cute conversion, that allowed spinning a Fomoco size oil filter on the exhaust, to contain the mess.  If you don't have such an arrangement, might I suggest you use a metal tray under your pump where making a mess is of concern.

If you are using the pump to cold boil, and aren't using an effective cold trap, you must regularly change your oil as the level rises beyond the safe level and begins to puke out the exhaust.

More on cold traps further down.

Tip#5  Test your vacuum pump by itself, before trying to vacuum test the complete system.

You can test your vacuum pump to see what level it will achieve in heaven, but simply attaching a refrigerant hose to the pump and the one to a vacuum gauge.

Make sure that the cord connecting the pump is heavy enough for the pumps rated amperage and avoid using extension cords.

Don't run multiple motors on the same circuit, because of the high locked rotor amp spike during startup.

Turn on the pump and see how far it pulls down.  If it doesn't pull down to its rated capacity at your altitude, check for leaks at each of the hose connection points.

To adjust for altitude, here is a good chart, taken from:

                        Pressure at Altitude-1-1


Tip#6  Vacuum test your complete system by first pumping it down as far as it will pull down, and turning off the pump isolation valve.  There should be no pressure loss after at least an hour.

Except for losses due to temperature changes, there shouldn't be any pressure losses overnight.  The key point is that the system needs to be hermetically sealed.

Tip#7  To keep the pressure of your chamber within the range that you have selected for optimum results, open the bleed valve and spill air into the pump line.

If you have both a 1/4" and a 1/2" hose connection on your pump, you can install a bleed valve on the unused one, to throttle vacuum at the pump.

Besides acting to limit how low your pump pulls the negative pressure down to, injecting dry air at that point, will help dry out the crank case vapors and slow down crankcase dilution.

Tip#8  Regularly close the ball valve to isolate the vacuum pump, and turn it off to give it a rest.

It isn't necessary to continue to run the pump, once the desired pressure is reached, but it is necessary to regularly start the pump and open the valve to burp it, so as to keep the atmosphere enriched by the cold boiling solvent removed.

You can also valve off the chamber and introduce a dry gas through the other vacuum pump port, to act as a ballast to dry out the crankcase.

Clearly you can't pull much of a vacuum venting a dry gas into the intake port, but the valved off chamber vacuum loss is tolerable for a few minutes.

Tip#9  Use a cold trap between the vacuum chamber and the pump when cold boiling solvent to recover the solvent for reuse, before it dilutes   the vacuum pump oil.

To insure that the solvent is recovered in the cold trap, and not cold boiled away there too, you must control the vacuum pressure at the cold trap, as opposed to the chamber.

The lower the cold trap temperature, the more efficient.  Besides ice water, antifreeze, dry ice and alcohol, and liquid nitrogen are used in cold traps to achieve desired recovery rates.

A trap with more face area to the flow, will decrease velocity and increase  transfer time.

A trap with the most surface area will transfer the most heat, at the same velocity and temperature.

Tip # 10  When you first pull a vacuum on a liquid, it will outgas the dissolved gasses and may boil explosively and violently, until the gasses are expelled.

To prevent making a mess, throttle the vacuum carefully, while observing the boiling action, until the gasses are expelled and the pool calms down.

Tip # 11  When cold boiling, use a glass sheet or equivalent cover over your boiling pan, so as to not blow specks of oil all over the interior surfaces of your chamber or oven.

Even though it may look like boiling has stopped, it is still sublimating, with occasional explosive bumping.

Tip # 12  Before paying good money for a cheaply made vacuum chamber, consider making something of your own like the one shown at:               


TIP #13  How come I had violent boiling when I first pulled a vacuum, and then the boiling slowed down to a crawl?

The liquid in the chamber has dissolved gases from the atmosphere in it and when you lower absolute pressure, they boil out and escape.

If it is boiling too violently to contain, try throttling the vacuum pressure back some, until most of it escapes, and it calms down.

TIP#14  How to calibrate a vacuum gauge without a reference standard

Here is another one from Skyhighler, on how to calibrate a vacuum gauge, accurately enough for our purposes, when you don't have a calibrated reference standard to compare it to.

Here is a chart showing at what temperature and pressure distilled water boils and some ideas on how to use it.

If you track the temperature of a pool of water, when you pull a vacuum on it, several things happen.

As the partial pressure is pumped closer to absolute zero, the dissolved gasses boil out of the pool.

The water itself starts to boil.

The temperature of the pool of water drops due to the refrigerative effects of the water boiling under vacuum.

The boiling slows down because the pools temperature drops closer to its boiling point.

Sooooooooooo, if you were to take distilled water and degas it by pulling a hard vacuum on it until violent boiling ceases, it can be used as a calibration standard that is close enough to track our process.

As it predictably boils at different temperatures at a specific absolute pressures, you can measure the temperature that the water is boiling at and find the corresponding absolute pressure in the following chart.

If you leave the thermometer or probe in the puddle and pull a vacuum on the degassed water, it will come to a boil at some vacuum level.  Look that temperature up on the chart, and it will tell you at what vacuum level you are at.

For instance, at -29.5" Hg vacuum, the water should boil at around  11C/52F after degassing.

Water boiling temperature in vacuum-1-1

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  1. Monty Lobo

    What would you recommend my temp and inHg pressure be for making shatter at 5,300 feet elevation? Full vacuum at 5,300 is -26 Temp is ?

    Reply       Edit
    1. Skunk Pharm Research,LLC

      At 5300 feet, there is only about 25.17" Hg to start with, so you have an adjustment factor of about 4.75", so would suggest -24.75" Hg at 115F. GW

  2. mike rycc

    Hi ive been reading alot on this site and love it but SWIM has been making bho and it seems to take a week to two weeks to vacuum purge this is bring the vac to full vac leaving pressure sealed at -29.5 and turning the vacuum back on for a half hour every 4 hours or so to help pop bubbles. SWIM leave the patty on one side till no more bubbles then flips purges till no bubbles wondering if he is doin sumthin wrong or if he is one of the few doing it right because ive yet to hear of anyone purging this long but his oil is always the smoothest thanks love skunkpharm

    Reply       Edit
  3. CropAndGarden

    Just had a thought and wanted to check if you guys have thought of / tried it already or know why it wouldn't work if done. I was thinking of setting up an apparatus to freeze dry my winterized extracts hoping it will enable me to regain the dry crumbly consistency by avoiding heat. I also noticed cannabinoids in my cold trap. I cold purge the ethanol from the cold trap and have an extract of clear cannabinoids.I didn't get them analyzed but it tastes and smells of them. freeze drying is the only process I can think of that would avoid heat or vacuuming off cannabinoids. Is there a resource to a list of cannabinoids and their vapor temps under vaccum ? I would like to isolate the cbd somehow or separate the thc in order to make an extract to help people without getting them high. I wanted to do that using liquid column chromatography ? Ive read about hexane / alumina being used as elutant/media. What elutant / beads have you guys used in your chromatography processes. Is there a media that would work with butane or ethanol ?

    Reply       Edit
    1. skunkpharmresearch

      Dry and and crispy is not a good thang, but it would ostensibly hold on to more of the lighter terpenes. We prefer to extract at around 25% water content, when it is still rife with terpenes and leathery enough to not break up into fine dust. Not sure what you have in your cold trap, but cannabinoids have very little smell. It is the mono and sesquiterpenes that provide the fragrance, so you most likely have a gin in your cold trap. We find the same thing when we recover the alcohol using a pot still. We end up with a terpene smell to the recovered alcohol, because it contains a bunch of them. Joe has separated the cannabinoids from the non targeted components of an oil extract using chromatography, but not the cannabinoids individually. I'll ask him to chime in on the last item.

  4. Tbone Pickens

    Hey there, I recently got the .9 cubic foot across international vac oven and a robinair vacumaster 8cfm pump. I am at 10500 feet and am having trouble understanding how to properly vac my extracts. I know that I am only able to pull around 540 mmhg max and was wondering what temp I should run at. Am I able to pull enough vacuum for low temp extractions or would I be better off setting it up at 5000 feet?

    Reply       Edit
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