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How to Vacuum Purge for Cleaner BHO Extracts

Butane hash oil (BHO) of the highest order is vacuum purged for better taste and aroma. As part of an optimized post-processing workflow, a vacuum purge removes all of the excess solvents, in this case, butane and/or propane, to deliver an oil worthy of medicinal or recreational use.

Cannabis Concentrate

What Is Vacuum Purging?

Vacuum purging, also known as outgassing, cannabis extracts is performed in a vacuum oven that evaporates residual butane and propane from the end BHO products. As a result, BHO processors reap a clean, flavorful, and aromatic oil without the harsh-tasting butane.

Purging systems used to remove butane and/or propane from cannabis and hemp extracts range from homemade contraptions using a large cooking pot as a vacuum chamber to industrial-scale systems capable of handling multiple batches at a time and efficiently heating BHO extracts.

So, how does vacuum purging exactly work? Once your crude oil is extracted, it is placed in a vacuum chamber that reduces the atmospheric pressure, thereby, reducing the boiling points of the butane and propane solvents. In essence, much lower temperatures can be used to purge the butane without degrading any therapeutic compounds.

At a normal sea level elevation, the weight of the atmosphere creates about 29.92” inHg of pressure. This unseen and omnipresent weight affects the boiling point of compounds. When the atmospheric pressure increases, such as at decreased elevation, compounds require more energy for the liquids to boil, which raises their boiling point.

In turn, when the atmospheric pressure goes down, such as when you are at a higher elevation, the boiling point goes down, as well. Using this concept, a vacuum chamber removes the weight of the atmosphere so that the solvent can boil at a lower temp. This is critical to remove solvents without needing to use as much heat.

At sea level, butane has a boiling point of 30.2º F. Propane boils at -43.6º F. While reducing the boiling point of both solvents is possible under a vacuum, you will also be reducing the boiling points of the cannabis compounds. At sea level, THC has a boiling point of 315º F. However, when in a vacuum at -29.9196” Hg, THC boils at 23º F.

Why Is Vacuum Purging Important?

Cannabis Tincture

If you are looking to create the highest quality BHO extracts with the least amount of impurities, you are not going to want to skip the vacuum purge. The best way to remove the harsh-tasting and inessential butane solvent from crude oil is through outgassing. Without it, you are left with a tainted oil that probably would not pass state-mandated residual solvent limits. In addition, oxygen is removed from the atmosphere in the chamber which reduces the oxidation of your oil making it fresher and longer-lasting.

What Equipment Do You Need to Vacuum Purge?

Of course, results from a vacuum purge vary by the type of equipment you are using. Differences in equipment may also affect the length of time needed to purge the solvent and temperatures needed to boil off the solvent from the shatter, wax, or whatever else you are making.

Vacuum Chamber

A vacuum chamber or a vacuum oven is where all the action happens. Vacuum chambers vary in their heating capabilities, but the best chambers make sure that every part of the oil is evenly heated. A vacuum purge chamber has a stopcock valve that regulates the flow of your gas. Inside the vacuum chamber, you will find a support plate or rack that you can place your BHO on. A vacuum chamber, on a commercial level, features shelving units where an operator can place multiple mats or trays containing a thin film (slab) of the extract.

The indicator on the pressure gauge marks the pressure inside the vacuum chamber. In a vacuum, the pressure should read zero which is when the outgassing can begin. The pressure should be about -29inHg. Most vacuums will measure the atmospheric pressure in inches of mercury. This will be designated in the form inHg or “Hg.

Vacuum Pump

The vacuum pump sucks out the evaporated butane from the chamber. For the highest quality BHO, choose an oil-free pump that can handle the pressures and temperatures used in the vacuum purging process. Choosing oil-sealed pumps can increase the risk of oil leaking onto the shatter or wax and ruining your products.

Now, there are many types of vacuum pumps that produce different vacuum levels. Diaphragm pumps are the best choice for the cannabis extraction industry because they do not use any oil. Plus, they are easy to maintain and are highly resistant to corrosion. Many diaphragm pumps are also quiet and can last for many years. While these pumps can be pricey, they can pay for themselves in the long run.

Rotary vane pumps are an affordable alternative to diaphragm pumps. They produce an excellent vacuum and can be run for long periods of time. They can reach as close to a vacuum level as possible and feature positive displacement characteristics. These pumps may have a lower initial fee, but may require additional maintenance costs because they use oil.

For the best of both worlds, a hybrid pump takes elements of a diaphragm and rotary vane pump to provide a highly corrosion-resistant system. The rotary vane pump technology allows for the creation of the deep vacuum while the diaphragm pump part takes out the moisture before it can degrade the shatter or wax.

Oil-free and hermetically sealed scroll pumps are also a great choice. The motor and bearings are completely separated from the vapor stream and many are approved for cleanroom use.

Finally, diffusion pumps are high-performance pumps that remove the gas molecules from the exhaust with a high-speed vapor jet.

Single-stage and two-stage vacuum pumps differ in price but two-stage pumps feature rotors and vanes. During the two stages, one stage creates the vacuum while the other stage cleans its system. In this case, the two-stage pump provides a deeper vacuum level compared to single-stage pumps. Two-stage pumps are a must if you want quicker outgassing of butane, a cleaner process, and less noise.

Isolation Valve

A ball valve between the vacuum pump and the chamber can help keep any oil from the pump crankcase from spilling into the chamber. The ball valve allows you to close off the vacuum line when the pump is not in operation. Extraction experts recommend installing an isolation valve, a vacuum gauge, and possibly a ball valve for backfilling from the atmosphere or a nitrogen tank with a filter to reduce contamination in the oil.

Pressure Gauge

A compound pressure gauge is a critical element of the vacuum pump system for making BHO. Invest in a high-grade gauge and calibrate it before starting the process and regularly. Vacuum ovens should never be operated at positive pressure.

Connecting Tube

The pipe that connects the vacuum pump to the chamber is usually composed of a durable plastic layer on the exterior that has a high-pressure resistance for the purging process. That means it will not get blocked with the suction power created by the pump. Most importantly, make sure your tube is designed to work with your specific vacuum system.

Silicone Mat

If you are using a silicone mat for your thin layer of resin to place in your vacuum chamber, make sure you are investing in a high-quality silicone. Low-quality silicone features a plasticizer that can absorb into your crude oil. One good way to tell if it has plasticizer in it is to twist and bend the silicone mat. If the twisted area turns a white or hazy color, it may have a lot of plasticizer.

A parchment paper or non-stick sheet can also work in the event you do not have a silicone mat. In addition, a glass sheet or a heat-resistant cover over the boiling liquid can help prevent the bubbling oil from making a mess inside the vacuum oven.

How Long Do You Vacuum Purge BHO?

Vacuum purges range in time depending on the quality of your crude oil and equipment. Vacuum purges can take anywhere from an hour or two to an entire half a day or full day.

What Temperature Should You Purge BHO?

Instead of boiling away your cannabinoids and terpenes at high temps, relying on a vacuum oven system allows the boiling to be performed at a significantly lower temp. Vacuum purging usually happens between 85 degrees and 105 degrees F. Temperatures vary depending on the type of oil.

How Do You Vac Purge?

Vacuum purging is the final stage in creating high-quality and flavorful BHO. Now, there are some main considerations to touch on about the vacuum purging workflow. Everyone’s process is different. Here is a general breakdown of the purging process when making BHO.

Run the Vac

When you have the BHO spread out on a thin layer on a silicone mat or parchment paper, you can place it in the center of the vacuum oven. When the BHO is inside the oven, set the temperature to your desired level and slowly run the pump to a full vac. Once it reaches near a full vacuum, set the oven temperature to 110º F.

Go slowly. As you increase the vacuum, you may start to see the BHO bubbling up and developing a muffin-like top that represents the outgassing of the butane. Over time, the boiling lessens and there are fewer bubbles (this may take a few hours or days).

How long you spend outgassing the butane depends on a few factors including the amount of butane present in the BHO after the water bath and the ambient room temperature. If your room is at a lower temperature, it will be harder to remove the solvent. Many extractors simply look at the BHO to determine its quality. When the bubbles stop and the texture and consistency of the BHO have no change under vacuum pressure, it means the vacuum purging is complete.

Remove All Bubbles

Cannabis Shatter Extract

After your desired purging cycle (can be up to 124 hours), you flip the thin layer of shatter on the silicone mat to repeat the process with the exact time and pressure parameters used before. During the second draw, your bubbling will not be as intense, however, there will still be some bubbling. When the boiling begins to subside, you can increase the temperature slightly (if you started at about 105º F, you may increase it to 110º F). Let the BHO sit in the full vac for your desired time (about 6-8 hours).

Repeat as necessary by flipping the slab of BHO and manipulating the temperature and pressure and time based on your desired end product (shatter, live resin, distillate, etc).

When flipping the BHO and to avoid a mess, invest in PTFE sheets over traditional parchment paper with silicone coating. Make sure they are heat resistant, food grade, and non-stick. PTFE sheets are solvent-proof allowing you to purge without absorbing solvent into the sheet. Plus, its non-stick characteristics won’t hold onto any terpenes.

Place a single sheet on top of the silicone mat. Transfer your BHO onto the sheet. When you are ready to flip, add another sheet on top of the BHO while it is still warm.

Place the BHO on a relatively cool surface such as a stone counter, chill block, or cool steel. Allow the cannabis extract to drop in temperature on the bottom. After a couple of minutes, flip over the thin layer of extract quickly and remove the top sheet. The cooling helps it not stick as much compared to a warm extract. If you cannot find this, parchment paper can work just fine. Any non-stick paper can do. Ideally, transparent sheets help you see the product better.

If necessary, consider residual solvent testing a sample of your end product to identify harmful levels of butane or propane solvent and other impurities. Labs use a range of chromatography equipment to identify each cannabis compound and contaminant.

Safety Tips: Outgassing With Care

As with any process that uses flammable hydrocarbons, taking the right safety precautions is critical to crafting high-grade BHO without injuries. Above all, your ventilation systems must be capable of handling the flammable hydrocarbons. Gas detection systems should be in place to make sure any leaks are quickly accounted for.

In addition, you must consider any electrical components used in or near the vacuum purging equipment and the equipment itself. Any out-of-oven purging should be carried out under a fume hood. Everything from non-classified equipment to any other source of ignition can increase the risk of an explosion in the event of a butane leak.

Perfecting the Art of Extraction With Luna Technologies

THCA Crystals In Terpene

Purging your BHO is a critical step of the refining process after extraction, but you cannot get good shatter, live resin, or any other extract if you do not start with high-quality biomass. In addition, you need an extraction system that can separate as many cannabinoids and terpenes as possible without as much butane and propane or undesirable compounds.

Luna Technologies’ automated IO Extractor is at the cutting edge of BHO extraction technology. Capable of producing any type of cannabis extract derivative, the IO Extractor is the crux of any BHO processing facility that wants to produce a clean extract with less of the bad stuff.

Optimize your cannabis extraction workflow with an automated closed-loop solution that streamlines your runs, reduces operator error, increases productivity, and extracts better quality concentrates than other methods. A final vacuum purge is the finishing touch to a high-quality BHO made with automated technology.

IO Extractor


Cut Labor Costs
Automated controls eliminate weeks or months of apprenticeship training required for manually controlled hydrocarbon systems.

Eliminate Operator Error
Pre-programmed recipe-monitoring system checks pressures and temperatures hundreds of times per second to remove risk of operator error.

Increase Capacity
Process 18 pounds of dried plant material or 25 pounds of fresh-frozen material per run. Single operator can process 400 pounds of biomass in a single day.

Improve Run Time
50-minute average run time with a 10-minute soak. Run-to-run changeover times of two minutes.

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