The new TVO-2B and TVO-5B ovens from Cascade Botanical both ship with digital vacuum gauges installed. The gauges are easier to read quickly since the scale on the old analog gauge was a little compressed.
It would be natural to assume that digital gauge is a lot more precise than an analog version because it gives you a very specific number on the LED screen. However, there are a lot of variables that can impact the number you see on your digital gauge. It’s important to have a good understanding of what those variables are in order to fully grasp what’s really going on inside your oven.
Here’s what you need to know:
1) Like the analog gauge in our older ovens, the new gauge is what’s known as a relative vacuum gauge, which compares ambient atmospheric pressure outside the oven to the pressure inside the oven. This means, among other things, that the gauge will read differently at elevation than it does at sea level. As a rule of thumb, for every 1,000 feet of elevation gained, your gauge will be off by about 1 inch Hg. This is something we’ve covered at length in a previous blog post.
This means at sea level, the gauge of a TVO-2B or TVO-5B connected to a CB 2052 pump that gives average gauge reading of 28.85 inches Hg will give an average reading of 23.85 inches Hg in Denver. It is important to note, however, that the actual level of vacuum inside the oven is identical whether you are at sea level or not.
2) There are several other factors that can impact vacuum level. For example under the right conditions, the setup mentioned above might be capable of providing an average gauge reading of 29.85 inches HG. However, the diameter, length and rigidity of the vacuum hose; the cleanliness of the oven’s internal plumbing; and the gas load (how much is being purged and what is being purged) on the pump can all reduce the maximum level of vacuum that can be achieved. That’s not necessarily a bad thing – keep in mind that beyond 1 Torr (29.88 inches Hg), terpenes, CBDs and Delta-9 THC become volatile and begin to be purged along with solvents.
If you notice fluctuation in the vacuum levels during purge, it is very likely responding to water or solvents out gassing. This does not happen in a linear fashion. It ebbs at flows as the change in pressure discovers and volatilizes pockets of gas or water.
What Does it All Mean?
While there are a lot of variables impacting the final numbers you’ll see on your vacuum gauge, it is still a useful tool. It’s important to keep in mind that the vapor pressure of the solvents used in botanical extractions is very low. The boiling point of butane, for example, is 30.2 degrees Fahrenheit at normal atmospheric pressure. It wants to be a gas, it just needs a little help getting free from its delicious golden prison. For all practical purposes, if your vacuum gauge is reading 28 inches Hg (adjusted for altitude as explained above) or better, you should be able to purge your extract of all solvents provided you are using the right temperature and technique.
For CO2 extractions, residual moisture in your dry plant material becomes “bound water” in your extract. This results in highly undesirable crackling and popping at vaporization temperature, as the water violently boils off. This water should be purged from your extract. The vacuum oven allows you to do that quickly and without elevated temperatures, considering the vapor pressure of water at approx. 85F is 28.7”Hg. This helps avoid oxidation- which darkens the color of your extract, and (in the case of botanical extraction) decarboxylation- which destabilizes consistency.
You will also find that, much like the varying profiles of extractions done at different temperature and pressure, two samples of the same extract exposed to markedly different vacuum and temperature conditions will show markedly different characteristics in terms of color, consistency, etc. This is the art of extract “polishing.”
As you get to know your oven and pump, you’ll have a pretty good idea of what your gauge should be reading when your vacuum valve is fully open as you begin to purge (vacuum level will increase slightly over the course of the purge as your gas load decreases). Over time, that reading may drop slightly due to residue in the internal plumbing of your oven, but the difference will be slight. If it drops significantly, it’s time to do some troubleshooting (check to make sure that your hoses are still attached properly, check for leaking, check your door seals and check to see that your pump is working properly, etc.).