The Convection Thread

[Dan Morrison]

I've been diving down the rabbit hole of convection heating techniques and I think there is a ton of cool stuff to talk about. So, this is an attempt to dump my thoughts and start some discussion.

This will only cover 100% convection heating. Everything changes when you introduce active conduction heating. Hybrid devices are relevant, but for the sake of keeping things simple I think it's best to omit them in this thread.

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The convection process, in theory, is simple. You've got hot air flowing through a chamber that is packed with loose leaf/flower particles.

In an ideal world the flow of hot air would be totally uniform over each individual leaf particle, resulting in a perfectly even extraction of the entire packed chamber.

In reality, convection heating can often result in uneven extraction.

When convection goes right, It's absolutely sublime...... so what can we do to increase our chances of success?!

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For the sake of this thread I think our goal could be to figure out what we can do to reach the highest possible "total extraction". Meaning the percentage of active compounds that end up in the vapor stream, relative to the weight of the extracted material.

This extraction should happen evenly throughout the entire chamber load.

And finally, that this extraction happens without manual stirring. (Perhaps it will be proven that stirring IS needed for absolute best results, but I am really hoping that is not the case) I'd still like to hear thoughts on stirring, as I know some people actually enjoy stirring.... but y'all are crazy. In my opinion, stirring is a sign that uneven extraction is happening. I believe it's better to treat the cause of that unevenness in the first place.

So, what are the variables that affect total/even extraction?

One obvious one is grind size.

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For this first post, I am going to focus on the variable of grind size.

Theoretically, the finer the grind size the more surface area you have available to extract, and so the more extraction you should be able to achieve with a fixed volume of hot air. From my experience this doesn't translate into the real world.

With a fine grind I typically get very quick extraction. Vapor density is high, seemingly because of the large surface area of the small particles. But the evenness of the extraction can be poor if you try to extract too quickly. Since I prefer faster extractions, I often get uneven extraction.

My working theory is that the fine particles restrict airflow, causing channels to open up where you get regions of high air flow, surrounded by regions of reduced airflow. This usually presents as "hot spotting", where one small area is over extracted.

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I recently watched THIS video discussing an espresso extraction study. The main focus being grind size and how it relates to the extraction of coffee beans.

While this is all dealing with coffee beans and hot water, I felt like there were a lot of parallels between this process and the process of extracting loose leaf with hot air.

When you think about it, an espresso machine functions similarly to a convection vaporizer, just replace water with air. Instead of the pressure coming from a water pump or hand operated lever, the pressure comes from our lungs.

Since "Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower" (wikipedia), we can likely still assume that there are useful bits of information to borrow from the espresso process, despite the fact that it's using water instead of air. Since espresso has been around for over 100 years, I feel like there are some insights that come with that maturity.

In short, the study found that total extraction (and more specifically the evenness of extraction) dropped with finer grind settings.

From the video,

....at very fine grinds you're inevitably gonna get channeling, water cannot pass evenly through that cake of coffee and so some water, quite a lot of water, is going to flow through just a little bit of coffee where channels form, you'll have a very high flow rate through a channel and the coffee around that channel won't see as much water. It won't be extracted as much and this is causing this drop in total extraction.

As mentioned above, I seem to have the same experience with fine grinds. If pushed to extract quickly, uneven extraction is almost guaranteed.

Conversely, coarse grinds, in my experience, have the opposite effect, resulting in more even airflow over the entire herb load. Even when pushed to extract quickly, the airflow remains more even throughout the load.

The above video cites that water flows more evenly through a bed of coarser coffee grinds.... so it seems like my experiences are mirrored in the espresso world.

But... you can only go so coarse. At the extreme end, you can vaporize whole flower. I find that this can work fantastically... but it really depends on the density of your flower material. With dense flowers you've got a lot of nooks and crannies hidden away inside the flower that are not getting good exposure to the hot air flow. These whole flowers may require mid-session grinding and stirring to achieve a high total extraction.

From the video,

....the finer you grind the more coffee you should be able to extract, but the coarser you grind the more evenly it's likely that the water will flow through the cake and so what you're looking for is kind of a crossover point. A point that's the best of both where you're going as fine as possible for an even flow rate

I think this pretty much sums up my thoughts on grind size and convection heating.

So, where is that crossover point?

I think it's somewhere in the medium - coarse grind size for my particle vaporizer and usage style. If I'm shooting for perfection, sifting the particles into uniform sizes will result in an incredibly even distribution of particles in the chamber. I use zero tamping, which is very important to preserve the air pockets around each particle.

Thoughts? What is your preferred grind size?

I also feel like it's worth mentioning what vaporizers you're using, so as to get an idea of how each device may affect these variables.

For future posts (and I invite anyone to start their own here) I think it'd be cool to discuss the other variables; chamber size/geometry, air speed, temperature, etc.. In fact, it might be useful to discuss exactly what variables exist in the first place, haha.

From the same creator of the above video, he has a series on "Understanding Espresso" where he outlines all the variables involved. He lists; Dose (weight of ground coffee), Ratio (coffee to water), Brew time, Grind size, Brew temperature, Pressure.

For our purposes, I'm not sure if we can use all those variables, or which ones make sense and how to translate them.. but I guess this is all a work in progress!

So, what variables do you find most important?

 

[Abysmal Vapor]

Other then grind i have found that if diameter of the heater/ hot air intake is wider then the bowl also helps. Stacking herb layers is counter productive .
Bowl matters too. My ideal bowl has a wide screen and enough space below it before the airstream narrows . Mesh size is also import,i have found 80 mesh to be my sweet spot.

 

[arb]

I like a thin layer finely ground for one hit extraction.
Dry is better than wet and takes a little practice but the vapor density is definitely better for me.

 

[Dan Morrison]

Other then grind i have found that if diameter of the heater/ hot air intake is wider then the bowl also helps. Stacking herb layers is counter productive .
Bowl matters too. My ideal bowl has a wide screen and enough space below it before the airstream narrows.

I've definitely found that it works better if the diameter of the heater/hot air intake is wider than the bowl.

I also agree that it's better to go wide than to go deep. There is certainly a depth maximum where uneven extraction is unavoidable.

I like a thin layer finely ground for one hit extraction.
Dry is better than wet and takes a little practice but the vapor density is definitely better for me.

How wide would you say the chamber is with this technique? I feel like I would agree with you, but given the size restriction of portable vaporizers sometimes you just can't fit a large enough dose in one thin layer.

Without that restriction I do think you're onto something. "One hit" extraction requires both a fine grind size to achieve high surface area AND the most even airflow. So in a sense it's the most demanding use case.

I suppose this touches on the variable of dose. Perhaps it's better to spread a large total dose over multiple one hit sessions, using a portion of the dose each session. hmmmm.

 

[Abysmal Vapor]

""I suppose this touches on the variable of dose. Perhaps it's better to spread a large total dose over multiple one hit sessions, using a portion of the dose each session. hmmmm.""

Yeah, i load as much as i can cash in one or two hits,i have a special scoop that matches my lung volume lol,also flavor deterioration is not something i enjoy . I have brainstormed many times on different loading systems and best one idea so far i've seen is this .

 

[arb]

1\2 inch id glass tube that fits around the heater core.
Unrestricted draw depending on pack and pure convection.
That's a log plug in I like 510 for portable convection in the shape of a stempod.
20 gauge coils and off she goes.

 

[Shit Snacks]

Yeah grind consistency is such an important variable regardless of how fine or how coarse exactly, along with how you load it of course! The other two huge factors I think with pure convection on demand, would be how you draw and how you use the power

 

[invertedisdead]

Fine grinder here: I think temperature attenuation is really important for consistent AVB. What I see a lot of is people driving at full throttle in vapes with lots of horsepower, often leading to hot spotting/charring. It ends up being more broiled than even convection heating. I always like to think of convection vaping like convection baking. You generally can't vape a deep bowl in one hit and get even AVB, just like a chocolate cake takes much longer to cook than a pita. If you try to rush a chocolate cake it will never cook evenly. Coming up on the Volcano, I also tend to side with a thinner, wider layer of herb, instead of a narrow, deeper bowl. (Think tortilla cook time vs a chocolate cake)

IMO trying to make convection vapes concentrate compatible leads to people using those temps for flower, and they end up electronically smoking. I think Smart temperature regulation is essential to even AVB. As soon as that directional air temp gets a little too high it's being broiled. This is where hybrid heating really comes in, as now you can increase vapor production without increasing the air temp.

 

[BabyFacedFinster]

My most used convection vape is my e-nano log. As they always say with this vape, less is more. I believe this has a lot to do with more space for hot air between the herb particles. Exposed surface area is a big deal regardless of the type of vape. I do a medium grind and stir after a few hits. Even with a fine grind I find that you get a slight increase in vapor after stirring. No matter how optimal the conditions, a little stir always helps, sometimes more sometimes less. I'm a little compulsive with this and if I didn't stir I would always wonder if I could have gotten a bit more out, so I don't fine grind because I'm gonna stir it anyway.
Even with concentrates wrapped in cotton, I will pull the cotton and stretch it apart to expose more trate to the hot air then continue. My concentrate version of stirring.

 

[Dan Morrison]

@Shit Snacks , Yeah... I think consistency is often overlooked. And I do think it's just as important, if not more, than particle size.

@invertedisdead , I do see that a lot, so I'll agree there. In general, visible vapor production seems to be the measuring stick for whether or not a vaporizer is good or not. I think this encourages users to crank up the temperature to get those monster clouds, regardless of what that might do to the quality of the vapor.

With the pattern I'm seeing so far it seems like fine grinds are:

- more evenly extracted when used in thin layers
- more evenly extracted at lower temperatures

This pretty much mirrors my experience.

I've tried to compare a fine grind at two different temperature ranges, for different inhale times. For example:

1. High temperature, 10 second inhale
2. Low temperature, 20 second inhale

There is a point where the temperature becomes too high and even extraction become very difficult.

If you take it to the extreme and imagine extracting at, say, 1000c, for 2 seconds... you're likely going to scorch the top layer and not much vapor. So there does seem to be a middle ground here.

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Where this all become interesting is that I've recently been experimenting more with coarse grinds, very coarse. This style works better with a higher temperature setting and faster extraction. The flavor profile is completely different compared to fine grinds.

As of right now I prefer a coarser grind. I feel like the flavor is more bright... if that makes any sense, haha!

It's made me think that there is not a best grind size, but only personal preference. I find that both extremes can give great results, but the extraction technique differs.

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@Shit Snacks , how are you loading your chambers? Do you tamp down? Are you sucking up leaf, straw style, or sprinkling leaf into the chamber.

Regardless of grind size, I prefer to pack the chamber up to the top, as loose as possible, no tamp. When the chamber is sealed there is no 'head space' above the leaf. In use the leaf cannot become so loose that it starts blowing around in the chamber. Packed loosely, I find that the leaf 'melts' and shrinks after the first hit, usually reducing in volume by as much as half. The leaf at this point sticks together and doesn't become loose in the chamber..... which is another reason why I avoid stirring.... since stirring would loosen the leaf and allow it blow around in the chamber. This blow by effect always gives poor performance IME.

 

[Farid]

I've noticed there are a couple types of hot spotting. One, which can occur in any part of the load seems to be connected to channeling, which as you mentioned is related to grind size.

The other - which I find more difficult to eliminate - is hot spotting at the center of the load, leaving a symmetrical circle of unvaped bud around the circumference of the load at the walls.

-----------------------------
This introduces another variable:
Flow Condition - (Laminar vs Turbulent)

The flow condition is the result of a number of variables working together. They include the flow speed of the fluid traveling through the pipe, the diameter of the pipe, and the viscosity of the fluid (which is relatively constant in our example, since we are always dealing with heated air). Flow can be laminar (with fluid traveling in smooth paths), or turbulent (with fluid traveling in irregular, fluctuating paths). This image shows the difference:

This second (symmetrical) type of hot spotting I think is related to the no slip boundary conditions that exist when you have laminar flow through a pipe.

The no slip boundary condition relates to the velocity of the fluid particles very close to the walls of a pipe. When you have this condition, the velocity of the fluid particles adjacent to the walls is zero. With laminar flow this results in a velocity profile in which the velocity of the fluid is fastest in the center of the pipe, and decreases as you approach the walls of the pipe.

If you compare the laminar and turbulent average fluid velocities through the pipe it will make a profile like this:

As you can see the laminar velocities end up with a parabolic shape, with a less even velocity throughout the pipe.

Now I could be totally wrong, but this leads me to believe that we should be exploring encouraging turbulent flow between the heater and the weed to reduce the hot spotting around the edges.

 

[MinnBobber]

@Farid , laminar vs turbulent:
In practice, I find laminar flow yields more even extraction, judged by ABV color.

IMO, the best example of laminar flow is the miniVAP with its long body / airflow channel to set up parallel air flow.

The best example of turbulent airflow may be the Tafee Bowle, where air enters at the bottom of the puck, flows to the top, and reverses direction to flow down thru the chamber.

Comparing ABV coloring, as a measure of even extraction, the laminar miniVAP yields perfectly consistent color while the turbulent Bowle gives variable colored ABV.

This may be related to the slow air velocity involved, of mere inches per second, where the wall / boundary layer issue is not a factor??

Bowle and miniVAP are opposites in their airflow but both are awesome.... love both of them.

It might boil down to proper design and engineering can make different flow work.

I generally don’t grind....leave the trichomes intact, gently tweezer off individual calyxes , temp step low to medium , open and pop the dry calyxes , finish on high heat.

 

[Alan]

While 100% pure convection heating is an ideal goal, the reality is that conduction and radiant heat transfer are always a part of the equation, even though a smaller part.
The walls of the chamber and the basket screen holding the ground material absorb heat from the hot air stream. As they get hot, they conduct and radiate heat. The mass of ground herb also conducts and radiates heat. Some materials conduct / radiate heat at different rates and hold more heat than others. I find that wood conducts less heat and holds less heat than glass, steel, or silicone. More thermal mass of the container requires more heat to achieve the desired temperatures within the ground material.

The process can be thought of as a thermodynamic equation. The controllable factors in the equation for convective heating are temperature, pressure, surface area, mass, and flow rate.
The mass, shape, and density of the ground material is important to the equation. A puck shape, similar to that used when making espresso seems to work the best. If we want even extraction, the entire puck must be heated to the level needed to volatilize the desired plant oils.

Complete extraction also seems to be a tough point to define. The plant material contains a wide spectrum of oil densities. There are some heavy oils and tars within the plant material that we really don’t want to volatilize. Some are only released with the heat of combustion. The most flavorful oils are released at lower temperatures. It all depends upon how dense of oils you want to volatilize to determine your fully extracted point.

Temperature is a fairly straight forward factor to control. The wattage level of the heating coil controls the air temperature.

Pressure within the space containing the ground plant material can be controlled with the draw speed and physical air intake restrictions. A fast draw speed will drop the pressure within the chamber. A lower atmospheric pressure surrounding the puck, lowers the boiling point of the plant oils. This allows oils to be volatilized at a lower temperature. The lower the temperature, the better the flavor. A lower temperature air will also require less cooling before inhaling. The air temperature really only needs to be just hot enough to volatilize the plant oil. Applying too much heat, only requires it to be cooled back down to be comfortable.

Surface area is controlled by the grind of the material. A very fine grind will create a lot of surface area, but won’t be of help if he material packs down so that the hot air can’t reach the material. There is a happy medium in particle size. I find the best way to get good air flow through the material is to hand crush it. The moisture content should be such that the material falls apart after being crushed. If the material has too much moisture, all the water must be driven off before it will be able to achieve volatilization temperatures. The more mass of material contained within the puck, the more heat is required to raise the entire puck to the desired temperature. There should be an ideal puck size for even flow rate and fast heat up.

Flow rate of the air controls the rate of speed that heat is being added to the puck. Too slow and the heat can build up to the point of combustion. Too fast and the heat can be drawn through without reaching the desired temperature. Drawing too fast can also pack down the material in the puck, which reduces the air flow through it. There is a point where the benefit of a lower pressure from a fast draw speed, drops off because too much heat is being drawn away. Getting the draw speed just right can make a big difference in performance.

I think that coffee does provide some good parallels for convective heating and not just from the brewing aspect. The roasting of the green coffee is almost exactly like what is happening with our convective heating units. So much so, that I refer to the process as roasting rather than vaporizing. My units are called “Herbal Roasters”. Hot air corn poppers can be used for roasting green coffee beans, and it volatilizes so much oil that it must be done outside. Turns the beans from green to brown, just like we do with herbal material.

The normal flow of air in a chamber containing the herbal material has almost zero velocity at the perimeter. This makes it tough to get an even air flow rate through the entire puck. Most of the air wants to go through the center of the chamber. If you can force the hot air to enter the perimeter of the chamber/bowl, that would change the profile of the hot air going through the puck. That is what I am attempting to do with my slit end glass heater covers. It has been working well so far. Any brown spots that I do get are at the perimeter, so there is no need for stirring. Getting the air hot enough for a quick and complete extraction can create air that is uncomfortable for inhaling. Having the ability to also use conductive heat transfer at the very end of the roast is nice. The end of the glass tube is closed and round, so it can be used to contact the entire puck for a complete extraction without the need to stir.

Another thing I have been doing is to install a ss foil disc in the center of the heater screen (sandwiched between two screens) to force all of the hot air to the perimeter of the roasting bowl. The ss foil doesn’t have much mass to rob a lot of heat away from the roast. Never any center hot spotting.

To summarize.

Hand grind and puck shape for good air flow, draw speed to lower pressure and control heat rate, force hot air to perimeter, and adjust wattage to suit.

Treat the material as you would a marshmallow. Low heat, slow roast provides the best flavor roasted marshmallow.


Happy roasting

 

[Dan Morrison]

@Farid YES DUDE! I'm so glad you posted that, you found some great example images as well.

This is something I've suspected as well.

I've found that encouraging turbulent flow JUST before the chamber does help reduce center hot spotting significantly. Adding an airflow diffuser that spreads the flow from wall to wall and also introduces turbulence definitely helps.

It's amazing, however, how quickly turbulent flow wants to change to laminar. Even a few millimetres of tube is enough to straighten out the turbulence it feels like. So it becomes difficult to know exactly when you're experiencing laminar or turbulent flow.

Another factor is how the air flows through the packed chamber itself... because fine grinds tend to increase central hot spotting... there is something going on there. IME, a deep load of finely ground material will show a distinct "cone shaped" heat pattern, where the flow of hot air creates a cone of flow that narrows the deeper you go into the material. Whether or not this cone shape has anything to do with laminar flow through the material is unknown to me...

There is an industrial chemical process that uses a Packed Bed of material for various reactions and heat transfer needs. This is the closest industrial process that I could find that relates to what we're trying to do here.

Here is a model for the flow of methane through a heated packed bed of catalyst pellets. This example shows cool gas flow over heated pellets, it's the opposite of what we are concerned with... so just imagine the reverse of this image, Blue is HOT, red is COLD.



Seems to line up with what we're talking about... the concentration of flow in the center, the cone shaped heat profile, and the dead zones near the side walls...

This model is a bit iffy as far as whether or not it truly has anything to do with convection heating of leaf material.. but it seems to me that there are similar laws of physics going on here, so it seems plausible.

@MinnBobber As far as I know the Minivap uses a flat metallic heater encased in a ceramic sponge that fills the heating tube. This sponge would impart a lot of turbulent flow. Additionally, the screen at the bottom of the chamber will also disrupt flow.

Depending on how you've loaded the chamber and whether or not you're using the glass core, you may also be benefiting from conductive heat at the side walls, which definitely help to reduce the center hot spotting.

I dig your style of loading "individual calyxes", this is totally something I've been doing as well, with great success.

@Alan , I'll reply to your post soon. Have to digest all of your great points!

 

[Siebter]

Too slow and the heat can build up to the point of combustion. Too fast and the heat can be drawn through without reaching the desired temperature.

While that is true for most convection vapes, I'd like to point out that this is not the case with the Tinymight, because it has a temp sensor that adapts the power to the draw (more precise: according to the temperature of the coil). Yet I still *can* char my herb with the TM, usually I get some hot spots when I draw very hard and have very even heating when I take moderate or slow hits. That shows me that the draw speed is indeed one more parameter that affects the extraction level, as @Farid has pointed out.

 

[Farid]

To add to what Dan pointed out, the turbulence will only usually be in one part of the airpath. By the time the vapor is produced and the air is exiting the puck of weed it will likely have returned to laminar flow (unless you're inhaling incredibly hard, but even then it may still be laminar depending on the vape).

This is especially true with convection vapes that have just a simple stem with no filtration, since usually those stems are on the longer side to reduce harshness.

I think Alan brings up a great point with the coffee roasting analogy. The difference that is important to consider is that there are some unique fluid dynamics properties that emerge at the smaller scale, that may not be as pronounced at larger scale. So there may be some variables that a coffee roaster does not need to consider as much as we do for vaporizers and vice versa.

The espresso analogy is also very good when discussing grind size, but the water aspect changes some things as well. With espresso sometimes a pre-infusion of water prior to applying the full pressure helps the extraction go more smoothly.

The other big difference that applies to both the espresso and the roasting analogy is the nature of the coffee bean compared to weed. Where coffee is much harder, it can hold it's shape throughout the processes much better. Weed on the other hand has a tendency to shrink as it is vaped and break down as it becomes drier. If it's good weed it also has the potential for the trichs to melt, clogging up areas which had previously allowed flow through the puck.

That said I like these analogies a lot and there is much to learn from them. I've learned a lot about the espresso making process from using a manual lever operated machine, which allows you to really play with every variable in a way that isn't always possible with a regular machine.

 

[invertedisdead]

Complete extraction also seems to be a tough point to define

I think there is a lot of focus on extraction, and not near enough on absorption.

We see that all the time in nutrition, where intake is obsessed about, when absorption is often the actual issue.

Rapid extraction rates can easily produce a supersaturated aerosol which effects particle size deposition and increased losses via nucleate condensation - which both affect absorption. There's an increased trend for vaping high moisture content flowers which have been stored with humidity packs to maintain a desired RH - I theorize this excessive water content also leads to "cloud seeding" and more losses to condensation.

IME, a deep load of finely ground material will show a distinct "cone shaped" heat pattern, where the flow of hot air creates a cone of flow that narrows the deeper you go into the material.

The Volcano vapes huge bowls of fine grind and is pretty much the gold standard to me still to this day for even convection AVB. IMO this is due to the consistent flow rate from the diaphragm pump and well chosen temperature attenuation. It's also not trying to extract the whole bowl in 10 seconds either which helps a lot.

I might argue a fine grind by nature gives you turbulent flow in the extraction chamber by using the cellulose material itself as a high porosity filter medium for air to travel thru, much in the same way the porous foam filter in a Vapor Genie or Minivap works. This also functions to increase the residence time and increase the rate of heat transfer from an "attached" flow. Whereas vaping a whole unground nug often functions like an airplane wing, with air flowing around it instead of through it, often leading to less impressive performance.

I tend to believe the hot spotting in my case with my vaporizer design is due to these conical glass connections functioning as a convergent nozzle, literally focusing the air into a hot spot.

If you want to see metrics on ground cannabis you might check out the data on grinding for commercial cannabis extraction. Here's some info prepared for Supercritical Fluid Extraction from Dr Markus Roggen.

ACS 2018 New Orleans: The Effect of Grinding on SFE

ACS 2018 New Orleans: The Effect of Grinding on SFE - Download as a PDF or view online for free

www.slideshare.net

 

[Shit Snacks]

@Shit Snacks , Yeah... I think consistency is often overlooked. And I do think it's just as important, if not more, than particle size.

Yes exactly, in pursuing it I think it often results in the finer grind, medium fine at best, though if a consistent medium coarse grind can be achieved it is very nice!

Where this all become interesting is that I've recently been experimenting more with coarse grinds, very coarse. This style works better with a higher temperature setting and faster extraction. The flavor profile is completely different compared to fine grinds.

As of right now I prefer a coarser grind. I feel like the flavor is more bright... if that makes any sense, haha!

Yeah I have noticed this as well, particularly with a whole nug, where I will take one hit at higher temp than I would normally start at, then flip the nug to take another, then break it up... The spectrum of flavors is a totally different experience and honestly even the effects sometimes... Even a chunky coarse grind is not quite the same!

It's made me think that there is not a best grind size, but only personal preference. I find that both extremes can give great results, but the extraction technique differs.

Yes that has been my overall experience I think, with pure convection on demand done right, most any herbal consistency can work. So it is fun to just mix it up, depending on the exact herbs, glass pieces being used, and personal preference at the time!

____

@Shit Snacks , how are you loading your chambers? Do you tamp down? Are you sucking up leaf, straw style, or sprinkling leaf into the chamber.

Regardless of grind size, I prefer to pack the chamber up to the top, as loose as possible, no tamp. When the chamber is sealed there is no 'head space' above the leaf. In use the leaf cannot become so loose that it starts blowing around in the chamber. Packed loosely, I find that the leaf 'melts' and shrinks after the first hit, usually reducing in volume by as much as half. The leaf at this point sticks together and doesn't become loose in the chamber..... which is another reason why I avoid stirring.... since stirring would loosen the leaf and allow it blow around in the chamber. This blow by effect always gives poor performance IME.

Yes what you described is how I tend to load my TM, using backwards RBT stems with rimless basket screens, I never tamp down really though I will load full, used to suck like a straw but now I use a scooper (mainly because I am much more diligent about maintaining lip balm lol) So I will typically load full and let the herb self tamp, once you take the first hit, I am also using it upside down with TMN in J hook... I do the same with Tetra and Splinter mainly loading rimmed or rimless basket screens, though with them I will straw suck sometimes depending on the mouthpiece and occasionally tamp if scooping in...

We're just talking about glass stem-based convection here? Yeah I never stir any of them anymore either, with bowls loaded like this and good temp regulation, it's just not necessary... When I'm using my heat islands, it's often one hitter bowls since they are smaller, I actually do those with SplinterZ too... Tafée Bowle I do tamp sometimes, also never stir though! Photos needed? lol

 

[OilOfSaints]

Just for kicks, today I tried a Fuego 510 cigar lighter mod on top of my O.B.S.S. Scientific Inhalations bowl w/WP. The mod is like a car cigarette lighter, but the temp is completely adjustable. I applied it and held it in position by hand. It fits over this bowl perfectly and it is somewhat of a snug fit. Both the bowl and the heating surface of the Fuego mod are 1" in diameter. The material did not stick to or touch the heating surface.

The bow is described as “THE 0 B SMART SYSTEM (OBSS)” Honeycomb Glass Screen Bowl. It allows (vaporization) more efficiently by putting the herb closer to the top in a thin layer. Glass screen bowls also provide a better taste, better flow and convenience over standard metal screens that get clogged and fall out when cleaning the bowl."

I was hoping it would produce hits similar to the Sticky Brick. The results were uneven vaporization. I tried to dial in the temp using CBD hemp material since it's cheap and can be wasted. I combusted several times. Once I got the hang of it I did get some decent hits. I tried 3 different diameter whips playing around with air flow. In the end I did not find it as a suitable option.

The purpose of posting my results is to provide some loose data.

 

[Dan Morrison]

The walls of the chamber and the basket screen holding the ground material absorb heat from the hot air stream. As they get hot, they conduct and radiate heat. The mass of ground herb also conducts and radiates heat. Some materials conduct / radiate heat at different rates and hold more heat than others. I find that wood conducts less heat and holds less heat than glass, steel, or silicone. More thermal mass of the container requires more heat to achieve the desired temperatures within the ground material.

Agreed on all fronts. I personally prefer wood chamber walls as I find it requires less heat input to evenly extract the load.

A puck shape, similar to that used when making espresso seems to work the best.

It seems like this is one of the variables that we can just all agree on. Haha.

Flow rate of the air controls the rate of speed that heat is being added to the puck. Too slow and the heat can build up to the point of combustion. Too fast and the heat can be drawn through without reaching the desired temperature.

In the espresso video I linked to in the first post, this idea was referred to as "contact time" (between hot water and the ground coffee). Given a heater that is capable of maintaining a specific temperature regardless of draw speed, I do think that there is an optimal flow rate of air. This become murky when we are dealing with temperature control vs analog heat sources, where the flow rate impacts the heat output to a lesser or greater degree.

Adding to what @invertedisdead was saying about the constant flow rate of the Volcano... it could be that constant flow rate is an important variable. In my experience I like to change my flow rate (with lung control) as I inhale to compensate for heat buildup in the chamber. I start to inhale slightly faster for the second half of my draw, which I am guessing helps maintain a more consistent temperature throughout the entire inhale.

I will say that I don't agree with the idea that heat can "build up" in the chamber to the point of combustion. But I assume you mean't that, slow flow rates cause greater contact time between the air and the heater and so that translates to hotter output with an analog heater.

@Siebter touches on the above with the temperature control in the TM. And I will mirror his experience as it relates to draw speed. Even with a TC heater, fast draw speeds will still produce central hot spotting.

Adding to what @Farid pointed out I believe that the parabolic velocity profile of laminar/turbulent flow is greater when the velocity is increased. Meaning, with faster air speed we should see a greater tendency for concentrated flow through the center of a pipe. I may be remembering incorrectly on that point...

One thing to keep in mind about the laminar vs turbulent thing is that it might not even matter that much. Both flow scenarios exhibit parabolic velocity profiles. So it's difficult to know exactly what's going on inside our vaporizers.



One thing that I have definitely found to be true is that in stem loaded vaporizers, if I put the leaf further up the stem and away from the end of the tube, I will get a much greater tendency to central hot spot. This is why I now put the herb load level with the opening of the stem.

You can see the velocity profile change in this diagram. The further you get from inlet, the greater the flow velocity through the center of the pipe.

I think there is a lot of focus on extraction, and not near enough on absorption.

Great point!

The Volcano vapes huge bowls of fine grind and is pretty much the gold standard to me still to this day for even convection AVB. IMO this is due to the consistent flow rate from the diaphragm pump and well chosen temperature attenuation. It's also not trying to extract the whole bowl in 10 seconds either which helps a lot.

I think this is a cool observation. With fine grinds I have found that lowering the temperature, slowing draw speed, and increasing inhale time definitely helps reduce uneven extraction. The volcano automates this process and gets rid of draw speed inconsistency.. so that all makes sense!

I might argue a fine grind by nature gives you turbulent flow in the extraction chamber by using the cellulose material itself as a high porosity filter medium for air to travel thru, much in the same way the porous foam filter in a Vapor Genie or Minivap works. This also functions to increase the residence time and increase the rate of heat transfer from an "attached" flow. Whereas vaping a whole unground nug often functions like an airplane wing, with air flowing around it instead of through it, often leading to less impressive performance.

As mentioned above I think you can still have turbulent flow but experience the same issues that come with more laminar flow. I think what is actually going on is some combination of the two. And perhaps with finer grinds you've got a homogeneous arrangement of many tiny little turbulent flows all squeezing through the fine particles... and acting as a whole more laminar.

____

@Shit Snacks Yep, I agree with everything you said. Seems like we have very similar vaporization styles.

Is there a standardized list of what each grind size looks like? I wonder if that would be helpful. For example:

Very Fine = Sand
Fine = Poppy seeds
Medium = Sesame seeds
Coarse = Quinoa
Very Coarse = Rice

@OilOfSaints , Try putting two SS screens (stacked together) between the heater and the herb load. This will help control the variable of airflow and heat diffusion. Try a medium - coarse grind with no tamping. I think that'll help a lot. Looking at the design of the heater it's clear that it needs a diffusion layer. Just don't touch the SS screens to the heater or you can short the heater and damage it.

 

[Hippie Dickie]

my 2 cents about a couple of your topics on convection vaporizer ...

okay, i claim my cube is mainly convection, although i acknowledge there is also radiant heat, directed into the herb that helps maintain the heater temperature throughout the herb with little of the herb being in contact with a hot surface, so minimal conduction.

even extraction ...
this is what i consistently see: the herb is uniformly dark brown throughout. so i take this to be an indication of complete extraction.

When convection goes right, It's absolutely sublime

i agree completely, and this is the feeling i have after the first draw: this is sublime.

grinding...
i agree with your analysis that fine grind is detrimental to extraction.

i never grind - makes the herb too small in the vial and it all falls to the bottom of the vial and the airflow through the flower is just not as good. i don't think grinding - particularly fine grind - is beneficial for exposing the trichomes to the airflow. yes, much more surface area, but that surface area is the inside of the plant material, not an increase of surface area of the herb's trichomes.

the goal is to melt the trichomes, pass the hot air, and vaporize the exposed contents. grinding may fracture the trichomes, but a lot of the trichome contents end up on the teeth of the grinder. i hate cleaning the grinder.

i use tweezers to pick apart the herb and drop bits and pieces into the vial - never pack, for maximum surface area of the herb exposed. and the larger size bits create more air channels through the herb for better extraction.

sometimes, if the herb has been sitting in the stash can long enough, i can crush the herb in my fingers to get uniform bits, but they tend to end up being too fine and there is trichome dust in the bottom of the stash can. so, before the stash can is refilled, i use the razor blade knife to scrape up the trichome dust and drop on top of herb in the vial.

stirring ...
but, the heat - radiant and convection - melts the waxes of the trichomes and the herb bits fuse together, it seems. the stir will allow the herb bits to rearrange and expose new surfaces.

so, after 3-5 draws (depends on the herb), i invariably put the cube down and stir the vial - while still in the oven tube, using two toothpicks (also in the kit) - one to hold the vial still and one to stir the herb. the next several draws will increase in density from the last pre-stir draw.

quick extraction ...
okay, first draw is 2 minutes from power on, this is 10 seconds to hit 400F and 110 seconds bathing in 400+F radiant heat as the heater hits a 430F setpoint. i will take 3 draws in the first minute - sublime - then, a couple more, a stir, and draws until the vapor is too wispy - all done in 5 minutes from power on. this is for .08g to .10g in the vial. repeat as needed.

screens and air flow...
no metal screen, but instead holes in glass. airflow is somewhat restricted at a factor of 6 to 1 - kind of like the pinch in the bamboo of the EVO - ambient air is funneled past the heater surface and then through seven 0.75mm holes, where it expands into the 10mm x 35mm vial - this is creating turbulent flow into the vial with the hole pattern directing the hot air to the perimeter of the vial, then out through seven more 0.75mm holes into the 14mm x 150mm draw tube. there is probably a formula for figuring out the cooling from that expansion. anecdotally, the vapor is not hot. i prefer a slow draw (10 to 15 seconds) to allow the ambient air to have time against the heater surface, and for the hot air to soak the herb.

bowl size...
the vial is 10mm inside diameter by 34mm height, but i usually don't fill past 20mm height (fill just 2/3 of the vial - no pack) - kind of the opposite of the wide shallow bowls mentioned.

temperature control...
this has been my main focus (besides only glass air path) - maintaining a consistent temperature during the extraction is what i want. measuring the heater temperature and reacting 3 times per second, with a sufficient power source - say, 20 amps - totally eliminates temperature sage during a draw and then waiting for the heater to recover, to get consistent results.

i think i am finally seeing accurate temperature readout with the BBQ thermometer. best temperature these days is 430F. i need to screw with the PID parameters (yet again) because the temp drifts downward about 10F toward the end of the session. or the conversion chip is getting hot and affecting the cold junction compensation.

i don't know if any of that is useful. i'm happy with the performance.

 

[Dan Morrison]

i use tweezers to pick apart the herb and drop bits and pieces into the vial - never pack, for maximum surface area of the herb exposed. and the larger size bits create more air channels through the herb for better extraction.

Since you're using a deep chamber, ("10mm inside diameter by 34mm height, but i usually don't fill past 20mm height") I'm guessing this is why the very coarse grind is working well for you.

It would be helpful to use a ratio for the puck size, perhaps. 10mm diameter x 20mm depth = Ratio of 1:2.

I like being more in the 1:1 - 2:1 range. Where the diameter is ideally less than the depth.

I feel like we can all agree that coarse grinds work better for deeper chamber loads.

I'm also seeing that there are a number of us who 'tweezer' pick apart our herbs, trying to maintain the natural plant structure. I think that this is definitely worth looking closer at. How to maintain structure and create a uniform particle size with the least amount of effort.

 

[invertedisdead]

okay, first draw is 2 minutes from power on, this is 10 seconds to hit 400F and 110 seconds bathing in 400+F radiant heat as the heater hits a 430F setpoint. i will take 3 draws in the first minute - sublime - then, a couple more, a stir, and draws until the vapor is too wispy - all done in 5 minutes from power on. this is for .08g to .10g in the vial. repeat as needed

How can you achieve 400 degree radiant heat with just a 430F set point? That seems extremely efficient, i have to run at 700-800F to produce radiant heat in the 400F range.

I'm also seeing that there are a number of us who 'tweezer' pick apart our herbs, trying to maintain the natural plant structure. I think that this is definitely worth looking closer at. How to maintain structure and create a uniform particle size with the least amount of effort.

I always said if Dry sift was more popular I would never grind again.

You get the benefit of undisturbed trichomes, high surface area, more terpenes, higher potency, less unvaporizable plant material, and less bowl space needed for increased heater throughput efficiency.

 

[Hippie Dickie]

How can you achieve 400 degree radiant heat with just a 430F set point? That seems extremely efficient, i have to run at 700-800F to produce radiant heat in the 400F range.

small geometry? the heater surrounds the vial, and radiant heat waves are redirected/focused by the cylindrical stainless steel heat shield back into the vial. it is all pretty tightly/closely contained - two 1mm glass walls and 0.5mm air gap separate the heater ribbon coil from the herb.

there is not much contact heating since only the bottom perimeter of the vial touches the oven tube/heater - there is a 1/8" gap between the bottom of the vial and the inside of the oven tube, which i think functions as a heat reservoir for the incoming heated air.

 

[invertedisdead]

small geometry? the heater surrounds the vial, and radiant heat waves are redirected/focused by the cylindrical stainless steel heat shield back into the vial. it is all pretty tightly/closely contained - two 1mm glass walls and 0.5mm air gap separate the heater ribbon coil from the herb.

there is not much contact heating since only the bottom perimeter of the vial touches the oven tube/heater - there is a 1/8" gap between the bottom of the vial and the inside of the oven tube, which i think functions as a heat reservoir for the incoming heated air.

If you hover a q tip in the oven tube while at your set temp, without the vial inserted, will it brown from radiant heat without touching anything?