Is the goal as near constant air temperature coming out of the vape heater? Or is is better if the temp drops over the course of the hit at a particular rate? All un-wired ball vapes will have a decreasing slope, but to various degrees based on the materials surface area and thermal mass. A wired ball vape probably could slow the decline; a strong enough direct heater in the vapor path would probably be able to keep the temp from dropping but would introduce potential overshoot of the set temp unless an expensive control circuit is involved. But maybe it is better to match the heat capacity to the expected load size. My best hits on the TM2 on demand involve releasing the button a second or two before I stop drawing. I could imagine that a ball vape that had an S-shaped heat release curve so the temp dropped after most of the vaping work was done would yield a smoother hit naturally. But of course you can also just remove the injector at the right time to achieve this, so maybe just as steady an air flow temp as possible is the ideal design goal. In which case the larger the heat capacity the better. Thoughts?
Attempting to engineer a declining thermal profile or an “S-shaped heat release curve” completely overthinks a simple physics equation, as a standard ball vape head holding 200–300 3mm spheres at 600°F possesses more than four times the thermal mass required to fully vaporize a 0.25g–0.5g load of flower (which only needs about 15–30 Joules to reach extraction temperature). Because the heat reservoir is already vastly oversized relative to the tiny mass of the herb, air temperature drops are negligible, and late-hit harshness is driven by the natural exhaustion of flavorful monoterpenes rather than heater overshoot. Consequently, maintaining a rock-steady, near-constant air temperature is the ideal engineering goal, leaving the user in total manual control of the extraction dynamic simply through draw speed and lifting the injector off the bowl the moment flavor degrades.
Good topic. I think this discussion has some things in common with a thread I started about convection vape technique On-demand vape technique: to fire or not to fire?
I agree with @SBuyer that a constant temp is probably best. Control of the heat is probably best left in the hands of the user, either through draw speed or removal of the heat completely.
I think this is true only because the definition of a “really good hit” is probably really different across the population. S-curves and other heater control systems might work for some people but there’s probably too much variability across users to program heat release curves that work for everyone.
I think it’s an art to get the most out of your convection vape through your draw and knowing when to kill the heater.
I’m like @TigoleBitties , I adjust constantly using my draw speed on convection devices, including my Universal Baller. It’s not as easy with more hybrid of conduction based vapes, but I think I would prefer a gradual increase, becoming a faster increase at the end. That is just conjecture though since I don’t have a device I can set profiles in, and I’m too lazy of a stoner to keep changing temp while I’m getting medicated.
Ok. So then perhaps it is best to have enough heat capacity so that if you slow your draw you get an increase in temp but not so much that you can’t lower the temp by pulling faster. At an extreme of heat capacity, neither decreasing, nor speeding up your draw would have an effect on the existing air temp.
Yeah I’m not 100% on the physics behind this but I think this statement is true and maybe it’s what we’re after. We don’t want the air temp to change much as we’re drawing so the governing variable of the rate of extraction is our draw.
I think of the ideal heater like the cherry on a cigarette. It will keep temps across the tobacco constant by burning hotter to combat the drop in temp from your draw. Pull hard and you get a big hit. Pull less hard and you still get smoke but just less of it.
I like my plug-in log vapes for this. I know the heater will “keep up” with my draw so I’ll always get vapor but my draw controls how much. I don’t want the temp of the hits changing much though and a good heater can do this.
My DaVinci IQ2 that I don’t much use anymore had profiles called “SmartPaths” (I think) and you had 4 different profiles that would change temps for you at specific times during the session. Nice idea but I never bothered with it and just did things manually. 
It’s important to distinguish a few things.
Even with a constant surface temperature (e.g. very high heat capacity, or tight control of the heater), it is possible to have different temperature differentials between the surface and the air. The air is always colder than the hot surface, and the higher the airflow, the more difference in temp. Otherwise, there would not be any heat transfer. These are related by “newton’s law of cooling” as it is sometimes called. This states that the rate of heat transfer to the air is proportional to the difference between the surface temp and the air temp. The constant of proportionality is known as the convection coefficient and depends on the geometry, material properties, and characteristics of the flow, so is quite complex to model. The other part of this is that the stored heat gets depleted if not replaced and so the temperature drops. For a lot of vaping situations, we can probably treat the heater as a “lumped capacitance”, meaning the temp throughout it is constant, and the surface temp and core temp are one and the same. But there are exceptions for glass heaters, since glass is a relative thermal insulator. So the point is heat capacity is not the only thing that causes the air temp to drop.
I disagree with this actually, as for convection you also have to heat up all the air that is flowing through the system, and that is where the majority of the energy goes. I think your larger point that a ball vape has plenty of heat capacity for what it needs to do is very true though. But I think the temp drop on the draw is inevitable and interesting to think about.
The thing about this though is that a log doesn’t actually have an active temp control that will increase the power dissipation as soon as the draw starts. It’s the stored heat that provides the on-demand power, that’s why the heatup time relative to the hit time is so long. For example one of my logs will idle at 10W, and reach equilibrium. But as soon as you start drawing the power delivered from the heater cover to the air could be 50-100 W. So the 10W that the heater is contributing helps, but isn’t really a substantial fraction of the total.
I do think the general question of an ideal heat release curve is interesting and relatively unexplored as a feature. I could see that there are some devices which benefit from the air temperature dropping during the hit and preserving flavor that way. For example the Ubik by @KeroZen is intended to be used releasing the button partway through the hit like @TigoleBitties refers to.
A little bit of a high temp in the very beginning of a hit might just be absorbed well by the cold bowl and herb, helping it get to temp faster without overshooting.
You also need a pretty extreme physical design of the heater in order to get the responsiveness needed for true on-demand temp control. You need either a super tiny thermocouple, like tafee, and/or some very thin ribbon heater with TCR control, like the tinymight and frolic. From there it would be interesting to vary the air temp according to different profiles.
I think fundamentally because of this most other vapes are “stored heat vapes” even if they do have temp control. In which case more stored heat is probably better, assuming you want a lot of heat at relatively low temps.
Add the fact that air temp does not equal herb temp, and that the actives actually have high boiling points, and so are evaporating rather than boiling, and it gets really complex. But at the same time simple devices can work really well.
Agreed.
Agree with this too. I think heat storage or a heat battery has proven to be an inexpensive addition to most vapes these past years. Corundum, zirconia and SiC are everywhere and their stored heat smooths out the inevitable temp drop from drawing. The Couchlog I use is no exception. The e-NanoXL though might be a plugin where the heater keeps up with little or no storage assist.
In any case I find that I enjoy vapes that can support a long string of draws with no appreciable temperature drop that alters vapor production.
Even without adding balls etc. there is a bunch of heat storage just in the heater cover. I learned this when I built my log with closed loop temp control. That’s where you get the peak power from. I don’t think it’s really accurate to say that the heater “keeps up”, since the power taken out by the draw is still much greater than the power going in from the heating element, and the temp drops, always. Just the nature of how the log regulates its temp passively. With that temp controlled log even 50W was not enough to keep up with a regular draw. Normal logs can definitely run out of steam with long hits especially on a water piece. Even ball vapes can, depending. But it’s about if they provide good vapor for long enough to satisfy until they can recharge. This might be controversial, but I find even a significant drop in heater temperature, say 50F, is not actually that noticeable with true pure-convection devices like logs and ball vapes. With conduction it matters more if you’re baking the flower, for better or for worse.
Glad to see some technical discussion. I think the process of how these devices work is not actually as well understood as we may think.
Yeah for sure - anything in contact with the heater is gonna get hot and store heat, especially a metal cover. I guess I just considered that whole piece the “heater” as opposed to balls that are added to help store more heat.
I think this is true too, and not so controversial but only if the resulting temperature drop isn’t enough to affect the ability to create vapor. That’s what I was trying imply by saying the heater can “keep up” with the draw.
Right. So since we are driving the evaporation of a significant fraction of the mass in the herb (cannabinoids, terpines, water, etc) that is going to happen a bit like drying clothes where the first stage is vaporizing compounds at the surface of the plant materials and then a second stage where we’ve got to get it to migrate out of the plant fibers to the surface. The tricombs hold most of the oil and probably burst quickly. High heat early is probably helpful to warm everything up and crack the tricombs and start stage one where we’ve got all the surface oil evaporation. This is probably the bulk of what we are interested in and the process of vaporizing everything should cool the herb relative to the air temp protecting against charring. In this first stage the rate of vaporization should be quite impacted by temp and airspeed (in that order). Then once the surface oils are about done the herb temp (and exist air temp) would start to rise and we’d move to the 2nd stage. This stage is less responsive to temperature and especially airspeed. Much more dependent on the characteristics of the herb, grind, etc. as we’ve got to move the oils inside the plant fibers out to the surface. Also the risk of charring or combustion goes way up in this stage so maybe we don’t even want to get into the stage two vaporization. Not sure, but a lower temp here could be useful. Probably one would ideally stop your hit at the end of the first stage and then wait a bit for the migration of the oil back to the surface before coming back for a short cleanup hit. Perhaps that’s the art that one targets intuitively for the perfect hit.
Well I don’t think vapes are extracting anything much that is not in the trichome heads. Pretty sure this is where like 90+% of the actives and terpenes are and the rest of the plant material and trich stalks contain mostly precursors. There was some info on this on FC, might be on the archive. I think it was Dan Morrison who found some sources for this. If you look at your herb before and after with a microscope or loupe (would definitely recommend) you can see that the trichome heads do quickly have their waxy coat melted. After this point they may spill their resin down onto the plant matter which provides an evaporation surface.
I agree that stage 2, if it does exist, might not be what we want to focus on. I do think that both temp and air speed are important for vaporization especially by convection and the evaporative nature of vaporization explains this pretty well. One thing I remember well from my heat transfer classes is that evaporation is a process limited by diffusion. This explains why even a conduction vape won’t really spill vapor until you draw on it.
I think that the proliferation of the ball vape shows a good solution to the problem of vaporization: expose the herb to a fast moving stream of relatively constantly hot air. With enough power, you can quickly flash the trich heads off and put less energy total into cooking the herbal material below, ultimately resulting in lighter colored ABV. I know @KeroZen and @invertedisdead have some opinions about some of this stuff as well.