Interesting. Why do you say the bigger ID will use more of the available internal airflow? To my mind the smaller ID (but wider coil) will be better cooled all round? The net effect might be the same due to the quicker heating up of the smaller ID coil.Haha, thanks for the shout @Silver! Glad its getting out!
I have that on my to-do list. Problem with vape-related research is you need the research atomizers for vaping as well... I'll have to scour the classifieds at some time for a few cheapies. I have some related data, but will keep that together for a bigger post.
But I've been speculating, and based on what I've seen thus far:
I agree 100% with you, @Silver.
If we compare two coils with the same material (28 AWG Kanthal), the same number of wraps (6), but different inner diameters (2mm and 3mm), and therefore different resistances (1.0 ohm vs 1.345 ohm):
In addition, smaller coils have more constructive/destructive interference when heating - in other words, all parts of the wire is closer to each other, therefore heating one part of the coil causes its neighbouring parts to be heated more, which causes the original part to be heated even more, which causes its neighbouring parts to be heated even more more... you get the idea.
- The 3 mm coil will (when stretched out) be longer (75.11 mm vs 55.85)
- Hence, it will have different surface area's (70.73 vs 51.92 mm2)
- Hence, it will have a lower heat flux (1.32 vs 1.78 Watt/mm2)
- Hence, it will require more power to heat up
- It will be able to wick more juice at a time
- It will utilize more of the available internal airflow
So we see that, with two coils with the same width (ie, same number of wraps), but larger ID, the larger ID coil will take more power to heat up to the same level. However, if you give it more power and get it to the same temperature as a coil with a smaller ID, you'll produce more vape since you've got more of the coil in contact with the juice.
The last two points in the above list I'll get to a bit later.
On the other hand, if we compare two coils in the same manner as above, but instead of keeping the number of wraps constant, we keep the resistance constant (@ 1 ohm), we see the following:
- Both coils will be the same length when stretched out (55.85 mm)
- The larger ID coil will have slightly fewer wraps (~4 instead of ~6)
- Hence, the surface area will remain identical (51.29 mm2)
- Hence, the heat flux will remain identical (1.78 watt/mm2)
- Hence, both coils will take the same amount of power to heat up
- The higher ID will wick more
- The higher ID will utilize more of the available internal airflow
The constructive/deconstructive interference I talked about above still applies here (in other words, due to the wires being closer to each other, will lead to quicker heating) - but I don't actually know how big an effect that is. My research thus far shows that it is not at all, and that the differences because of this effect comes down to fractions of a second. But it is there, either way.
Looking at the list above, we see that if we have two coils with different ID's but the same resistance, they will take the same power to heat up to the same temperature. The only difference between the two coils will be the last two points:
Lets scratch the first one, and assume our wicking technique is even better than Zeus', and that both of our coils wick fast enough for the power we are throwing at them. Now, we are left only with the last point, but arguably the most important one.
- The higher ID will wick more
- The higher ID will utilize more of the available internal airflow
If we have two coils with the exact same physical properties (in terms of how much power it requires to heat up), the larger one relative to the airholes (as well as how the air flows inside the atty) will be cooled more, which generally means more vape - depending on our setup.
For instance, lets say we take the two identical resistance coils from above, and throw 40 W at each of them while drawing as hard as we can. The smaller ID one will reach say 220 Celcius, at which point the airflow across the coil cools the coil almost as fast as the electricity is heating the coil, and the temperature remains more or less constant. On the other hand, with the 3mm coil and at 40 W, there will (usually) be more airflow across the coil, so it is cooled more, despite having the same heat from the electricity (since the two coils' physical properties are identical). So this coil might max at around 200 Celcius, maybe, and therefore produce less vapour. But that means we can bump up the power to 50 W using this coil - despite the fact that both coils heat up at the same tempo - because we have more cooling, and overall we'll produce more vapour at the same temperature in the bigger ID.
Unfortunately, things aren't always that simple. Using a larger coil (and more wick, accordingly) might mean that you'll be blocking of some of the indirect airflow, which can help cool the atty, create bigger clouds (albeit slightly less dense), and cool the final airflow, giving a cooler vape. In addition, a larger coil can actually mean blocking off the route air would take which would give maximum exposure to the coil, and rather force air to follow pathways which give less cooling and less vape. Or it might mean breaking a vortex effect.
In summary, I have found the following regarding coil size. These are mostly still heuristic, but I'm working on backing these up with experimental data:
- Larger coils equals more power equals more vape
- Usually, larger coils gives me an overall larger vape
- Usually, larger coils heats up my atty and mod more
- Too large a coil will give airflow problems - it will get hot too quickly but with very little vape, and the atty will heat up immensely
- Larger coils tends to give me less crisp flavour (not always though, and not necessarily less flavour)
So all in all, I'll say what I always say - larger/smaller isn't better/worse, it will only depend on the atty. Certain atty's like smaller coils, certain atty's like bigger coils.
Btw, in terms of the electrical properties.... I haven't found any difference in any resistance range. As long as you stay safe, and you're mod is more than capable putting out higher voltage for higher resistance builds, I haven't found any difference in vapour/flavour production based on resistance, if the physical properties of the wires (heat flux, heat capacity ect) remains the same. So you don't necessarily have to think higher resistance means more voltage means super-fast moving electrons which will blow the crap out of that poor little Suicide Bunny - it generally is all the same, as long as the power (W) is the same. I think some of the pervasive ideas that lower resistance equals more vapour stems still from the days before variable voltage or variable wattage. At much very high or very low resistances there are some thresholds and funky patterns in terms of electricity flow and the thermodynamics of the material, but I believe these occur outside of vaping ranges.
That said, if you are building a high resistance build, just check whether your regulated mod and batteries can handle it - they can be as dangerous as a low-ohm build if your regulated mod doesn't have the correct safety features. And keep a watch on the mod's temperature - some mods can handle a high resistance build at high wattages only once, if you get what I'm saying.
all 26 guage kanthal because students be poor. will be getting some 32 guage for some claptons soon.
