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The practical applications of heat flux, wire gauge, parallel wire count & net resistance

Published by State O' Flux in the blog State's Stuff. Views: 4028

The practical applications of heat flux, wire gauge, parallel wire count & net resistance as they relate to... the simplicity of a mech mod vs. the independent wattage control of regulated mod.

In simpler terms... the Ohm's law limitations of mechs vs. the ability to "force" wattage with an APV... as it effects user vaping performance / satisfaction.

I'll keep this relatively short. If you've read the Steam Engine User Guides - parts 1 and 2... then you (probably) have a good handle on the fine tuning aspects of your builds. If not... then much of this may be beyond your current knowledge level. We're dealing with potentially high wattages and current values, so be smart and play it safe... until you really have a good understanding of the variables.

This is directed at both mech mod diehards - who, much like guns, won't surrender them until they're pulled from your cold, dead hands - to more rational folks, who may be on the fence about mechs and APVs in general... or those considering a first time purchase of a high wattage APV.

If you're a complete noob, with little to no understanding of Ohm's law, wire selection and so on, start with the "Advanced Education" article, and proceed in what ever direction suits you. A good bit of reading, but your brain will be bigger for it. :p

Trust me when I tell you, although I'm a true lover of mech mods... in actual fact, if you don't give a crap about the Ohm's law knowledge prerequisite, "battery holder" simplicity (and "style"?) that mechs provide... the high-output, regulated APV is the way to go... if you want more build freedom than Ohm's law will allow.
(But you should still know your OL formulas... m-kay?) [​IMG]

Without independent regulation, a battery will discharge wattage/amperage at a rate determined by net resistance and voltage.
  • For wattage ("Power") the formula is V² ÷ Ω = P.
  • For amperage ("Current") the formula is V ÷ Ω = C.
The calculated wattage doesn't care about your wire gauge or coils in parallel count... just the net resistance and battery voltage.

Heat flux is the coil(s) radiant heat, expressed in milliwatts per millimeter of coil surface area... squared. For our purposes... it's simply how warm you perceive your vape to be.

Heat flux and capacity do care about the wire gauge (AKA wire surface area & mass), coils in parallel count and net resistance, and is directly affected by them, both together and independently.

With that out of the way, lets move on to examples. All values are arrived at using the Steam Engine vapist's calculator.

With a mech mod... if you want to run at 0.5Ω with a heat flux of approximately 300 mW/mm² +/- .

Ohm's law formula calculates this to be a discharge of 35 watts @ 4.2v (and less as the battery voltage diminishes) ...

Disregarding wrap count and surface area for now, for a single coil, you must use 24 gauge (or thinner) wire... for dual coils you must use 28 gauge (or thinner) wire. That comes out to 309 & 311 mW/mm² respectively.

With a regulated wattage mod (for consistency, we'll call this a 250 watt output APV) you can use what ever wire and build you want... or at least a broader spread of possibilities.

A 300 mW/mm², 26 gauge dual parallel build at 0.7Ω? No problem. 95 watts will provide 300 mW/mm2 even. With a mech, you simply can't do it (at least, not with 26 gauge - try 30 ga.)... Ohm's law dictates that you will discharge only 25 watts, for a stone cold 39 mW/mm² with 26 gauge wire.

Another example... lets go deep sub-ohm, say 0.2Ω, with a quad parallel 24 gauge build. From a mech mod, Ohm's law dictates 88 watts @ 4.2v.

Run those numbers through Steam Engine, and we get a very cool 121 mW/mm². With a regulated mod, all we need to do is run at 218 watts to obtain our HF of 301 mW/mm².

Can we get 300 mW/mm² from a 0.2Ω, quad parallel 24 gauge build with a mech? Yes, we can. but it can only be done with 27 gauge wire, for a slightly hotter than desired, 345 mW/mm².

This last example is through the eye of maximum net coil surface area, with the lowest acceptable heat flux.
Our resistance limit is 0.20Ω on a dual parallel coil build. We'll use the resultant 88 watts from an OL calculation at 4.2v. 23 gauge wire provides a HF of 343 mW/mm², and a net coil surface area of 177.59 mm² per coil.
How low a coil temperature can we accept? 200 mW/mm²? 150mW/mm²? Lets split the difference and see where we end up. By accepting a 171 mW/mm² using 21 gauge wire, we increase our net coil surface area to a whopping 253 mm² per coil!

The point to that last example is that if you have enough wattage, you can increase net coil surface area, without trading off a loss of heat flux.

The most important values to the vapist are heat flux, parallel coil count net coil surface area, wire gauge and net resistance.

My personal preferences are, in order - coil surface area, heat flux and resistance. What ever wire gauge will provide those values will be what I use.

Mech mods pretty much require that order... and I'm sorry for you, if all you have is 24 gauge and want a dual parallel, mid sub-ohm net resistance build. :(

Secondary values are heat capacity ("lag time" to desired temperature) and leg power loss... which is the percentage of power wasted heating the legs. This last value is based on wire mass/density... the greater the mass, the more energy wasted to the coil legs.

I'd not get too spun up if something is a bit high, but keep an eye out for extremes. Steam Engine will actually change color in the value box if something is extraordinarily off the mark.

In either case, we want low secondary value numbers... and to obtain those low numbers (and good primary values), we need to perhaps use thinner wire than we'd prefer... or possibly a higher net resistance, or a single coil build, when we'd prefer a dual parallel build.

With a regulated mod, you don't have total freedom in the sense that you can do stupid stuff, but... the range of possibilities opens up greatly when you can force wattage above that of the unregulated Ohm's law discharge.

Again, I'm a mech fan. Don't even own a hi-watt regulated mod... Yet. ;-)

That's it. Ciao for now!

Mooch and Ryedan like this.
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