is long legs generally undesirable? (coil legs)

[paulw2014]

When building coils, is it better to keep the legs short? For example, for the same resistance, one coil has longer legs (but thicker gauge Kanthal), another has shorter legs (thinner gauge Kanthal), is the latter better when it comes to performance?

Note: resistance is directly proportional to coil length, and inversely proportional to coil wire diameter. So although the first coil has longer legs, it has thicker coil wire, so resistance is the same as the second one.

 

[State O' Flux]

Interesting question. Interesting enough that I'm curious, and so have spent a bit of time running numbers. You don't provide any specific values, so I've run a few of the more common or popular wire thicknesses... so we have a broader view of the possibilities.

The short answer, if resistance remains unchanged, is... it depends. ;-)

I'm using "Steam Engine" to calculate the numbers, the link to this coil calculator can be found as my sig line. So we don't go to extremes with wraps at either end of the wire gauge spectrum... In other words, so much wire that it won't fit into anything, or so little that there's not enough to wrap less than 3 coils.
I'll be using 1.Ω as a constant and I'm not going to go to extremes on leg length... more than double is sufficient to see a trend.

26ga wire 92.6mm long - 15/14 wrap - 1.5mm ID - 7mm leg = 7.6% power loss.
26ga wire 92.6mm long - 15/14 wrap - 1.5mm ID - 3mm leg = 3.2% power loss.

28ga wire 57.8mm long - 9/8 wrap - 1.5mm ID - 7mm leg = a 12.1% power loss.
28ga wire 57.8mm long - 10/9 wrap - 1.5mm ID - 3mm leg = a 5.2% power loss.

30ga wire 36.5mm long - 6/5 wrap - 1.5mm ID - 7mm leg = a 19.2% power loss.
30ga wire 36.5mm long - 7/6 wrap - 1.5mm ID - 3mm leg = a 8.2% power loss.

32ga wire 23.3mm long - 4/3 wrap - 1.5mm ID - 7mm leg = a 30.1% power loss.
32ga wire 23.3mm long - 4/3 wrap - 1.5mm ID - 3mm leg = a 12.9% power loss.

So... we can conclude several things from producing these examples.

To first answer your specific question - For a given resistance which thickness wire performs better - thicker wire with longer legs, or thinner wire with shorter legs?
It's pretty close when there is only a few gauge separations. Slightly further apart when there are several gauge thicknesses in separation, and only in extreme do we have a substantial power loss (26ga/7.6%~32/12.9%)

For all practical purposes, it may not be that relevant, when resistance is the common denominator.... because, there is a wrap to wire length ratio that we may choose as most effective - within the given build platform physics we have to work with.

Extreme spread has more interesting information. With 26 and 32 gauge wire, the "rounded" wrap value is the same, ("exact" wrap, which I've not shown is slightly different to compensate for the added leg length) the leg and power loss only change. This is another possible indicator that, at extremes, either wire may be a poor choice for the desired resistance.

Potential power loss increases as gauge becomes thinner. This might change if I adjusted the resistance up or down, but might put either of our extremes out the running... at upper and lower resistance values.

I'll be putting this information in my blog so I don't lose it, and so I can run some additional formulas I've an interest in.

(If I've made any errors in math... someone please bring it to my attention)

Take it for what it is, do with it what you will. Cheers.

 

[scrappy]

When I first started using micros I would have said long legs don't matter. But now I can detect some harshness that I confused with throat hit when my legs are too long (irregardless of the wire size) . I keep them as short as possible now. It helps if you're using an atty with a small chamber so the coil isn't far from the airhole. Ie quasar, igo-s, kayfun, etc.

 

[steel bender]

In your first two lines, with the 26g, you have the same wire length, wraps and ID, but with different legs. How is the same wire length with the same amount of wraps producing longer legs with the same 1 ohm resistance?

Without doing any math, I would think, using the same g wires, same end resistance, but one coil having less wraps with longer legs, would be less efficient vaporizing juice, because you're losing some coil to wick surface area.

ETA - Comparing different g wire, with different legs, is probably over my head...