Insomnia, and nothing to do... but write.
There are only two things that determine resistance in single wire coil construction... wire length from terminal to terminal (this will include the leg length), and the gauge/thickness of the wire.
Wrap count or coil ID are relevant only in the physical sense. Some of the more useful considerations... does the coil fit in the atomizer and use the available dimensions to best advantage, and does it provide an optimized surface area for the wick material used.
As an example, and using the Steam Engine coil modelling program... a length of 28 gauge Sandvik Kanthal A1, between 44~45mm long (again, terminal to terminal) will provide a resistance of 0.8Ω+/- ( or, 0.018Ω+/- per mm)... and will always provide this resistance, regardless of ID or wrap count.
So what would be a good build for your desired or target resistance - you might ask? On a 1/16" (1.6mm) mandrel, with a 2mm leg length... you can do a 7/6 wrap. All else remaining the same, but a target resistance of 1.0Ω... the wire length increases to 56mm and the wrap count to 8/7.
With dual parallel coils, the gross resistance of 1 coil is halved to provide the net resistance. The value previously described, in a DPC build, should have a net resistance of 0.4Ω+/-.
With parallel coils, the net resistance will always be the resistance value of one coil, divided by the number of coils mounted.
Inversely, to maintain a specific desired resistance for X number of parallel coils, the length of wire used (or as described semi-correctly by your local B&M guy - the wrap count) per coil must be multiplied by the total coil count.
You
must have an ohm meter... be it a build box or a DMM (preferably both), to accurately measure and verify net resistance. To go without is to assume the target build has no shorts or opens, and that the atomizer has no added resistance. Potentially, your safety, and the safety of those around you, are in your hands... literally.
Once confirmed, you need to insure you have adequate current output for the target resistance... or visa versa. Simple Ohm's law calculations will provide these answers.
What you use for a wrapping mandrel isn't all that relevant. You can use a toothpick, a Q-tip, a micro-screwdriver shank, a probe from a DMM test lead, a ball point pen ink tube... as long as it's sturdy enough not to compress or break under wire tension.
Coil IDs in the 1~3mm range are common. Smaller IDs
may provide a higher heat concentration and a more rapid vaporization of the liquid within the coil/wick... and requires wick material with excellent capillary action and retention.
Technically speaking, the "micro" coil is considered by most to have an ID of 1/16" (1.6mm), to slightly less... with no air gaps between wraps (compressed).
There are always
a few wire gauges that would be considered as "optimal" for any desired resistance/wick material/applied current and... the number of coils fit in parallel, if any.
Click the two sigline links below if you want to learn more. ;-)
