Start filling out the input fields from the top left. If you're American, you may want to switch to imperial units (inches instead of millimeters). If you're unsure about something, try leaving it at the default value. You can always correct it later if it turns out to be wrong.
If you're new to coil winding, your wire is probably Kanthal A1, and it is probably round. Conveniently enough, these are the default values.
The wire diameter should be printed on your spool, in either AWG or in millimeters. Enter this in the AWG field
or the field to the immidiate right of this, labeled ⌀r.
Finally, select the target resistance of your choice. It is advisable to stay above one Ohm until you're fairly certain of what you're doing. You need to know how much current your batteries are capable of providing safely. Please read up on battery safety anyway, this stuff is important.
As you update the input values, the results will be updated in the table on the right.
Video tutorials
Reading the results
Resistance wire length
This is the length of the resistance wire after you've installed it in your topper and trimmed the excess.
Number of wraps
If you're making a coil for an atomizer where both the coil legs point in the
same direction, the "Number of wraps rounded to half wraps" is the result you want. If you're coiling for an atomizer where the legs point in the
opposite direction, use the "Number of wraps rounded to full wraps" result.
Heat flux
Generally you want to stay somewhere between 120 and 350 mW/mm². Some like a cooler vape, others like it hot. The color of the flame icon will give you a rough idea. Adjust to your own taste.
Heat capacity
The higher the heat capacity, the slower your coil will be to heat up (and to cool down).
Leg power loss
Wasting power on heating the coil legs can make your vapor taste metallic or harsh, so keep your legs short whenever you can. Interestingly, the leg length is not the only value that affects the percentage of power loss in the legs. The wire gauge and the number of wraps also come into play, so keep an eye on this number. With most coils, you generally want to keep it below 10%.
Advanced
The rest of the result values will probably start to make sense once you get used to using the calculator. If you need an input option or a result that you haven't seen in Steam Engine yet, try clicking the Advanced button. You might be in luck. A second click on the button will bring you back to the basic mode. Note that any changes you made in the advanced mode will be remembered even if you exit the advanced view. If you want to start from scratch, use the Reset button.
How the coil calculator works – what it does, and what it doesn't do
Platform and precision of engine parts
All calculations are done in
JavaScript, which uses
64 bit floating point. This yields a precision of 15–17 significant decimal digits, which is more than sufficient for the purpose of modeling a coil build.
Internally, all variables are stored and calculated in
metric units. Unneccessary unit conversions are avoided in order to prevent accumulation of rounding errors when using
imperial units.
Three values are written to the input fields during use (advanced mode): Wire diameter, wire resistance per mm, and resistance wire length. These numbers are rounded in the input fields, but still preserved with full precision in memory. If you manually override a value, you can enter your own number with any precision you want. When you save, and subsequently load the settings, rounded values will be displayed, but the number will still exist with the full precision in memory.
Inner workings – a peek inside the engine room
Resistance wire length
AWG is converted to diameter by using the formula that defines AWG. This should make the AWG conversion more precise than the numbers stated by many resistance wire vendors.
Wire resistance per length is determined by the
specific resistivity of the wire material, and the cross section area of the wire. The specific resistivity for each material is looked up in a small table of constants.
The resistance wire length is your set target resistance divided by the wire resistivity per mm. Leg length is subtracted before calculating the number of wraps.
Material Specific resistivity (Ω mm²/m)
Kanthal A1/APM 1.45
Kanthal A/AE/AF 1.39
Kanthal D 1.35
Nichrome N20 0.95
Nichrome N40 1.04
Nichrome N60 1.11
Nichrome N70 1.18
Nichrome N80 1.09
Ni200 0.096 (@ 20°C)
Wraps
When you input the inner diameter of the coil, the outer diameter is simply the inner diameter plus twice the wire thickness. The circumference of your coil is then by multiplying the outer diameter with π, and we have length of a single wrap. The wrap does not go in a straight circle around the mandrel, but rather in a helix, making it slightly longer than the coil circumference. For twisted coils, the 2–4 strands are combined into one diameter using the diameter of an outer circle encompassing the 2 4 tangent circles of each strand.
Heat
The
heat flux is more or less evenly distributed over the resistance wire. Hot legs are undesirable, so the power used to heat the legs can be regarded as "lost".
The density of the coil material is used to calculate the wire mass and
heat capacity. Because of lacking data on the density of different Nichrome alloys (except N80), the density of the Nichrome qualities are interpolated from the densities of the main alloy elements.
The heat capacity of the wire materials does not vary much between the alloys used. Therefore 0.46 kJ kg-1 K-1 is used for all kanthal, and 0.447 kJ kg-1 K-1 is used for all nichrome.
Possible error sources – or spherical cows in a vacuum
This coil calculator is a pretty simple and straightforward digital model of the geometry and electrical properties of an atomizer coil, and can be expected to be concistent with at least itself. Real life, on the other hand, involves a myriad of ways to introduce error to your numbers:
- Depending on the quality, the resistance wire might be slightly thicker or thinner than specified, or the alloy might be slightly different, which would affect resistivity.
- When you wrap a coil, the wire is also being stretched, increasing resistivity. This is seldom very significant, but that depends on how small the inner diameter of your coil is, and how much tension you put on the wire while you wrap it. Thinner wire stretches more easily, but it also bends more easily, requiring less tension on a small mandrel.
- In a coil with touching loops (e.g. a micro coil), a little current will flow between the loops. Even though the oxidation of kanthal creates an thin insulating layer of alumina around the wire, no insulator is perfect. The amount of current that will "leak" depends on the thickness of the alumina layer, which in turn depends on the alloy used, and how much you torched it. It also depends on the area of the loops actually touching, how hard they are touching, the voltage potential between each loop, etc.
- E-juice does not conduct electricity very well, but like everything else, it does conduct a little. Burnt juice leads to carbon buildup on the coil, and carbon conducts electricity fairly well.
- When building with Ni200, the resistance of the coil is typically so low that the "internal" resistance of the atomizer itself can become significant. As a result, the resistance may read higher than expected when everything is put together on a mod. Examples: One of my favorites, the eXpromizer, has a spring loaded center pin. The spring also acts as a conductor, and because of the high currents involved, it can become warm if it is not clean. The Squape R is also known to not "like" Ni200. High or erratic resistance readings are not uncommon. If you can, try to stay well over the 0.1 Ω limit of the DNA 40. With a higher resistance coil, the current will be lower, which means that you lose less energy heating up the electrical pathways in the atomizer. Your resistance readings, and as a result the temperature control, will be more accurate. Your battery life will probably be a little bit better as well.
The maximum resistance for the DNA 40 in Ni200 mode is 1.0 Ω. Reaching this high is difficult with Ni200, and not a goal in itself, but keep this in mind: There is plenty of headroom at the top. Don't be afraid to take advantage of this fact.
