Here you go
@BrentMydland - something I had been meaning to write about for a little while anyway.
TC Inaccuracy from Atomizer Resistance - Static and Variable
Static Resistance (SR)
SR = any amount of resistance read by the mod which is not accounted for by the wire in the coil itself, and which is always present (static)
*.
Therefore it is primarily the resistance of the atomizer itself - its positive pin, its positive and negative block/posts, and any intermediate metal. The positive is usually the higher resistance, as the negative goes through the body of the atomizer which is much thicker and so lower resistance (though the positive is often partly brass or copper to balance this out.)
SR can possibly also include the wiring to the mod's 510 and the 510 itself, depending on whether this has been measured and automatically subtracted by the mod/chip maker. (Users of the DNA 200 can adjust this themselves via eScribe's Mod Resistance setting.)
The impact of static resistance (SR) depends entirely on the base resistance of the coil. It is irrespective of the TCR, and therefore also, technically, of the wire used.
However as the wires we use have different resistivity, in practice the wire used does impact the likely effect from SR. Ni200 coils are almost always very low resistance and thus experience high inaccuracy from SR, where Stainless Steel resistance is almost always much higher and therefore much lower inaccuracy, and so on. Titanium and NiFe are in between, with Titanium less affected than NiFe.
I consider 0.01Ω to be the maximum amount of static resistance I will willingly accept. Most of my tanks that I have checked are no more than 0.005Ω, but I haven't been particularly bothered to test them all quickly. I'll do it over time, as I feel like it, and I wouldn't stop using a nice tank if I did find it was more than 0.01Ω. I would though see if I could improve it. I have already improved a few tanks by replacing stainless parts with brass, in particular my Squape Rs clones. My best tanks for SR are my authentic Aqua v2s, which have an all-copper positive path that I measured as 0.0004Ω!
As mentioned, the inaccuracy caused by SR is relative to the base resistance of the coil. It is also relative to the target temperature - it is best expressed as a % of the target temperature.
Here's the inaccuracies caused by 0.01Ω of static resistance for the following coil base resistances as a % of the target temperature, and actual inaccuracy at target temperature = 230°C (446°F)
- 0.08Ω: +11.41% | +26.25°C
- 0.12Ω: +7.61% | +17.50°C
- 0.15Ω: +6.09% | +14.00°C
- 0.20Ω: +4.57% | +10.50°C
- 0.40Ω: +2.28% | +5.25°C
- 0.60Ω: +1.52% | +3.50°C
I have expressed the inaccuracies as + numbers, by which I mean they are the amount by which the mod will
overheat the coil. So a 0.08Ω coil with 0.01Ω SR (read by the mod as 0.09Ω) set to 230°C will actually be heated to 256.25°C, which is +11.41% of the target temperature.
The percentages change linearly with SR. So 0.02Ω SR will cause +22.83% inaccuracy with the 0.08Ω coil, and +3.04% with the 0.60Ω coil. They also change linearly with decreasing base resistance - for example note 0.40Ω being half the inaccuracy of 0.20Ω.
As SR always leads to overheating the coil, the natural fix is to set our target temperature lower than we want to achieve. To compensate, we can simply set it lower on the mod by the amount indicated above - eg if we know we have 0.01Ω SR on a 0.20Ω coil, and our actual target temperature is 230°C, we set the mod to 220°C. Rounding as appropriate to get the nearest °C or °F setting the mod will do.
* The exact definition of the 'static' in SR is: present at the time the cold base resistance of the coil is measured by the mod, and then present inthe same amount throughout all subsequent vapes until the next base resistance measurement.
If the resistance is not present during one or the other occasions, or its amount changes, then that (or some part of it at least) becomes Variable Resistance:
Varying Resistance (VR)
VR = any amount of non-coil resistance which comes and goes, for example caused by the atomizer not being screwed on as tightly as possible to the mod, or by screws not being tightly secured onto the coil, loose posts, or any other resistance that cannot be guaranteed at a single figure.
VR should be avoided wherever possible because it is unpredictable and thus cannot be easily compensated for.
VR affecting the base/cold coil resistance reading
Let's look at a 0.01Ω varying resistance that is there when the cold resistance is read, and then disappears later; perhaps the atomizer was not fully tightened at first and was then adjusted later.
- Two examples, two different atomizers on two different identical mods
- The atomizers themselves have 0.01Ω SR
- Coil One is really 0.20Ω, but the SR + VR causes the mod to read it as 0.22Ω.
- Coil Two is really 0.40Ω, but the SR + VR causes the mod to read it as 0.42Ω.
- The user has a desired target temperature of 230°C
- Some vapes are taken before the VR disappears. There is 0.02Ω SR+VR in place, therefore the inaccuracies are:
- Coil One, 0.20Ω : +9.13% = +21.00°C = heated to 251°C on a 230°C setting
- Coil Two, 0.40Ω : +4.57% = +10.50°C = heated to 240.5°C on a 230°C setting
- These are just the same as if there had been 0.02Ω SR.
- Now the VR disappears, but the mod still thinks the base resistance is 0.42Ω
- Coil One, 0.20Ω : Will be heated to 265°C = an inaccuracy of +35°C
- Coil Two, 0.40Ω : Will be heated to 247°C = an inaccuracy of +17.5°C
- The fact that the VR has gone makes things a lot worse, an extra 50% inaccuracy compared to when it was there, because now the base resistance reading is out of sync with the in-vape resistance readings.
VR of this nature - present for base resistance, gone later - should be removed over time on mods with Refinement: the DNA 40, DNA 200, and some clones of the DNA 40. It will detect the resistance has gone down, and adjust its base reading accordingly.
It will not be removed, and will cause ongoing inaccuracy, on "Set Resistance" chips such as the Yihi, Dicodes, etc - or on the DNA 40/200 if the "lock resistance" feature is used.
We could regard base readings taken on non-cold atomizers (eg moved from another mod and not cooled) as also being VR. Well in fact it
is VR, just of a different cause to the mechanical components we normally associate with SR/VR.
VR not present during the base/cold coil resistance reading but then appearing during vapes
For this case, let's assume the base/cold coil resistance was read correctly but then 0.01Ω VR suddenly appears in the middle of a vape. For simplicity, this example assumes there's not also any SR:
A 0.20Ω coil, read correctly by the mod as 0.20Ω, but then exhibiting +0.01Ω VR during the vape, will have an inaccuracy of -14.3°C; it will be
underheated, not overheated as in the SR cases.
This case would include situations where the atomizer was accidentally unscrewed a touch from the mod, or otherwise became slightly looser - causing the base reading of the coil to increase.
I am not yet sure if DNA Refinement helps correct this: it's much harder for the mod to detect, because it has to be sure that the rising resistance isn't just normal heating of the coil, but actually the base resistance rising. Maybe the DNA 40/200 does address this, perhaps by detecting base resistance rose for a long period during which no vaping occurred? I'm trying to test it now and will update.
Summary:
- Varying Resistance: if you've got it, check your atomizer/mod and fix it; it is, as the name suggests, variably impactful and there's no good reason to put up with it.
- It might underheat, it might overheat, and either way it will do much more damage than SR
- Static Resistance: every atomizer has some amount, and unless you're using Ni200 it's rarely a big deal
- It applies fairly minimally for most resistances you are likely to build to with Titanium, Stainless Steel, and (to a lesser extent) the NiFes.
- With NiFe70 single coil you might get down to 0.15Ω or so, and thus be subject to 6%, or around 14°C at common TC temps.
- NiFe52 single coil should not usually be worse than 4.5%, or 10°C at 230°C/450°F, and will often be better (my standard coil of NiFe52 will be 0.30Ω)
- SR maximums: I consider 0.01Ω to be a good guideline maximum: with at most this a mount of SR, at the resistances I build, I don't have to think about offsets.
- I set 235°C every time and my vapes are pretty consistent (on accurate mods) regardless of the atomizer in use
- If you've got more than 0.01Ω you can use the figures in this post to decide how bad it is at your usual base resistances and either try to reduce it, or just account for it in your target temperatures.
- Adjusting for SR: In all cases of high(er) SR, the result will always be that the mod over-heats the coil
- So it can be counteracted by lowering your target temperature by the amount indicated by the inaccuracy
- Remember that the inaccuracy is a % of the target temperature, so the offset will vary according to that.
- But not many people will want an actual achieved temperature outside the range 215°C - 240°C (420°F - 464°F), so the offsets are going to be very similar at all these temps, not different enough to worry about checking the % each time.
As an aside, it's this over-heating of the coil - and thus under-setting of our target temp - that explains why it was common in the earlier days of TC to see YouTubers like Busardo talk about "this tank tastes burnt at more than 300°F, while this RDA needs 450°F". The joys of Ni200
