How does TC work and what is the Temperature Coefficient of Resistance?

How does TC work and what is the Temperature Coefficient of Resistance?

Temperature Control works by monitoring changes of resistance in the coil and using this to estimate temperature changes. It works because resistance increases linearly and predictably with temperature, although the amount that it increases varies greatly between wires.

TC is possible only with wire that has a reasonably high Temperature Coefficient of Resistance (TCR). The Coefficient is a numerical value that indicates how much resistance will rise for a given temperature increase relative to its starting resistance. When the coefficient is high resistance will increase a lot as temperature increases. When it is low, resistance rises a smaller or negligible amount.

Ni200 was chosen by Evolv as the first TC wire because it has one of the highest coefficients amongst common metals/wires. Kanthal has an extremely low coefficient - its resistance barely rises at all even with hundreds of degrees of heating. (For its intended purpose, this is a benefit; I think it may even have been engineered specially to achieve this.)

The TCR is relative to the starting resistance, meaning that the higher the starting resistance of the coil, the greater rate of increase in resistance with temperature. So a 0.10Ω coil heated hundreds of degrees will only increase in resistance by a few 0.1's, whereas a 1.0Ω coil heated the same amount will increase by multiple ohms.

The TCR of pure Nickel is approximately 0.006. This means that for every °C hotter a Ni200 wire gets, its resistance rises by (0.006 * )Ω. So for a coil that starts at 1.0Ω, every degree °C of heating will increase its resistance by 0.006Ω (0.006 *1). For a coil that starts at 0.1Ω, each degree will increase resistance by 0.0006Ω (0.006 * 0.1).

  • A coil is at 0.10Ω at room temperature (20°C / 68°F)
    • You vape and the chip sees its resistance is now 0.22Ω
      • So it knows its resistance rose by 0.12Ω
    • Then it calculates using the coefficient of 0.006 that a resistance rise of 0.12Ω equals a temperature rise of 200°C (392° F)
      • And therefore the coil temperature is now 220°C (428°F)
      • (0.22Ω - 0.10Ω) / (0.006 * 0.1Ω) = 200°C (392°F)
      • + 20°C (68°F) [starting temp] = 220°C (428°F) [coil temp]
  • A coil is at 0.5Ω at room temperature (20°C / 68°F)
    • During vaping the resistance rises to 1.136Ω
    • This is a temperature rise of 212°C / 413°F, from the following calculation:
      • (1.136Ω - 0.50Ω) / (0.006 * 0.5Ω) = 212°C (413°F)
      • + 20°C (68°F) [starting temp] = 232°C (450°F) [coil temp]
  • Of course it is performing these calculations many times a second, constantly monitoring the temperature and adjusting the power it sends accordingly.

DNA 40 and Yihi TC devices have Nickel's (or Ni200's) Coefficient hardcoded, so they only work accurately with Ni200 wire.

TC vaping with other wire is thus inaccurate by default, unless the mod allows the coefficient to be adjusted to suit the wire.

What wires besides Ni200 could we use?

At this time, there are three that I know about and I've tested two:
  • Titanium (Grade 1)
  • Stainless Steel (SS317L)
  • Dicodes' Resistherm NiFe30


Titanium has a coefficient of 0.0035, just over half compared to Nickel's of 0.006.

It is already being used for TC vapes (existing ECF thread: Titanium wire, vaping and safety.) It's possible to do this on normal TC mods but to do so requires a temperature offset of around 90°F - ie. one would set 340° when one wanted 430°.

With a corrected coefficient this will no longer be necessary - the real desired temperature can be set.

The advantages of Titanium over Ni200 are various, including enabling the use of micro/contact coils (not spaced) and higher resistance ranges. It's also stronger and won't break easily like Ni200 often does. Downsides with currently available wire is that it's springy as hell until (lightly) torched/pulsed.

Another potential advantage is safety; some metallurgists are identifying Titanium as being particularly safe, perhaps on a par or even more so than Kanthal. But understanding of vaping wire safety is still very much in its infancy.

Much more info is in the Titanium thread linked above.​

Stainless Steel

Stainless Steel has a coefficient of 0.00094, less than one-sixth of Nickel's.

This means that SS can not be properly used for TC without adjusting the coefficient - its resistance increases with temperature too little for normal mods, that are expecting Ni200, to use effectively.

Some people have vaped SS on normal TC mods, and it does somewhat reduce dry hits, but it will still burn cotton. To use it effectively, a coefficient adjustment is necessary.

Once the coefficient is adjusted, Stainless Steel works well - no burnt cotton. However my testing thus far has still required a temperature offset, albeit not as much as people have had to do with Titanium. When using the Infinite Nickel Purity feature, I have set an offset of around 50°F when using SS.

The advantages of Stainless Steel versus Ni200 are the same as for Titanium - micro/contact coils, higher resistance ranges, stronger. The advantages of SS versus Titanium is that it's much easier to work with - malleable, easy to coil. It is also readily available, and cheap. Crazy Wire/The Mesh Company in the UK sell SS 317L under the brand TMC.

The safety aspects of it are unknown at this point in time, though theoretically it should be safe at TC type temperatures (and perhaps safer than Ni200, though don't quote me on that!)​

Dicodes' Resistherm NiFe30 (IsabellenHütte)

This is a new wire that Dicodes are popularising along with their new mods. Its coefficient is 0.0032, very similar to Titanium.

They are calling it "Dicodes' Resistherm.." as if they invented it, but they did not. It was created by IsabellenHütte - their info page on it is here: RESISTHERM® - Isabellenhütte Heusler, and here the datasheet.

In one of their manuals Dicodes state that it is easier to work with than Ni200 (what isn't?) I don't know how it will compare to SS and Titanium. Its resistance at 29G is 5.5Ω/metre, making it less than Titanium and Stainless Steel but 4x greater than Ni200.

The data sheet lists it as usable up to 600°C (1112°F) so it certainly has a far higher ceiling than Ni200.

One potential downside is that it currently only exists in one size - 0.28mm/29G - and that may be the thickest that is available any time soon. The manufacturer states the wire goes from 0.02mm to 0.25mm/30G, so Dicodes have already made it available slightly thicker than the manufacturer normally offers. Maybe 32+G would become available in future, but it seems unlikely that 28 and below will, at least unless/until it becomes very successful for vaping.

I have ordered some for testing (€13/10m !! :( )​

Infinite's implementation of TCR modification : Nickel Purity

What is Nickel Purity?

Nickel Purity is a setting from 10-100 that Infinite have added to their new DNA 40 clone chip. It can be found in a number of mods, including VaporFlask clones, Zero clones and VaporShark clones. Some of these mods are 60W only, and others are available in 40, 50 and 60W variants, which are identical besides the wattage. The 60W variants are by far the best value as they are almost the same price as the lower watages.

The stated purpose of Nickel Purity is to allow users who have lower quality Ni200 wire - such as is commonly found in China - to better use TC. However, it has far more use besides that - and thus is rather poorly/inadequately named.

It works by by directly modifying the Temperature Coefficient of Resistance (TCR) value that the mod uses to estimate temperature increases from resistance increases in the coil wire. If nickel wire is less pure, it will have a lower coefficient - the resistance will rise less with each degree of temperature increase. The TCR for less pure Ni200 will be lower than 0.006, perhaps 0.005 or whatever.

In fact, the feature is much more useful than its name suggests. In most of the world we do not suffer from poor Ni200. But as Infinite implemented Nickel Purity as a scale across a wide range of TCR values, it has much more interesting and exciting uses: it allows us to use different wire altogether, including Titanium Grade 1, Stainless Steel, and the new Resistherm wire.

My testing has indicated that this works well, enabling better and easier use of Titanium without temperature offsets, and making the use of Stainless Steel properly usable for TC for the first time.

How does Nickel Purity work?

My testing indicates that it works very well, and very logically.

It would appear to quite simply be a Temperature Coefficient of Resistance (TCR) scale.

The scale of Nickel Purity is 10 - 100. The default on the new Infinite mods is 70.

I have found that a value of 10 is roughly equivalent to a TCR value of 0.001 and a value of 100 is 0.01.
  • Ni200's coefficient of approx. 0.006 would suggest a value of 60.
  • Titanium's coefficient of 0.0035 would suggest 35.
  • And Stainless Steel, whose coefficient is around 0.00094, would use a value of 10.
I have tested all these wires and various values, and found that they mostly work as expected:

  • Titanium on a setting of 35 vapes well, and much better than it does on a normal TC mod (DNA 40/Yihi).
    • However I have found a slightly better vape from increasing the value, up to around 42.
  • Stainless Steel on a setting of 10 also vapes well, and becomes properly usable for TC for the first time
    • But a temperature offset is still required - around 40-50°F I am finding.
    • I still need to do more thorough testing, including with other attys
  • With Ni200, a value of 60 would seem appropriate from the scale, but in practice I am finding the default of 70 works a bit better (no doubt that's why it's the default)

I still need to do more testing to understand these results fully. It did seem from my first testing that the scale was fairly linear with coefficient, but in practice I am finding that I want to set the value a little higher than suggested by that correlation.

I am in the process of doing further testing and will update as I find more.

But the great news is that it definitely works, making vaping on Ti much easier and more convenient, and enabling proper TC use of Stainless Steel for the first time.

How to test/tune Nickel Purity settings?

The new Infinite chip mods have an additional excellent feature that made it very easy to test for this. Unlike most/all standard TC mods, the Infinite chip displays live resistance readings during the vape.

So say you put on a TC coil that measures 0.10Ω, and then you start vaping. Most TC mods will continue to show the static value of 0.10Ω, hiding the real (increasing) resistance value of the coil.

In contrast, the Infinite shows the live updating resistance. It also shows, as usual, the live updating temperature (or rather its estimate of it.)

By comparing these values together - what is the resistance, and what does it think the temperature is? - it's possible to work out what coefficient value it's using for the temperature calculation.

The only downside is the annoying "Temperature Protection" message it often flashes, which obscures some of the readings. But this can be worked around by setting a temperature too high, so the message is not flashed or flashed more rarely.


Credit: TheBloke


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