Do Dual Coils Affect TC Accuracy?

[Jalcide]

Do dual coils, in mods without a "Dual Coil" setting, affect the temperature control accuracy for a given TCR?

This thread was created to continue a discussion run amok, in the RX200 and eVic VTC Mini threads.

Many dual coil builders have been having issues with anemic vapes in temperature control.

I suggested it might be due to the low ohm builds common with dual coils being in a range more inaccurate for mods that employ single, linear TCR values. There may be some merit to this, but let's put that aside.

It was then suggested that it was the parallel circuit that a dual coil creates that was the culprit. That it somehow "changed" the effective TCR value of the circuit, from the mod's point of view.

Chaos ensued.

I was first to point out that the TCR for a material, itself, doesn't change. That is correct, but was not the assertion.

It seems there is some evidence for the net TCR of the circuit, viewed as a whole, being effectively changed when the resistors (coils) are in a parallel configuration.

It started with this graph:
WISMEC Reuleaux RX200 TC

Summary version: A parallel circuit will increase about 1/2 TCR per degree C. A single coil nets about 0.438 ohm delta from 0 to 300C. A dual coil nets about 0.234 ohm delta from 0 to 300C.


Continued with these equations:
WISMEC Reuleaux RX200 TC

And continued on with my (incorrect?) interpretation that Heat Flux being shown (by Steam Engine's Coil Wrapping Calculator) to greatly decrease, when in dual coil, was the underlying mechanism (or at least a key one).

Panic ensued.

My assertion was that if two builds with an identical base resistance, one single build, one dual build, with the same wattage pushed at both of them, the dual coil build would exhibit a lower Heat Flux (would run cooler) and therefore the resistances read would be lower, resulting in inaccuracy.

What now seems wrong-headed about this notion, is that if a mod sees the resistance being lower it would assume it should push more wattage at it to reach the target temperature per the TCR it's set for. So, it would be expected that the vape would be too hot rather than cold. But the reports of inaccuracy are that vapes on dual coils are too cold.

It was also asserted by someone in the forum that as a mod pushed more wattage at the dual coils, they would eventually heat up in a manner consistent with the TCR of that wire type.

Some more debunking of dual coils affecting TCR was posted here:
WISMEC Reuleaux RX200 TC

Another debunking contribution:
eVic-VT mini?

I'm now at the point where I've lost confidence in the assertion that a dual coil config behaves differently with respect to the TCR of its underlying coils.

Let mayhem by all ensure.

But that's hopefully peppered with smart facts and folks smarter, with definitive math, physics and electrical engineering prowess on this useful topic.

Do dual coils affect temperature control accuracy? How?

 

[BigEgo]

You already have your answers. The posters in the threads you linked explained it already.

The TCR is an inherent value of the element in question; it doesn't change no matter what mass of that element you use. What will change is the heat flux and thus the amount of power needed to increase the temp of the coil. The base resistance of the two coils does not matter either as that has nothing to do with the TCR. The only thing that changes the resistance is temperature (which is a result of heat being applied since temperature itself is the average amount of heat in a body).

As for why dual coils might feel "cooler" to some people. It's probably in the heat flux. As you already stated, a dual coil at X resistance and a single coil at the same resistance will have different heat fluxes. This means that even though they are at the same temperature, the dual coil will dissipate the heat faster due to it being a larger coil mass and having more surface area to dissipate the heat.

It's sort of like a heat sink on a PC -- the bigger mass of copper/aluminum, the better.

Perhaps druckle will chime in (he is a metallurgist with lots of academic knowledge of metals and their properties).

 

[Jalcide]

Thanks. Yes, I've been focused on the heat sink concept, too.

So even though the dual coils are doing a better job of dissipating heat, which = getting less hot, which = less change in resistance, perhaps the "must make it hotter now, says the mod" factor doesn't offset the overwhelming heat dissipation, and we end up with an anemic vape.

 

[BlueridgeDog]

IMHO: Dual coils have one major impact to TC. Double the connections, twice the potential for hot legs and twice the potential impact of poor wicking. Taken all together, dual coils have 8X more potential to have vaping problems (typically cool vape as results in one wick not keeping up or a hot leg, causing ONE side of the setup to reach temp and shut down the wattage).

 

[Jalcide]

So, do we think this graph (and its underlying assertion) are wrong?

It started with this graph:
WISMEC Reuleaux RX200 TC

 

[Spirometry]

In the graph that you posted, although the data was correct, the poster came to the wrong conclusion.

If he had done a single coil and a dual coil at the same resistance, the two lines would have laid on top of each other. So it would be impossible to say that a dual coil has a different temperature rise for the resistance because of being parallel.

What can be said about the graph is that if you have a higher starting cold resistance, you will get more rise in resistance in your vaping range. Depending on the accuracy and resolution of your device, having more rise in resistance can result in less margin of error. Steam Engine gives a value for this called "TC precision"

Generally, your dual coils will have less resistance than their single coil counterparts. But you can't make the broad statement that dual coils are less accurate than single coils.

 

[KenD]

In the graph that you posted, although the data was correct, the poster came to the wrong conclusion.

If he had done a single coil and a dual coil at the same resistance, the two lines would have laid on top of each other. So it would be impossible to say that a dual coil has a different temperature rise for the resistance because of being parallel.

What can be said about the graph is that if you have a higher starting cold resistance, you will get more rise in resistance in your vaping range. Depending on the accuracy and resolution of your device, having more rise in resistance can result in less margin of error. Steam Engine gives a value for this called "TC precision"

Generally, your dual coils will have less resistance than their single coil counterparts. But you can't make the broad statement that dual coils are less accurate than single coils.

Exactly. In terms of percentage the resistance-increase was identical.

Sent from my M7_PLUS using Tapatalk

 

[Spirometry]

Exactly. In terms of percentage the resistance-increase was identical.

Sent from my M7_PLUS using Tapatalk

And in this case, the poster used a TCR of 0.0052. At 300°C this would be a factor of 2.456 for both coils.

.15 x 2.456 = 0.3684 @ 300°C
.30 x 2.456 = 0.7368 @ 300°C

 

[Croak]

TCR is TCR. To think otherwise is similar to thinking a pound of feathers weighs less than a pound of lead.

 

[Robert Cromwell]

IF the dual coils are VERY well matched in resistance TC should be fine. If there is much difference in the resistance of the 2 coils then TC can be impaired somewhat.

 

[Mactavish]

IMHO: Dual coils have one major impact to TC. Double the connections, twice the potential for hot legs and twice the potential impact of poor wicking. Taken all together, dual coils have 8X more potential to have vaping problems (typically cool vape as results in one wick not keeping up or a hot leg, causing ONE side of the setup to reach temp and shut down the wattage).

Indeed. More connections the potential for problems increases. As far as TCR issues on dual coils, since TC vaping has been available for sometime now, one can assume if there were a huge problem, it would have surfaced long a go. One should also take into account some atomizers as well as RDA's, have connection issues that apply to their particular construction and end up not being suited for TC in the first place. I've personally experienced this with both the Skyfall and Fountain V2, both oddball squonking tanks, better suited for Kanthal power vaping.

 

[Jalcide]

I've been given permission to add some thoughts from an Evolv forum member, Jaquith, of whom I greatly trust his opinion. Here they are below:

"I've been doing dual TC coils since the DNA 40, it's one circuit; one positive and one negative with spliced wire. Assuming the same alloy or same metal the TCR is identical for 30 gauge as it is for 20 gauge as it is for 0.05 Ohm or 0.20 Ohm coil or coils. The problems caused are in the 'skill' of the builder not the properties of the wire. I can make a dual coil out of a single wire as I can have one 'part' of the coil cooler than the other part IF I change the RESISTANCE part(s) of the coil(s). Likewise even in a single coil I can have hot-spots or hot legs .. symmetry and building skill. The net percentage of Base (Resistance, Temperature) to Calculated (Resistance, Temperature) from TCR or TFR table. Example 0.50 Ohm vs 0.25 Ohm Base (0.5, 75F) .. Set (0.75, 500F) is the same as Base (0.25, 75F) .. Set (0.375, 500F). Actual math Calculated°C = (ReadingΩ - BaseΩ) / (TCR or TFR * BaseΩ) + Base_Temp°C
Further, you're assuming that all of the CSV (TFR) or even TCR data is correct, it's not, especially in alloy (mixes) of metals.

Evenness has more to do with the: how the wire is formed e.g. I'm not a fan of cold pressed; TCR of the metal or alloy e.g. I prefer 0.00200 to 0.00300 or higher (Resistance, Temp); gauge of the wire e.g. I prefer low gauge (24 gauge); electrical flow, coils that are spaced or older are less likely to have hot-spots e.g. ideally I prefer to pulse my coils but that doesn't mean that I have to make them so hot they form oxides but instead to remove stresses either caused through the coiling process or often (cold press SS) how the wire is produced.

Lastly, no wire is precisely represented as a TCR. A TCR is an approximation, often an averaging of the (Resistance, Temperature) changes. Instead, ideally a TFR with many plot points is the most accurate with its irregular 'curve' particularly in the temperatures most people vape e.g. 400F to 600F. "

And, his other reply...

"Most people don't fully understand Heat Coefficient (W/mm^2) which is different than a TCR or TFR, but both are relevant to 'Temperature.'

If your dual is 'anemic' then set your (Power) Wattage to something ridiculous and compare, verify conductivity, and your cold Lock. On new builds I always perform 470F cotton burn test. In an 'ideal' world too much Wattage on a 'good' TC board doesn't matter. However, that's a much different wattage than Preheat; it's different because of a flaw in the DNA 200 in that it often over shoots the Set Temperature exasperated with lower TCR metals and or alloys. Stainless steel with its low (any metal alloy <0.00200) TCR I disable Preheat and I'm extremely accurate in my proper Power setting; Heat Coefficient for the coil(s).

If you're going to be a Temperature Control aficionado then I'd recommend adding the $5 MicroCoil Pro application to your arsenal. Example http://i.imgur.com/bMng5hR.png"

 

[Mactavish]

In the graph that you posted, although the data was correct, the poster came to the wrong conclusion.

If he had done a single coil and a dual coil at the same resistance, the two lines would have laid on top of each other. So it would be impossible to say that a dual coil has a different temperature rise for the resistance because of being parallel.

What can be said about the graph is that if you have a higher starting cold resistance, you will get more rise in resistance in your vaping range. Depending on the accuracy and resolution of your device, having more rise in resistance can result in less margin of error. Steam Engine gives a value for this called "TC precision"

Generally, your dual coils will have less resistance than their single coil counterparts. But you can't make the broad statement that dual coils are less accurate than single coils.

Your post brings up a question I have as a fairly new TC coil builder. Heat flux values aside, for the best experience and ACCURACY in TC mode, does building ones coils to higher ohms result in more accuracy? My basic 24 gauge titanium builds are about .22 - .30 ohms, but I also get a great Vape on some pre-made titanium coils, and noted most of those are built to .4 to .5 ohms. So basically, Are higher ohms a better target to build to for better TC accuracy?

 

[Jalcide]

Your post brings up a question I have as a fairly new TC coil builder. Heat flux values aside, for the best experience and ACCURACY in TC mode, does building ones coils to higher ohms result in more accuracy? My basic 24 gauge titanium builds are about .22 - .30 ohms, but I also get a great Vape on some pre-made titanium coils, and noted most of those are built to .4 to .5 ohms. So basically, Are higher ohms a better target to build to for better TC accuracy?

I do know that total "TC Precision," as the Steam Engine creator calls it, is higher for wires with a higher base resistance. In some cases, even a wire with a lower TCR can end up having greater accuracy if its base resistance is higher than a low resistance wire with a higher TCR.

For example, Ni200 has a high TCR of .00600, but low base resistance. Its TC Precision is: 768.
Whereas SS430 has a lower TCR of .00138, but high base resistance, resulting in a TC Precision of: 1105.

Also, increasing the resistance of the build will increase the accuracy. This was discussed in this post:

TC beyond Ni200: Nickel Purity, Dicodes; Ti, SS, Resistherm NiFe30; Coefficient of Resistance

So, yes, in theory, a higher ohm build has the potential to be more accurate. Whether a given mod does this, I can't be sure.

I have seemed to notice higher ohm SS builds working better and smoother on the DNA 200.

 

[Spirometry]

Your post brings up a question I have as a fairly new TC coil builder. Heat flux values aside, for the best experience and ACCURACY in TC mode, does building ones coils to higher ohms result in more accuracy? My basic 24 gauge titanium builds are about .22 - .30 ohms, but I also get a great Vape on some pre-made titanium coils, and noted most of those are built to .4 to .5 ohms. So basically, Are higher ohms a better target to build to for better TC accuracy?

Accuracy is a big word to be throwing around. I would say with higher resistance builds (with the same material) you will have less margin of error. It is still possible to get very accurate results using low resistance builds.

The accuracy, resolution and sample rate of the device taking the resistance readings has to play into this also. There has to be a point where increasing your base resistance isn't going to make a differance.

 

[Mactavish]

Accuracy is a big word to be throwing around. I would say with higher resistance builds (with the same material) you will have less margin of error. It is still possible to get very accurate results using low resistance builds.

The accuracy, resolution and sample rate of the device taking the resistance readings has to play into this also. There has to be a point where increasing your base resistance isn't going to make a differance.

Thanks, I remember that post by "TheBloke", before he left that ECF thread. I'm going to try increasing my titanium builds from .20-.30 to .4-.5 ohm coils.

 

[Spirometry]

My current dual Ti 24 awg is has a cold resistance of 0.168Ω on a DNA200.

 

[Mactavish]

My current dual Ti 24 awg is has a cold resistance of 0.168Ω on a DNA200.

I'm using single coils for now. Input a TCR of .0366 (the streamingengine site), and getting similar Vape to my DNA200 at same settings and tank.

BTW: Observations so far. Wish the left button allowed for lowering wattage in TC mode, as it does in power mode, it's odd. I believe this has been mentioned a lot before, even in the original PBusado video review. Those little silicone beauty dress up colored rings you can put on the top of the TRON tank are way too big and loose, lost and found my black one, but they rub off the tank easily, no big deal.

Biggest difference I see so far is this chip does seem to increase the Vape temp with wattage increases, while the DNA200's wattage setting just gets me to my set temp slower or quicker, and the temp setting has a majority influence on flavor and vapor on that chip. On the VTC I hit temp limit properly but it seems I can get a bit warmer vape with higher wattage settings, I may be wrong, still playing with it as I just got it today.

 

[Jalcide]

I think I figured it out!

If not, then this is at least some brilliant folly.

(Edit: It was folly. I was wrong. The math ensures the temps are the same in each coil between a dual and single build.)


Coils in parallel, being resistors, will still be seen by an ohm meter (the mod) as a single resistor. To keep the math simple, let's assume they are perfectly identical, 1-ohm coils.

Two 1-ohm coils in parallel create the equivalent of a .5-ohm resistor.

Let's assume they heat and cool mathematically perfect, in perfect unison, and therefore this "variable" resistor (controlled by its temperature) is easier to discuss.

We now have "one" resistor for the mod to analyze hundreds of times a second, as it does. It's looking for a change in resistance.

That change has to be expressed in milliohms, or some such finite unit of granularity. It's looking for so many units of change per unit of time.

A Heating Cycle:

Each increased milliohm that a single, serial wire would report back, is now taking twice the milliohms (for each coil independently) for the circuit as a whole to report back as 1 milliohm of change.

Not a problem for the mod, it will happily push more wattage at it until the heat increases enough in both coils to increase the ohms enough to meet the TCR's delta enough to decide it's reached the target temperature.

A Cooling Cycle:

I think this is where the "magic" happens (the magic of dual coils being anemic).

The power is cut (or reduced) and waiting for the milliohms to drop sufficiently so that it can kick in again.

But just as before, it takes 2 milliohms of change in each coil, to produce 1 milliohm of change for the circuit as a whole.

Since the cooling is a "static" function of the thermodynamics of the coil mass and surface area (and wick), it doesn't have the benefit of a mod to push more cooling at it. It just has to cool at its own natural rate.

The cooling that happens on each coil, independently, is identical in behavior to its single coil counterpart; even when each coil, of a dual, has more wraps, they don't work to retain the heat more, due to the topology of a spiral, spaced coil.

So the cooling cycle is nearly identical between the single build and dual build, but the dual build is required to cool longer, a full 2 milliohms drop in resistance worth of cooling, longer, to report back only 1 milliohm worth of cooling as the mod sees it.

I believe it's this extended cooling cycle, repeated over and over (perhaps dozens or even hundreds of times a second) that results in a net cooler temperature, as compared to the ohm-unhampered single coil build.

The single coil, reports "I'm cooler now," faster.

The dual coil reports "I'm cooler now," slower and therefore, in fact is cooler than the single coil would be, at that reporting.

 

[Croak]

No.