Do Dual Coils Affect TC Accuracy?

[Jalcide]

It's been asserted that the parallel circuit of a dual coil will effectively change the TCR, from the mod's point of view.

Is there any merit to this?

I'm looking for the science behind the often anemic dual coil TC experience.

[Jaquith]

No differences, as long as the coils are symmetrical. There's no doubt that single coil is easier. If the coils are asymmetrical or otherwise uneven or there are differences in connection (eg leads / posts) then there will be a difference in Temperature between the coils.

[Jalcide]

Thanks much, as always, Jaquith.

Yes, let's assume they are mathematically perfect.

There has been some assertions that the TCR effectively "changes" when viewed as a parallel circuit. It has some pretty convincing charts and equations tossed in as supporting "evidence."

I bought it hook line and sinker, and thought the reason had to do with Heat Flux differences in a dual build (per Steam Engine).

The discussions that ensued, on two ECF threads, no less, have all but detailed them.

I've now started a new thread to get this confirmed or debunked once and for all -- by those with enough of a grasp of the physics behind it, to put it to rest.

https://www.e-cigarette-forum.com/forum/threads/do-dual-coils-affect-tc-accuracy.721108/

The graph that is shown to support the idea of the TCR being different in dual configs, is of particular interest (see the first post of the thread if you're interested). It is real?

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.

He seems to imply the actual TCR effectively becomes "different" when viewed as a parallel circuit by the mod.

[Jaquith]

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 = (Reading? - Base?) / (TCR or TFR * Base?) + 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. 

[Jalcide]

Thanks, Jaquith. Feel free to cut and paste your thoughts over on the ECF thread, if you're so inclined.

I'm hoping to get the "TCR is different" originator's response to all this, as well. He was so very confident, and a seasoned ECF member, I'm quite interested in his rebuttal.

My current thinking is focused around heat dissipation as a possible culprit to why the dual coil TC builds are anemic, all other things being equal.

[Jaquith]

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

[Jalcide]

I greatly appreciate your thoughts, David. Thanks, I just may add these good points to the conversation.

(Feel free to add more, if you think of anything. And I could append them over on what is already turning out to be lively discussion.)

[Jaquith]

Bottom-line, it's more about getting a feeling for your 'board' and its firmware in addition to knowing your wire of choice. Example the reason I like 24 gauge Titanium is because of its higher TCR (0.00350) and the lower gauge stabilizes the energy applied to it. Fast Power adjustments to the wire are absorbed more readily which provides a dampening effect; smooth and accurate temperature. Higher gauge wire reacts faster but at a cost of what I call "TC Jitter" which your sensitive mouth can easily detect. In other words, good Temperature Control is boring. BK picked up on that phrase, but it's true. To sum it up, I don't notice any measurable temperature differences between a single or a dual coil at the COIL. If I do then it's user error on my part: uneven leg lengths, uneven coils, poorly tightened leads, overly stressed or partially cut leads or Atty issues. I have a ton of pictures of cotton burn tests demonstrating even and accurate temperatures. In addition I have a temperature probe...

[Powerman]

So this bleeds into something I've been trying to figure out on contact or spaced coils. I got some 430 and built some dual 24g coils. But I had to run the TCR up to 170 to get it to fire right. Too cold. I've been trying to wrap my head around why. TCR is TCR. If I have to fire harder, then it is not seeing the heat rise. In TC, it's going to keep giving power until it sees it. But it's seeing a resistance rise and cutting power. So some of it here is helpful concerning heat Flux and symmetrical coils. What I'm not getting is that even though it isn't giving desired temp, it is seeing a rise in resistance. If there is a hot spot or uneven coil, there could be spots of higher resistance, but combined it is not that way across the whole circuit. If resistance is higher in one coil, current should flow to the other one heating up to equal resistance.

[Powerman]

Jaquith said:

Bottom-line, it's more about getting a feeling for your 'board' and its firmware in addition to knowing your wire of choice. Example the reason I like 24 gauge Titanium is because of its higher TCR (0.00350) and the lower gauge stabilizes the energy applied to it. Fast Power adjustments to the wire are absorbed more readily which provides a dampening effect; smooth and accurate temperature. Higher gauge wire reacts faster but at a cost of what I call "TC Jitter" which your sensitive mouth can easily detect. In other words, good Temperature Control is boring. BK picked up on that phrase, but it's true. To sum it up, I don't notice any measurable temperature differences between a single or a dual coil at the COIL. If I do then it's user error on my part: uneven leg lengths, uneven coils, poorly tightened leads, overly stressed or partially cut leads or Atty issues. I have a ton of pictures of cotton burn tests demonstrating even and accurate temperatures. In addition I have a temperature probe...

So my post above is a little disjointed. This is what I mean... Single coil, TC applies power within set limits, monitors resistance, and modulates temp. Yet contact coils do not seem to work so well. I could understand if it is basically shorting the circuit. Power cuts across wraps, TC sees resistance rise in one part, cuts power. But if it sees the rise in one part of the coil, and not across all of it, then why does it not see shorted resistance for base line? If it is only regulating one part of the coil, it should only see part of the resistance. Now dual coil. Identical for this argument. Half the resistance. But for same results of single, it need twice the power. TC sees resistance rise, but across double the mass. So double the mass requires twice the power to input heat to get temp to set point. Yet many report cold vape on dual. I can understand greater heat Flux and heat transfer/dissipation, but that's the point of TC... Continental to apply full power within setting until it reaches temp. So if dual half the resistance, then resistance rise is also shared and temp will be reached. It's just that it's taking more power to do that. Yet contact coils, and with same effect dual coils, that seems to be bypassed. It's only applying less power for cold or anemic vape. So it is seeing a resistance rise, and that doesn't matter what wire it is, yet the full coil mass, being contact or dual, isn't heating to full temp. In power mode, it doesn't matter. Full set power is delivered for full time and you get what you get. So why?

[Jaquith]

To be clear you can indeed have a good TC experience using 'contact coils' however there's absolutely no difference in the hotspots you see using a new Kanthal coiling when pulsing. So you have a couple choices: Annealing or Spacing. The problems with Annealing is: Oxidation and Damaging the coiling. My scope isn't to explain Annealing, Thermal Dynamics, recrystallization, metallurgy, etc all of which are the underlying reasons for Spacing as a reduction to the problem of hot spots. As mentioned, if a dual coil is too cool ( anemic ) and the correct TCR / TFR is selected then either the requirements of Power aren't being achieved or the Ohm's Lock is too low to the actual resistance. Review what I typed before.

[Powerman]

Thanks. It was an interesting read. I read this thread, and the links to the other threads. 8m not using ohm lock. And that's what's confusing. The discrepancy between what resistance the mod reads, cold, and what TC reads to limit power. I made contact coils, and had to use 170 TCR. I made 3 spaced builds, and the 138 worked. But even that isn't stable. I've had to walk the two I'm using up to 600 for same results. I analyzed all and the resistance was stable. Fine if it changes, but TC should account for that on baseline readings. I'm just not getting how resistance can stay the same, but TC is going off something else. Meaning anemic or having to keep bumping temp up. Even letting it sit can give different results hot or cold. Too much power or not enough. Very inconsistent.

[Jaquith]

If you want contact coils, anneal to a high glowing temperature with TC off, retighten posts, disable Preheat period (1,1, 0 seconds), set Power to 100W or more, set temperature to 470° F using scrap cotton and "cotton burn test" until light golden brown. Adjust TCR until passed. DO NOT OHM LOCK! If your Atty is unstable then don't use it in TC .. get a new Atty. Whenever I have a new wire I use a rock solid and resistance stable RDA with one spaced coil until I have a TFR or TCR that I know works! Reduce your variables. Once you have TC stable and an accurate TFR / TCR then dual coils or Tanks or whatever. If you have problems then you know it is NOT a TC issue. I find lower TCR metals or alloys don't do too well with Preheat .. eg Stainless Steel...

[Powerman]

So I have heard about not using pre heat. But have been reluctant. That's one reason I like the DNA. But I see what you mean. Turn up power and let the board do it. I'll give it a shot. I have been using a couple Crius tanks. The ohms are stable. But I couldn't tell you there are no issues. I have a couple others coming so we'll see how it goes. I like your logical approach. Trouble shooting 101 in my line of work. But I threw some builds with a new wire and here we are. So I will back up and see. But is there anything to what I was asking? Why does TC not see the whole coil? Or am I just thinking wrong. I have another issue or multiple issues and TC just is getting what it needs to work right. Build problem and not a physics problem?

[Jaquith]

Powerman said:

Why does TC not see the whole coil? Or am I just thinking wrong. I have another issue or multiple issues and TC just is getting what it needs to work right. Build problem and not a physics problem?

That's the 'problem' TC does only see the 'whole' coil(s) .. it looks for X% Ohm Change. The hottest parts of the coil(s) are the Legs and the Center. The right and left sides of the coil are cooler.

Try it without Preheat. As you've read, I prefer lower, e.g 24, gauge wire because of the dampening effects. If you want near perfect TC then I highly, as in with the exception of Platinum or TiPt, Titan Wires Grade 1 Titanium 24 gauge wire!

'IF' an RBA has either 1/2 Coils or longer legs then I recommend coils as pictured below (stupid fast examples, lol not my best work):

[Powerman]

I really appreciate the advice. Yes, all my decks will be velocity style. I keep the legs straight just because that's convenient for split post. I'm using 24g now. I got some bigger tank decks so will be going 8/9 wraps at maybe 3mm. You've given me plenty of tips to try out. So, TC sees whole coil. There are hotter parts of the coil, but the resistance rise is the averaged/summed hot/cool spots of the coil. I guess I just don't get why dual or spaced coils have such an effect. That's is for improperly built coils. I mean you have given plenty of reasons... Low TCR, pre-heat, unequal coils, misrepresented or poor forming of material... And that proper builds will act right dual or contact. Now I'm just trying to figure out what about the improper build is changing performance.

[Jalcide]

Thanks for the continued discussion, guys.

I'm still thinking there are raw physics that play into why a dual coil build is often anemic, when driven by a TCR based control system.

I think I figured it out!

 

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

 

 

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.

[Powerman]

While I understand the thinking, heat exchange is directly dependent on surface area. During the cycle, say 100 times a second, you are still pulling air, the coil is still driving vapor production (read transferring heat) and the dual coil has twice the surface area of the single coil to do that. Heat exchange, cooling, will be twice the rate. Effectively making the dual coil cooling equal to the single.

[Powerman]

I did have a minute to change settings before going to work. What was I thinking.... I was just thinking I didn't want "no" preheat.... I put everything at 1 for 0.1 sec. I turned my crown profile up to 150w and I'm at 500F. Ya, it's fast. Not waiting on warmup. And yes, it's much smoother. Not by the graph, just by the seat of my pants. Haven't looked at device monitor yet. But the vape is plenty fast and smooth.

[Jalcide]

Yeah, I was thinking along the same lines, the 2x mass thing.

But I'm now thinking it's all about how that mass is distributed. I'm guessing there's not twice the cooling with a coil twice the length, for the same reason a 3 inch wire and 6 inch wire would both glow about the same when heating an end (or any part of it) with a blowtorch; the heat dissipation across a wire is not so good.

We see this heat gradient at the hotspot, but it drops off pretty quickly.

I think there is an effect of the longer coil dissipating more heat, just not 2x.

At the same time, the ohms (and temperature) is about twice in each resistor, because of how linear most TCRs are.

So we have twice the heat, but not twice the cooling.

[Powerman]

That might be in the case of ambient passive cooling, but that is not what is happening in our application. If TC is stable and putting out 30w at 450F, only 1 or 2 of those watts are going to maintaining the wire at temp. The other 29 is going to vapor production and changing juice to vapor. And that is dependent on how much air flow is going past the coil. The air flow is carrying heat. So in our application we are actively cooling the coil with juice and air. And in that case, we are carrying away plenty of heat. Twice the surface area, with twice the juice and air flow will cool it twice as fast.

[Jalcide]

Well, strictly-speaking, all of the watts go toward heating the wire. The electrons flow nowhere else. It's a subtle point to make, but helps to frame what the the vapor production really is: part of the cooling process (evaporation) of surplus heat that was there to get rid of. The vapor isn't providing additional cooling.

There can be more heat than vapor (dry hit), but not more vapor than heat. The wicking makes sure there's not more liquid in either system, for a given segment of coil.

Sorry, I digressed, back to the main points...Yes, I agree we're carrying away more net heat, for the larger amount of power we're putting into the larger mass of the system.

But let's pick a small, arbitrary segment of the coil (and the wicking material it touches) for a thought experiment. Let's say a single wrap of a spaced coil (of any length).

I suspect each wrap-segment, when viewed under close, isolated inspection is not behaving much differently during the cooling cycle. The only "connection" it has to the thermodynamic system around it that represents the "2x part" is through heat conduction across the 2x length of wire.

The other methods of heat transfer, evaporation, convection and radiation are the same in both systems; same amount of wicking material and liquid in direct contact with any given wrap-segment of coil, same amount of airflow through the atomizer (actually, slightly less, but we can ignore that).

The only contribution toward cooling we see is the longer length of wire. And I don't think it helps much, certainly not twice as much. It would help slightly more, the larger the wire gauge.

In fact, the additional heat generated inside the atomizer for a dual build is working to slow the cooling, but this is probably a lesser effect than my supposition; 1 ohm of cooling as the mod calculates it, taking 2 ohms of cooling of actual time = temp is always cooler than mod calculates. This is only during the cooling cycle. The mod then blasts power at it again to compensate. So it's the net effect of that battle. But where the mod is slow to reapply power, it's quick to cut the power when the TCR delta is reached.

[Powerman]

So, single coil... Match air flow to vapor production to preferred taste. Now add another coil. You need more air to produce same density with the increased vapor production. Double the coil, double the power, double the air needed. Double the surface area, doubles heat transfer. It's not a guess. It's an equation. Double the surface area, double the heat transfer. The whole point of multiple coils is to increase surface area to increase vapor production. So regardless of the hypothesis, heating cooling isn't slowed. It's not a static environment cooling by ambient conditions. And we are talking many times a second. There is not a slow down in cooling. It is heat. And it is heated then not, then heated then not. The biggest influence is vapor production and air flow.

[Jalcide]

Okay, if you're going to assume increased airflow (harder draw), that's going to increase cooling as it relates to the extra heat production.

Probably a smart idea, too. Although temp control will help lower vapor production when not drawing as much air.

Anyway, this seems pretty linear and in balance with the larger build. More vapor, more airflow taking that extra vapor away.

In other words, the extra airflow balances the extra product of the build. We're still not accouting for the drastic increased cooling we're seeing with dual TC builds (on mods without a mystery "dual coil" TC setting -- which I suspect accounts for what I'm suggesting).

So, I don't think any of this overshadows the mechanism of: 1 ohm of reported cooling, equaling 2 ohms of actual cooling time.

Because that's happening at the sampling rate. It's got to be a huge factor, when averaged over time.

In keeping with the shift in linear graphs that some others have posted, that happen with dual TC builds.

The accuracy of said graphs is somewhat in question, however -- which is why I'm trying to confirm or debunk it with smart people like you (thank you, btw, for entertaining me on this, on Xmas Eve no less! I owe you a beer or three.)

As for doubling the heat transfer with doubled surface area -- sure, but the system also has double the power pushed to it to get to that state.

In a parallel config, the volts stay the same across the coils, so it would be amperage that does the deed.

In other words, that extra heat transfer is not adding to the cooling effect, it's just keeping what's being thrown at it, in check.

[Powerman]

Well, I'm no electrical guru. I do know a couple things about heat transfer and fluid flow. I'm not seeing it. Single or dual, they are doing the same thing. If its a build problem, I would like to understand what is actually happening. I'm not used to not get things. But I'm not doing a very good job explaining what I'm not getting... well... because I don't get it. If it's a build problem, well, thats still physics. And its not clicking why spaced coils or dual coils seem to be such a problem with some. And if SS is at the ragged edge of usability as far as low TCR, then fine. I still like SS. If the low TCR simply shows problems where other materials easily cover them them that's fine too.