Do Dual Coils Affect TC Accuracy?

[Jalcide]

I guess it's all moot now. I just ran the math. 

I was wrong in a key assumption.

The rise and fall of twice the ohms in the parallel circuit does happen, like I suspected, but the math of the TCR formula, when paired with parallel resistor math, works out to keep the temperature the same in each coil, between dual and single.

Here's the math for a .15-ohm Ni200 build, with a .1 ohm rise and drop in resistance:

TCR formula:
(Rref + rD) = Rref [1 + a(T - Tref)]

 

R = resistance at temp T

Rref = resistance at temp Tref

a = TCR

T = temp

Tref = ref temp for the TCR

rD = resistance delta (rise)

Single coil build TCR formula:
(.15 + .1) = .15[1 + .006(T - 20)]
A .1-ohm rise and fall equates to 131.1C, or 267.9F.

For the dual build to read at .15-ohm, each coil needs to be .3-ohm.
So the same .1-ohm rise of the circuit, means each resistor inside it must rise .2 ohm.
Dual coil build TCR formula:
(.3 + .2) = .3[1 + .006(T - 20)]
A .2-ohm rise and fall required for each coil equates to 131.1C, or 267.9F


I still don't fully have my head wrapped around why this is true. But I'm going to accept the math, for now.

[addappt]

From my experience the main issue with dual coils in TC is twice as many possibilities for a bad connection. Usually if I have any issue it is to do with a post connection being bad or uneven. Retentioning of post screws usually fixes he problem.

[JaySoCal]

Aww the dreaded dual coil temp control lol. Been doing alot of hybrid complex builds in dual using 24g grade 1 ti. Been loving it but using kanthal with temp control causes more issues with hot spots and by nature pulling heat out of the temp wires. Stability of atomizer is first and for most. They are all contact coils but due to the no dry burn getting hot spots out is hard normally 10 to 15w pulse but no glow. Then alot of times I'll vape them in non tc mode at 30 to 40 watts for a few days. Then they seem to run fine after I give them a little time to work themselves out. Leg length is probably the second most important part.

[Indi]

I look at it a simpler way because I honestly don't really understand half of the technical stuff Jaquith is coming out with (but I completely respect his opinion and knowledge) , I think TC is just super sensitive , the resistance changes your dealing with are tiny and until there is some kind of actual temp sensor within an atty or at or near a coil temp control is going to remain flaky at best.
I build my TC coils super anal , make absolutely sure it is as perfect as possible , make double and triple sure its tightened and seated correctly as well as the wicking has to be "just right" and if its not perfect do it again and again until its perfect , that's includes the subtank builds on my ladys reuleaux and the dual coil crius and velocity builds on mine , its a miniscule difference in a coil that makes a huge difference to the vape , I even inspect the coil with a jewellers loupe for gods sake lol.
Using kanthal on any of those attys with claptons or spaced coils or touching coils or parallels you can literally just throw it together in seconds , stick cotton in there virtually any old way and off you go but you don't get the lovely consistent heat , flavour and vapour you like EVERYTIME like you do with TC when you get it right.
All I'm saying basically is I've found that patience pays off with TC and I've had no anaemic vapes with single or dual coils using 24g Ti when I take my time and make sure it's as perfect as possible especially with dual coils.
Don't like Ni200 , funny flavour.

[VapingBad]

Nothing at all flaky about temp limiting on any of the DNAs IMO, it works great with single and dual Ni & Ti.

You do need to ensure dual coils are very closely balanced, but other than that they are they are as rock solid as single, ss needs more effort as it is barely a temp sensing alloy.  Obviously you need the right materials profile for your wire and good connections, and as the resistance is typically lower so having the mod resistance set is more important.  I have been dual coiling Ni200 for over a year and the only extra challenge has been sourcing thin enough Ni200 (twisted 34 AWG, 0.16 mm) for the DNA40, you can also use 0.4 mm Ti and drop the temp by 90 F on the DNA40.   A good tip it to cut both lengths of wire exactly the same length before coiling and check the tails are the same length before trimming when fitting the coils.

[Guest]

Have any of you guys been running quad TI coils? I've jumped from sub-tanks to a dripper, I've noticed the ohms starts to move up after vaping ( like .15 to .18 ) it's 28g wire, my coils look about the same but not perfect there spaced 6/5 wrap but after vaping on it for awhile the coils start to gunk up and the cotton starts to discolor but it's all uniform in color. I have 4 profiles set ( different watts and temps) for when it starts to vape differently I try to maintain what I like to vape at, refilling a dripper also changes everything to. But if I put my 30mm Goliath clone on my under powered DNA 40 everything is stable ( just takes longer to get a good vapor production) I think a lot of it has to do with running more power, more coils. It's like tuning a 1 cylinder vs v8 vs v12. The only way I see for multiple coils to perform the same and stable is to have ( in my case) 4 dna's powering 4 coils separately. But as where it sits now I'm happy the way the 200 performs. If we can get away from the whole 510 connection setup that's where will see the ultimate vapor device.

[Dabada]

Jalcide said:

I guess it's all moot now. I just ran the math. 

I was wrong in a key assumption.

The rise and fall of twice the ohms in the parallel circuit does happen, like I suspected, but the math of the TCR formula, when paired with parallel resistor math, works out to keep the temperature the same in each coil, between dual and single.

Here's the math for a .15-ohm Ni200 build, with a .1 ohm rise and drop in resistance:

TCR formula:
(Rref + rD) = Rref [1 + a(T - Tref)]

 

R = resistance at temp T

Rref = resistance at temp Tref

a = TCR

T = temp

Tref = ref temp for the TCR

rD = resistance delta (rise)

Single coil build TCR formula:
(.15 + .1) = .15[1 + .006(T - 20)]
A .1-ohm rise and fall equates to 131.1C, or 267.9F.

For the dual build to read at .15-ohm, each coil needs to be .3-ohm.
So the same .1-ohm rise of the circuit, means each resistor inside it must rise .2 ohm.
Dual coil build TCR formula:
(.3 + .2) = .3[1 + .006(T - 20)]
A .2-ohm rise and fall required for each coil equates to 131.1C, or 267.9F


I still don't fully have my head wrapped around why this is true. But I'm going to accept the math, for now.

I think the things are simpler than that. If you apply voltage and current to a resistor the latter will heat up. This heat depends on the power dissipated by the resistance in Watts and the time in seconds during which you hold the trigger fire.

For a resistance of 1 Ohm and traversed by a current of 5 A the power dissipated will be 25 W (P = R * i ^ 2). If I hold the trigger for 2 sec, the dissipated energy will be 50 Joule (E = W * t). It is the energy applied to the wick (without taking into account the losses).

Now, if I apply a current of 5 A on two resistors of a value of 2 Ohm each, I will have the same power on the whole circuit that is to say 25 W (2 Ohm in parallel = 1 Ohm) . The question is: what is the power dissipated by one resistance and therefore applied on a wick?

In a circuit in parallel and with two equal resistances the current will be divided by two. We thus have 2.5 A by resistance and therefore a power P = 2 * 2.5 ^ 2 = 12.5 W. In conclusion the dissipated power is divided in two as well. Each resistance therefore delivers only half of the total power. For a trigger held for 2 sec, the energy applied on each wick is 25 Joule! The Half !

It is for this reason that for a double coil installation it is necessary to double the power or the temperature to maintain the same energy applied per wick.

It will be interesting, if Evolv, adds an option in the Escribe software that allows the user to specify the number of parallel coil used.

Sorry for my english ! Google Translate.