Low/High resistance, at equal wattage: what's the difference?

[godzilla93]

I was wondering: what is the difference between a low and high resistance atty (or carto, or whatever), at the same wattage? In other words, if the only difference is that the same wattage can be achieved with lower voltage for LR attys, then LR attys would seem to be the way to go in terms of battery life... What's the point in using medium or high resistance with a VV device? Does it have to do with vapor heat? Help please...

 

[A17kawboy]

Weirdly and counter-intuitively, the way electricity works (I read something about it on here and just now had to prove it to myself on paper) the higher resistance atomizer running at a higher voltage actually draws less amperage from the battery and therefore the battery lasts longer. Seems strange but the formulae don't lie.
Low resistance atties were developed mainly for people with constant voltage devices that wanted more wattage (heat) from their vape. That's the only way to do it if you can't crank up your volts.

 

[godzilla93]

Interesting (and, yes, totally counter intuitive!). Thanks!

 

[dragginfly]

Does it have to do with vapor heat?

This is part of the difference... almost.

More accurately, it has to do with the vapor temperature.

Heat is energy. The energy stored in a PV is expelled as heat.

A given battery will provide a fixed total amount of heat regardless of
the voltage, resistance, or output wattage. At higher wattage the heat
is generated more quickly.

The power output of a PV is measured in watts, which is how fast
energy (heat) is dissipated. The heat can be dissipated slowly or more
quickly depending on the wattage (rate of energy output).

Vapor quality is not dependent on heat, but on temperature.

At a fixed vapor temperature, the vapor quantity depends on
the overall amount of heat.

Transfer more heat to a given quantity of e-liquid and the vapor
production is warmer (or hot). Or with more liquid in contact with
the heat source, the vapor temperature is cooler.

A fixed amount of heat transferred to one drop of e-liquid will
have a higher temperature than the same amount of heat
transferred to two drops of e-liquid.

Now this is where the difference between a "small" coil with
low resistance and a "big" coil with high resistance at the
same power output becomes evident.

The "big" coil comes into contact with more e-liquid and creates
more vapor at a lower temp than from a "small" coil even with
the same power output rate (watts).

Now carry this one step further...

Compare a 2.5 ohm coil to a 2.5 ohm dual coil setup.

In the dual coil setup you have two 5 ohm coils operating
at the same time in parallel.

At 5 volts and 2.5 ohms, the current is 2 amps and the total
power output is 10 watts in both the single and dual coil
configuration. But in the single coil all 10 watts are laid on
a small 2.5 ohm coil, and in the dual coil the power is split
between two larger 5 ohm coils with 5 watts each.

Same power. Same heat. Totally different vapor temperature,
quantity, and quality.

Finally, comparing a single low resistance coil to a single
high resistance coil at the same power output, the heat
is dissipated over a greater area (and volume of e-liquid)
in the high resistance coil resulting in a lower vapor temp.

The HV setup can push more power (watts) and generate
more vapor with a given (desired) vapor temperature.

Or at the same power output, the HV setup will generate
more vapor at a lower temperature.

 

[Rader2146]

Weirdly and counter-intuitively, the way electricity works (I read something about it on here and just now had to prove it to myself on paper) the higher resistance atomizer running at a higher voltage actually draws less amperage from the battery and therefore the battery lasts longer. Seems strange but the formulae don't lie.

Unfortunately, that is only half the formula. Below is a post from another thread, but it boils down to voltage doesn't matter; as long as wattage is the same, the battery drain current will be the same....regardless of the resistance.

A boost regulator has to transform low voltage into higher voltage. This can only be done by using more current on the input (battery) side. The amount of additional voltage needed is expressed as:

(Volts out - Volts in) / Volts out = Percentage of voltage increase, also know as the switch duty cycle.

Now that we know the duty cycle, we can figure the additional input current required to obtain the desired output voltage.

Amps out / ( 1 - Duty Cycle) = Amps in

Example...

Known factors:
3.7v in
8 watts out
3.0 ohm carto

Ohms Law tells us that we'll need 4.9v and 1.63 amps output to achieve 8 watts.

(4.9-3.7)/4.9 = .24 = 24% increase in voltage = 24% switch duty cycle

1.63 / (1-.24) = 2.16 amps input drawn from the battery.
-----------------------------------------------------------

Now for validation.
As deemed by the Law of Conservation of Energy, power (watts) in must equal power out. (The true statement of the law is power in equals power out + efficiency losses. But for simplicity sake, we'll get to efficiency below.)

Power = Volts * Amps

Input:
3.7 * 2.16 = 8 watts input

Output:
4.9 * 1.93 = 8 watts output
------------------------------------------------------------

But what about efficiency?
Typical efficiency for a boost converter is in the 75-90% range. Efficiency is not constant. It varies with the desired outputs. You can find the efficiency for certain [manufacturer chosen] situations in the regulators data sheet. Using an optimistic value of 90% efficiency we can figure our adjusted input current.

Power out / efficiency = adjusted power in

8 / .9 = 8.89 watts input.

Adjusted power in / Volts in = adjusted amps in

8.89 / 3.7 = 2.4 amps input.
-----------------------------------------------------------
And comparison:
A fixed voltage device @ 3.7v will achieve an 8 watt output using 2.16 amps (Ohms Law)

8 / 3.7 = 2.16 amps input
------------------------------------------------------------
The above calculations explain why I say that boost regulators will get less battery life than a same size fixed volt, and also that you will not achieve better battery life by using a higher resistance coil.

-----------------------------------------------------------

This holds true for buck (multiple battery, high voltage "bucked" down to a lower voltage) devices as well, but the math is slightly different.

 

[A17kawboy]

@Rader2146: Thank you once again sir! Very much appreciate your time and patience explaining these fundamentals. So why the heck are manufacturers not working on variable resistance atomizers that will work on fixed voltage devices?
If I understand the concept correctly that would enable users to vary power and still have maximum battery usage efficiency.
"Early days..." I guess.

 

[Nurzrachit]

Just look how far we have come in the past few years! At this pace the e-cigs of the future will not resemble what we have today at all.

 

[cozzicon]

Coil size... coil size... coil size..... Many times an LR atty has a small coil. Not always... but often.

Not many people talk about this... but the size of the coil matters in addition to the wattage supplied or ohms of the atty.

If you have one coil, three ohms, four wraps- and another three ohm coil with eight wraps... they vape differently.

It's not *just* wattage. It's also the coil, wicking material, and variations in the atomizers- none of them are the same.

So while the electrical math is useful. It doesn't paint the whole picture... which is why people have preferences in atomizers. Some coils are tight around the wicking material. Some coils are loose around the wicking material. Some coils have no wicking material. Some coils are large.... small.... ect.

So really... wattage is very much a general measure for the purposes of vaping. Which is to say something like "I generally prefer 6.5 watts". But the particular hardware you might be using can change that drastically- a good example being Vision Clearomizers which perform nicely at lower wattages but are about 2.4 ohm.

 

[Slurp812]

I was just going to mention coil size. A longer coil would spread out the heat a bit more.

 

[dragginfly]

A longer coil would spread out the heat a bit more.

Excellent way to state it!

It took me nearly a full page of explanation to conclude with this above...

Finally, comparing a single low resistance coil to a single
high resistance coil at the same power output, the heat
is dissipated over a greater area (and volume of e-liquid)
in the high resistance coil resulting in a lower vapor temp.

I have GOT to learn to be more concise.

 

[hrtserpent6]

Holy smokes, I was *JUST* wondering the answer to this very question. I've got a Provari on the way, and looking at the Safe Vaping Chart, I came to the same thoughts as the OP.

Thanks so much for the great explanations!

 

[Shub]

great, but slightly confusing post! Thanks

 

[Moueix]

I learned something here, I'm just not sure what...

 

[johnlacie]

Weirdly and counter-intuitively, the way electricity works (I read something about it on here and just now had to prove it to myself on paper) the higher resistance atomizer running at a higher voltage actually draws less amperage from the battery and therefore the battery lasts longer. Seems strange but the formulae don't lie.
Low resistance atties were developed mainly for people with constant voltage devices that wanted more wattage (heat) from their vape. That's the only way to do it if you can't crank up your volts.

Folks, this is mostly correct. But what most people forget is the internal resistance of the battery and power circuitry. For the same wattage setting, a low ohm atomizer will cause the battery+circuitry to waste more power on the internal resistance than a high ohm atomizer. This applies to PWM circuitry as well. Hope, that helps.

 

[godzilla93]

Thank you all for your answers! I gave me a better grasp of the theory; now back to vaping! I just got a provari, and am experimenting with the Phoenix rebuildable (BTW, using hildicat's http://www.e-cigarette-forum.com/fo...oenix-chalice-vertical-cotton-wick-setup.html -- it works great! )