Low/High resistance, at equal wattage: what's the difference?
Page 1 of 2 12 LastLast
Results 1 to 10 of 15
Like Tree6Likes

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

  1. #1
    Full Member Verified Member godzilla93's Avatar
    Join Date
    Sep 2012
    Location
    Brooklyn
    Posts
    73

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

    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...

  2. #2
    Senior Member Verified Member
    ECF Veteran
    A17kawboy's Avatar
    Join Date
    Jul 2012
    Location
    Northern BC, Canada
    Posts
    226

    Default

    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.
    Last edited by A17kawboy; 10-02-2012 at 08:18 AM.
    skelley and johnlacie like this.
    Keep your powder dry and your wick wet!!

  3. #3
    Full Member Verified Member godzilla93's Avatar
    Join Date
    Sep 2012
    Location
    Brooklyn
    Posts
    73

    Default

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

  4. #4
    Full Member ECF Veteran dragginfly's Avatar
    Join Date
    Nov 2010
    Location
    Missouri, USA
    Posts
    140

    Default

    Quote Originally Posted by godzilla93 View Post
    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.
    Last edited by dragginfly; 10-02-2012 at 06:57 PM.
    "You know me to be a very smart man. Donít you think that if I were wrong, Iíd know it?" ~ Sheldon Cooper

  5. #5
    Ultra Member Verified Member
    ECF Veteran
    Rader2146's Avatar
    Join Date
    Mar 2012
    Location
    Waco, TX
    Posts
    1,197
    Blog Entries
    4

    Default

    Quote Originally Posted by A17kawboy View Post
    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.


    Quote Originally Posted by Rader2146 View Post
    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.
    | Brass M16 Clone | #8 | OKR-T/6 | Trident Clone | IGO-L |
    "The illiterate of the 21st century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn."

  6. #6
    Senior Member Verified Member
    ECF Veteran
    A17kawboy's Avatar
    Join Date
    Jul 2012
    Location
    Northern BC, Canada
    Posts
    226

    Default

    @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.
    Keep your powder dry and your wick wet!!

  7. #7
    Ultra Member Verified Member
    ECF Veteran

    Join Date
    Sep 2011
    Location
    Gray,GA.
    Posts
    1,358

    Default

    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.
    A17kawboy likes this.

  8. #8
    Ultra Member Verified Member
    Registered Reviewer/Blogger
    ECF Veteran

    Join Date
    Jun 2010
    Location
    Chicago IL
    Posts
    2,559
    Blog Entries
    30

    Default

    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.
    dragginfly, A17kawboy and MikeA5 like this.

  9. #9
    Super Member ECF Veteran Slurp812's Avatar
    Join Date
    Jun 2011
    Location
    Northwest Ohio
    Posts
    392

    Default

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

  10. #10
    Full Member ECF Veteran dragginfly's Avatar
    Join Date
    Nov 2010
    Location
    Missouri, USA
    Posts
    140

    Default

    Quote Originally Posted by Slurp812 View Post
    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...


    Quote Originally Posted by dragginfly View Post
    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.
    "You know me to be a very smart man. Donít you think that if I were wrong, Iíd know it?" ~ Sheldon Cooper

Page 1 of 2 12 LastLast

Tags for this Thread