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Viewing blog entries in category: Ohm's law in application

  • State O' Flux
    So many questions, so much misinformation - and all it will take to clear up at least 80% of both... is a bit of knowledge.

    If you can do basic math... the most useful Ohm's law formulas are:
    • For wattage ("Power") the formula is V² ÷ Ω = P.
    • For amperage ("Current") the formula is V ÷ Ω = C.

    This isn't going to be a class in all things Ohm's Law, so much as a few things you need to know about resistance and power... so that both your vape and your safety is well considered. First... lets open up a very easy to use, uncluttered with BS, on-line Ohm's Law calculator, or 'OLC'. Got it? Good. :D

    Most common misconceptions I run into are:

    "What wattage do I need for X resistance?"
    If we are using a regulated APV, then we need a few of it's specifications. The maximum wattage and amperage maximum or limit.
    Lets say you want to run up against the "edge of sub-ohm"... but you have a 15 watt limit. Can we go sub-ohm with just 15 watts? Yes... yes you can. :p

    Lets put 0.9Ω into the appropriate box of our OLC.

    Insert the 15 watt value, the 0.9Ω value... and hit the button. We get 3.7v and 4 amps filling out the other boxes.
    Can we use this? Yes we can... because the specs of our APV indicate an amp limit of 5 amps - and we're under than value.
    If we were over that value, you could still run your wattage, but at the selected resistance, there would be no additional benefit.
    Most VW APVs have lower resistance limiters as well... for example, the Innokin MVP2 has a 1.3Ω/3 amp/11 watt limit. If you insert any two of those numbers into our calculator, you'll see that they line up fairly well with Ohm's Law.
    If you try to use a resistance less than Ohm's Law indicates the maximum power of your APV is capable of supporting... there is no benefit.

    The above doesn't take into account a value called "heat flux"... which is the coil radiant heat temperature. Heat flux is one of our most desirable tunable elements... second only to net coil surface area for value. You'll learn a lot more about heat flux when you read the two part series on using the Steam Engine coil modeling program.

    "What's the smallest battery I can run with X resistance?"
    This is the kind of question a new mech mod user will ask, because they don't really know anything about battery ratings or how Ohm's Law applies. Because unregulated mechs are used mostly with low resistance, lets use 0.5Ω as our reference value.
    The batteries we have on hand are AW IMR 18350 (6a), 490 (16.5a) and 650 (24a/1600mAh version).

    Now... we need to know how much amperage we need for 0.5Ω. Because an 18XXX series battery has the same base voltage regardless of size, we can use that as a constant. I use 3.9v as it's not a full charge, but still above base voltage. Insert 3.9v and 0.5Ω into the OLC. Our required amperage draw is 7.8a.

    Because our mech mod is unregulated, we need a margin of safety. I use 10% of the rated battery maximum continuous amperage drain value... so, a battery rated at 15 amps, I would subtract from that, 1.5 amps.

    I would seriously suggest that new mech users use 10%~15% margin of safety... until you have a good understanding of what you're doing.

    If we look at our battery amp drain ratings, we see that the "smallest" battery we can use is the AW IMR 18490, rated at 16.5 amps... leaving us a substantial margin of safety.

    This begs the secondary question... "So, how much lower can I go with that 18490 battery?" Insert your base voltage, and our AW IMR 18490, with a 10% margin of safety subtracted from it's max. amp drain rating. 14.8 amps is the number we're "pushing up" against. Run the numbers, and we can go as low as 0.26Ω.

    "How low a resistance can I run with X battery?"
    This is a variation of our last question. Lots of folks want to run an 18350 battery to keep their mechs compact. Nothing wrong with that, but there is a resistance limit you can't ignore.
    Using our AW IMR 18350 6 amp limit... we can easily calculate our lower resistance limit. With our safety margin, we have 5.4 amps to work with... so, lets insert amps and volts. What did you get? I came up with 0.7Ω.

    If we had one if those new Efest "purple" 18350s with a mfg. rating of 10.5 amps... run the numbers again and we come up with 0.4Ω.

    "How can I use Ohm's Law to figure out the right voltage for my mechanical mod?"
    This question is coming from a place of confusion. Can you control the voltage of a typical unregulated mech mod? Nope... battery voltage is what it is. Even with a Kick, you only control wattage.

    I run 0.3Ω dual coils... which Kick should I buy?"
    Ahh... the kind of question that makes you go :blink: :closedeyes: :facepalm: To begin, how is it that someone "runs 0.3Ω dual coils", yet apparently has no concept of what a Kick is, or how Ohm's Law works? What are they currently running "0.3Ω dual coils" in - on what APV and why the hell do they think they want a Kick?

    Kicks were designed to allow mech mod users the look, feel, style and "simplicity" of a mech... with the safety advantage of a wattage regulated, amperage limited APV. Kinda like turning your $200 Nemesis into a Vamo. :laugh:

    All Kicks, from the genuine Evolv Kicks 1 & 2, to the various clones, have lower resistance and maximum amperage limiters... just like any other VW APV.
    Toss on your atomizer, crank it up and go. If you didn't do your OL math, installed a atty with too low a resistance... it will either not work, or run your battery down pretty quickly.

    What wire is best for X resistance? (Sorry... not everything is about Ohm's Law)
    A good question this one... and one encountered often. There are a few ways to go about it. You can read posts, look at YouTube videos, see what others are doing and experiment with those examples.
    You can refer to Super_X_Drifter's coil building thread with examples of differing resistance values for a common coil build... or you can figure out the perfect, optimal build yourself using "Steam Engine"... a multifunction vaping calculator that may be the most valuable vaping tool you ever use.

    Heat flux (HF) and heat capacity (HC). These are two values available found in Steam Engine... that are far more important than you might first imagine.
    You can use HF to determine the optimal wattage value for a given build, or the optimal build for a given wattage.
    Although not as useful as HF, pay attention to the HC... if you selected the wrong wire, your HC value will go up, and you'll wonder why it takes so long for your build to heat up and cool down.
    More information on these two values... can be found in the Steam Engine Advanced User guide. Yes... yes I have promoted the user guide twice in one article. Steam Engine will be that valuable to your vaping happiness. :thumbs:

    What is the momentary or "pulse" rating of a battery?
    Batteries may have two ratings. A Maximum Continuous Current Discharge (MCCD) rate... and a pulse rate. Pulse can be anywhere from 1.5 to 3 times that of continuous, and is base on the amperage potential within a short time frame... from as little as one second to as long as 30 seconds.
    Pulse rating can be used for an unregulated mech mod to calculate resistance. For example, if a 10a battery has a 20a pulse rate for 10 seconds... then you can calculate resistance for 20a... for 10 seconds. After that, amperage drops rapidly to the continuous rate.
    Not all batteries have an advertized pulse rate, and although you might feel comfortable extrapolating your own, based on the continuous rate... you are only guessing.

    In the end, I do not recommend using pulse rate values to beginners. Stick with continuous ratings until you have confidence in your knowledge and abilities. Don't go supersonic, until you first learn to fly. :D

    Addendum 5-1202015: These fairly recent, unusually high (35, 38 & 40A) 18650 ratings are not in fact MCCD ratings, but rather pulse or "momentary" ratings.
    Granted, they are relatively modest pulse ratings within a time limit likely exceeding even a deep draw vapists usual application... but they are still a lie, and not the true Maximum Continuous Current Discharge.
    The same applies to the the smaller popular vaping batteries, where the listed amperage is above the norm.

    That's it for now. If anyone who reads this has a specific question... feel free to ask and if it's a good one, I'll include it in the above. Same for errors... see any? Let me know.

    As always, take it for what it is, do with it what you will. :2cool:
    FilthyPazuzu likes this.
  • State O' Flux
    RBA, RDA, RTA... oh my! For those folks that are new (and semi-new) to RBAs, what are they and how do they work?

    I could delve into the history of the RBA, but I imagine that most of my "gentle readers" don't really care about history - rather a bit of definition for what is available now. I'll also toss in a few bits of editorial commentary for the reader to ponder.

    I apologize if I don't mention "your" atomizer. In the scope of this short article, I can't cover every atomizer design made, so I'm focusing on what the blue collar "Joe Vapist" is buying now.
    If this annoys you, write your own elitist POS article. :2cool:

    First, lets define what an RBA is. An RBA is any atomizer, designed so that the wick and coil elements are, to a large degree... at your discretion.
    Beginning around the fall of 2014, you can buy RTAs with your choice of ready to go coil/heads, or the option of a DIY RBA head. You can start off with know RBA experience and just use the supplied coil/head assemblies... and at your discretion, learn how to produce your own builds.The Kanger Subtank and Subtank mini are good examples of this relatively new genre of "user optional" atomizers.

    Beats the hell out of trying to rebuild a clearo/glasso that was never intended to be rebuilt... don't it?

    Some will require, due to their design, a specific shape or even specific coil and/or wicking material, but for the most part... it's builders choice, hence the name "rebuildable atomizer".

    They can be a rebuildable dripping atomizer, like the ubiquitous and still hugely popular JPGE Nimbus... or a genesis type atomizer - the Smoktech RSST being a popular entry level example, or a tank type atomizer or "RTA" - as represented by perhaps the most popular RTA ever made - the SvoeMesto Kayfun.
    In the last few paragraphs, I'll include a few RBAs that fall into a, mostly undefined category - the "top-filler-to-wick" atomizer.

    The RDA.
    In basic design, a dripping atomizer or RDA, is the simplest form of atomizer. They do not have a built-in, semi-sealed or sealed juice container or tank. You build your coil(s) and wick(s), drip juice from your bottle onto the wick... and vape away. When the draws start to taste a bit dry and less flavorful, it's time to drip in more juice.

    The upside of RDAs are that they allow practically any coil - shape, quantity and resistance values are limited only by the number of terminal posts and the size of the build deck. The wicking materials used are unlimited as well - from "roll-yer'-own" cotton ball to ceramic rod or rope... it's your call.
    The downside to RDAs is that they don't have a juice container - with even the deepest juice wells holding no more than 30 or so drops of juice. RDAs can leak, and will require that you have a separate juice bottle at your disposal.

    "Drivin' and drippin' can be a dangerous habit to develop - particularly with a manual trans, or motorcycle. :ohmy:

    Still hugely popular, but... as drippers become larger, more refined and complicated, what will they turn into - something else entirely?

    The Genesis RBA
    A genesis atomizer is a device made from necessity - and a "Eureka moment". We look at the dripping atomizers greatest negative, a lack of juice container - and think, "how do I get around carrying a juice bottle and dripping?" - with the answer being... "lets just attach a juice tank to the bottom of an RDA, and run the wick down into it!!!!" And that's how the "genesis" was born.

    OK... maybe it wasn't quite that simple. :p

    Genesis atomizers have some limitations and requirements. Because the liquid tank is not sealed, if you tip it over or upside-down, juice may leak... or even pour out the wick hole or tank vent port.
    Because they essentially feed from the bottom up, you may need to "tip" the atomizer to get juice up to the portion of wick that is wrapped by the coil. Wick materials and design become more important in that capillary action is a critical consideration.

    Stainless steel mesh and rope, as well as ceramic rods are popular wick materials, as they can provide a balance of adequate, but not excessive, capillary action.
    There are new and interesting variations of genesis atomizers introduced every day. Some that are guaranteed not to leak and some with tanks on top.

    "Genny's" have lost a lot of love in the past year or so... mostly due to the next RBA on our list.

    The RTA - "tank" atomizer
    All of the popular tank type atomizers can be defined using another term... they all operate on the principal of "pressure differential". The hyperlink is to an article on how pressure differential or "PD" tanks work.
    For those that don't want to read the entire PD article, in short and sweet terms... positive pressure (in the air venting system) and negative pressure (in the liquid tank) are held separate - yet balance... serving to feed juice, in appropriate quantities, to the wick. The wick itself may aid in this function, acting as a seal between the two pressures.

    Added 7-1-14: (Sorry... I'm about 2 months behind in adding this)
    If you've not heard of it, don't feel too bad... because the Highwinds Ruzgar, and it's recently introduced clone (first one... anticipate better quality ones) have been a bit under the radar. Think of it as a top fill, glass tanked evolution of the KFL. It doesn't have AFC, but the genuine one I tried had air flow on par with a stock Kebo R91%.

    The previously mentioned Kayfun is one of the 'true' pressure differential atomizers, in that the design does not use the wick as a seal, but rather only the "boundary layer", established by juice channel orifice size, to keep the pressure, and juice, separate.
    Other popular PD tank atomizers, like the Stattqualm sQuape and Taifun GT use orifice size restriction as well, but in concert with the wick, which, not unlike a glassomizer coil/head assembly, serves to define that restriction.

    "Filler" type RBAs.
    OK... I'll be honest here, I'm not sure what to call them. Gravity tanks, top-filler-to-wick tanks, capillary tanks - are any accurate enough to stick?

    This category is populated with atomizers like the VapourArt GP Spheroid, early version GP Heron and Gus Aiolos... which can be considered closer to genesis atomizers than RTAs, because they do not use pressure differential or a true liquid tank, but rather a filler material, usually "cerawool", to both hold juice, and feed the wick.

    Interestingly enough, the latest version GP Heron, with the addition of a spacer/separator, or as VapourArt calls it - "a topper that works as a reducer and cotton feeder at the same time" - this after-the-fact part, aside from the stated design intent, creates the potential for a seal between liquid storage and vaporization chamber.
    This seal, depending on the build, may be sufficient to create and hold a negative pressure, effectively turning the atomizer into a semi-PD type tank... but one where the pressure variance is not used to feed juice. There is a Heron clone available now from FT.

    5-5-2014 Addendum.
    There's a relatively new atty on the market made by GUS in Greece, called the Estia.
    It is a PD atty, and is designed around 3 coils with 6 wick tips "in the juice"... with the highest air flow of any RTA to date. It's currently the very best RTA I've ever used... and as close to a good RDA as one can buy.
    Not much else I need to say, because qorax has done a very well photographed, slightly gushy and subjective, but never the less informative review here.
    If you get a chance, take a look at the Inverno Erlkönigin review too. I don't know qorax, but I know he does two things very well... buys the latest cool attys, and produces some very well photographed reviews.

    The Fogger V4 came out a month or so ago. This is a dual coil atty... and I mention it in the same breath as the Estia, because it's, well... quite a flavorful vape for such a low cost atty.
    It doesn't have the air flow, features or quality of fit and finish of a genuine RTA, but it's 1/5th the price... and easily one of the best performing clones you can get. Only the single coil, genuine Taifun GT is on par.
    Due to it's low price, there are many threads in the RBA forums on this atty... so find one, and read.

    I return and update as things change... stay tuned.

    As always, take it for what it is, do with it what you will. Cheers!
    FilthyPazuzu likes this.
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