Notes on Sub-Ohming

by atmizone
on

One of the most important elements of every atomizer build is the total resistance of the atomizer. Different atomizer types work best with different resistance ranges for a number of reasons related to: i) the diameter of the resistance wire, ii) the diameter of the windings and iii) factors related to heat transfer. These properties are also dependent on the particular building method.

A thick resistance wire will always have a lower resistance compared to a same-length, thinner wire. The heat transfer characteristics of the thicker wire will also be different due to its higher mass and higher surface area. In general, Genesis-type, single-coil atomizers work best with lower resistances than traditional Silica-type atomizers.

The use of sub-ohm resistances in Genesis-type atomizers has become quite common for some time now. Sub-ohm Silica setups are also popular, often using two or more resistances wired in parallel.

Some vapers have often voiced concerns about the use of sub-ohm resistances, mainly focusing on the issue of safety. Most of these discussions fail to communicate clearly what about sub-ohm vaping is unsafe, or, better, how sub-ohm vaping can be made safe.

So what are the risks of sub-ohm vaping, after all? How should a sub-ohm kit be handled and what is needed to build and operate a sub-ohm capable mod safely and efficiently?

1. Sub-Ohming: Safety and Efficiency

Most of the issues associated with the use of sub-ohm resistances can be discussed with only fundamental knowledge of electrical engineering. As we have discussed previously, a mechanical mod is a rather simple electric circuit which consists of a non-ideal voltage source (the battery), a medium that transfers power (the mod + atomizer body) and an ohmic load (the resistance of the atomizer).

The basic elements where we need to focus our attention are: i) the load (our atomizer resistance), ii) the source (the battery), and iii) the transmission medium (the mod). Then, the question we then need to ask is this:

“What do we need know in order to operate such a circuit in the safest *and* most efficient way?”

Battery Performance and Safety

The first thing we need to ensure is that our battery is able to provide the power that the resistance “needs”, in a safe and efficient manner.

From this post, you may remember that the amount of power that a resistor will consume is a function of i) the current that flows through it and ii) the voltage applied on it. The current that flows through a resistor can be found by the simple formula I = V / R, where V the voltage applied on the load and R its resistance value. Knowing the current, we can then calculate the power requirements of the load with the formula: P = V * I.

As an example, when powering a 0.8 ohm resistance with a 4V DC source, the current that will flow through the resistor will be 5A and the power dissipated on it will be 20W. In this case, we need to use a battery that is able to supply 20W.

Batteries are characterized by their discharge rate capability and energy capacity. The C rating of a battery is an indication of the maximum safe continuous discharge rate. If you see 10C on your battery, it means that the battery can be discharged at 10 times its capacity. Capacity refers to the milliamp-hour rating of the battery, which is listed as a number followed by mAh (2000mAh, for example).

For example, most red AW IMR 18650 batteries have a 15C rating and a nominal capacity of 1600mAh. This means that its max continuous discharge rate is 15 * 1600 = 24000mA = 24A.

Similarly, we can find that the limits for the AW IMR 18490 (15C), AW IMR 18350 and AW IMR 16340 batteries are: 16.5A, 6A and 4A.

Based on the above, this means that the lowest resistances that can be safely powered by these batteries are around 0.15 Ohm (18650), 0.23 Ohm (18490), 0.65 Ohm (18350) and 1.0 Ohm (16340).

On a side note, the black (protected) AW ICR 16340 is rated at 750 mAh but has a ~3A limit, which corresponds to a resistance of around 1.3 Ohm. In this case, the discharge limit is imposed by the integrated protection circuit.

With safety put aside, there are still two limiting factors — the capacity of the battery and its efficiency.

The capacity becomes a limiting factor when batteries are used to power resistances close to these minimum values. For example, the AW IMR 18350 battery cannot be realistically used to power a 0.7 Ohm resistance, since the battery will need to be recharged so frequently that it will be practically unusable. For the same reason, a 16340 battery will not last long when powering a 1.0 Ohm resistance. This fact applies even more to the higher C-rated 18490 and 18650 batteries.

In practice, a discharge rate usable for real-world vaping is around 5-6C. Roughly, this corresponds to a minimum resistance of around 0.4-0.5 Ohm for the 18650, 0.6-0.7 Ohm for the 18490, 0.9-1.0 Ohm for the 18350 and 1.2-1.4 Ohm for the 16340 (AW IMR).

Last but not least, the efficiency of each battery depends heavily on its internal resistance. Smaller cells have a higher internal resistance, which always translates into voltage loss, a subject which has already been discussed here. In simple words, this means that if we compare the performance of an AW 18350 vs an AW 18650 when both batteries power the same 1.0 Ohm resistance, we will find that the AW 18350 results in a voltage drop that is about double that of the AW 18650. In terms of efficiency, this means that not only the 18350 lasts less due to its lower capacity, but more power is wasted on the way compared to the 18650. This can be witnessed intuitively if you notice that an AW IMR 18350 will always warm up more than its 18650 counterpart when powering the same resistance.

Mod Performance and Safety

A mechanical mod is nothing but a series of fixed electrical contacts that connect a number of parts, as well as one normally-open, make-break contact: The switch. In our recent discussion about voltage drop, we mentioned that all these contacts and parts introduce some resistance, which is placed in series with the resistance of our primary load – the atomizer.

Fixed Contacts

The greatest chunk of the combined resistance of a mod can be traced to the fixed electrical contacts between its parts — not to the bulk resistance of the material of these parts. In other words, a copper-alloy mod performs marginally better than its ss-alloy version *not* because the bulk material of its parts conducts noticeably better, but because copper-to-copper contacts perform noticeably better than ss-to-ss contacts on average.

In any case, the higher the combined mod resistance, the higher the combined voltage drop on the mod. In a similar way to the internal resistance of the battery, the connections of a mod play an important role to the efficiency of the circuit, which becomes more and more important when using sub-ohm atomizer resistances. There is one very important thing to keep in mind here:

The performance of a mod’s fixed connections / contacts depends primarily on their number and condition, and less on other factors. This means that a mod with i) few, ii) clean / non-oxidized, and iii) tight connections will always perform better than a mod with more, dirty or loose connections.

From this, it is obvious that the exact same mod may perform very differently if its connections are not clean, tight and properly assembled. More connections mean that a higher probability exists for a connection to be dirty or loose, while a bad connection may have a huge impact on performance, and potentially on safety, too.

In the sub-ohm world, the importance of these facts becomes so important that a mod with un-maintained connections or loosely assembled connections may be unusable or – in extreme conditions – dangerous. At higher currents (lower atomizer resistances in the sub-ohm range), some connections might actually perform better and some progressively worse for a number of reasons. A slightly resistive spring-loaded connection may dissipate so much heat when a lot of current passes through it, that the spring might lose some of its flex. When that happens, the electrical contacts on both ends of the spring will be weakened, making things even worse and resulting in overheating, voltage drop and eventually a collapsed spring. When using spring-loaded contacts for sub-ohm vaping, it is necessary to ensure that the spring is hard enough to provide a firm connection and that it is always clean and in mint condition it terms of flex. Otherwise, it is a far better idea not to use a spring-loaded contact at all. Thermal shock is one of the factors that can affect the performance of spring-loaded contacts in a very adverse way – springs and pogo pins should not be used in places where rapid changes of temperature may happen.

All fixed contacts must be tightly assembled and kept in top shape in order to: i) maximize the voltage that will reach the atomizer, and ii) to prevent any localized overheating which could damage your equipment or affect the operation of your batteries. The contacts against the battery terminals are the most critical and, usually, the ones that can easily result in voltage drop and overheating unless they are treated with attention.

The Switch

The make-break switches of all mods require special attention to remain operable when using sub-ohm resistances: Normally, DC contacts must be switched fast to minimize the negative effects of arcing, which damages electrical contacts, while the force applied on the contacts must be: i) sufficient for the specific operating current, and ii) ideally applied by a dedicated mechanism in a controlled, repeatable way.

The switches in all mechanical mods are simple, rebuildable designs where the position and force of the contacts is directly controlled by our hands. These switches are less than ideal in many ways: They are not making and breaking as fast as they should, they do not operate in a repeatable manner and they are not environmentally sealed, which means that dust and other contaminants can freely roam on the contact surfaces. It may sound like a paradox, but these tradeoffs have been made exactly to avoid placing off-shelf, sealed microswitches into mech mods: These switches are usually bulky, sensitive, non-rebuildable, require cabling / soldering and will only fail once – and for good.

The most important and simple parameter to ensure a reliable operation of mod switches is contact force and the material / condition of the mating surfaces. Sealed industrial switches are usually designed in a way that minimizes surface area for the rated current of the switch, in order to i) maximize the effect of the applied force (hertz stress) and ii) minimize cost. Larger surface areas become more meaningful at higher currents / higher available forces and when the mating surfaces are even. Contacts with large surface areas should be avoided unless: i) the current magnitude absolutely requires the increased surface, ii) the mating surfaces are even and iii) enough force is available to electrically “activate” the larger surface.

The switches in many mech mods are so well designed that the angle of operation will have no effect on circuit closure; some switches feel so responsive that vapers get into the habit of pressing them with feather-light forces. At higher currents, though, a very light activation force may actually be insufficient, resulting in contact overheating.

Additionally, the plating material on all contacts tends to wear off with heavy use – and mod switches are used very heavily indeed. The role of the plating material is exactly to bring down the force required to operate a contact well – once the plating layer starts to wear off, the contact will no longer perform well and will require higher forces to operate without becoming resistive.

So:

  1. Sub-ohm vaping requires a consistent, firm operation of make-break contacts due to the high current.
  2. A worn or dirty contact will overheat and get damaged much more rapidly when used to conduct a high current.

2. Sub-Ohming: Good Practices

Summing up, here is a list of good practices when using a sub-ohm resistance. The lower the resistance, the more important it becomes to follow these tips religiously in order to prevent equipment damage and maintain a high level of safety.

Batteries
  1. Use this guide to select batteries that are suitable for the resistances you are building.
  2. Don’t use a cheap multimeter to measure sub-ohm resistances – the measurement might be much higher than the actual value of your resistance. Low-resistance measurements require dedicated equipment! Do not experiment with super-low-resistance builds unless you can accurately measure your resistances. Once you have the necessary experience, you will be able to rebuild the same setup in a repeatable way even without measurement tools.
  3. Check the condition of your batteries often. Stressed batteries will gradually discharge very fast and show a higher voltage drop on their internal resistance. Don’t use batteries for sub-ohm vaping unless you are monitoring their condition.
Fixed Mod Contacts
  1. Just like with batteries, you need to be able to monitor the condition of your mod by measuring voltage drop using a battery that you know. When you notice an abnormally high voltage drop, immediately inspect all connections, starting with the atomizer-to-mod contacts and with the battery-to-mod contacts. It may take time to find the bottleneck, but it will save you from other headaches. If necessary disassemble all fixed contacts of the mod and clean all threads and connections individually.
  2. Avoid spring-loaded contacts for sub-ohm vaping, or use harder and preferably plated springs. With use, springs tend to lose their flex, which weakens contact at both ends, resulting in voltage drop and overheating. If you have to use a spring, stretch it often to ensure firm contact at both ends.
  3. The collapsible springs used in some mods are a sacrifice of performance in favour of safety. In most cases, you can remove them, though. If you have such a mod and have damaged more than a few of those springs, you shouldn’t be using a mech mod in the first place.
  4. Keep all parts tight and clean and, as a rule of thumb, prefer simpler mods with fewer connections. If that means sacrificing things like air-flow control, extensions or other gimmicks, so be it. Keep things spartan, especially at the lower end of the sub-ohm range.
  5. Keep a list of all connections from the positive battery terminal to the atomizer resistance, and from the negative battery terminal to the atomizer resistance. You need to know which of these connections are the most sensitive, handle them properly and inspect them often.
Mod Switches
  1. Handle the switch properly and keep it in top shape to ensure that it is able to conduct the high current without overheating.
  2. Proper maintenance and prevention is important. Wipe the contacts of the switch with alcohol every week, blow all parts with compressed air and avoid touching them with bare hands.
  3. At lower resistances, remember to operate the switch firmly, especially if it’s a double-break design (all top-firing mods).
  4. Very light, accidental presses of the switch can be catastrophic at high currents. Always lock your mod when carrying it and avoid throwing it in dusty pockets.
  5. When sub-ohming, you will have to re-plate the contacts of your mods at least every few months. The more serious you are with vaping, the more you must be informed and prepared to do what’s necessary to enjoy it when pushing the limits.
  6. With regular maintenance and proper use, the contacts of your mod are very unlikely to suffer from overheating incidents, but if any black spots appear on them, they should be cleaned with “silver wipes” or a non-corrosive cream suitable for silver/brass/ss-ware as soon as possible.
  7. Some mods use thermoplastic materials designed to handle temperatures up to 300*C. This means that they are – by design – able to operate in overheated switch conditions that can possibly be an issue for battery safety. Although that means that the mod won’t break easily, don’t take the matter lightly – just because the switch can be operated in unsafe conditions doesn’t mean that you should neglect its maintenance. Industrial switches at these ranges are built to be operated well below the 80*C threshold — high temperature always translates into rapid contact damage, high voltage drop and compromised safety. Monitoring the voltage drop of your mod is the best way to ensure that the switch is in mint condition ***.

    *** Many battery chemistries will start to become unstable when temperatures in excess of 150*C reach the cell. Our approach at Atmizone has been to use materials that are designed be operable in those temperature ranges that all batteries operate safely. For safety reasons, all components in direct contact to a battery, such as a mechanical switch or an electronic component, should not be allowed to operate at temperatures beyond this range. This is a common industrial practice which is based on a simple principle: If a low-cost component is not used or serviced properly, to the point that its operation becomes a potential safety issue for other components, it is better if that component is designed to fail. Atmizoo mods use thermoplastic materials that will not sustain temperatures in excess of 150*C for precisely that reason. A switch allowed to operate at the 250-300*C range is a big safety and performance compromise. Mechanical switches should be designed, handled and maintained so that they never develop such temperatures in the first place, not the other way around. Allowing a component to operate under conditions where it’s no longer safe or efficient is a two-edged sword: It is a poor approach to reliability and a poor attempt at making mechanical devices more appealing to non-technical vapers (Updated: Jan 6, 2014).

3. Sub-Ohming with Atmizoo Mods

Like most fully mechanical mods, Atmizoo mods are suitable for sub-ohm vaping as long as all this information is kept in check. Until now, top-firing mods like the Roller are by design more complex than the simplest bottom-firing mechs, which means that they require more attention. Additionally, most top-firing designs have a spring-loaded battery-to-mod contact, which may result in headaches when used for sub-ohm vaping by inexperienced vapers.

Older versions of the Roller were shipped with a thin diameter, un-plated spring which was quite sensitive to thermal shock and rather soft for high-current use, especially in 18350 mode where the spring cannot be compressed firmly. 18350 batteries are mostly unsuitable for sub-ohm vaping, not because they are borderline capable to handle the high current, but also because of their limited capacity, as outlined previously. Many vapers who ignored the limitations of spring-loaded contacts had trouble using Rollers for sub-ohm vaping, especially in 18350 mode.

For a few months now, all versions of the Roller are shipped with thicker diameter (optionally silver- or rhodium-plated) springs which are able to maintain a more firm contact in all modes but are still not ideal for super-low-resistance vaping. In the vein of keeping things simple, experienced vapers who use Rollers for sub-ohm vaping usually remove the spring and use exclusively 18500 / 18650 batteries, remembering to keep the telescopic tube tight. Safety-minded sub-ohm vapers replaced the spring with a Bug, which is a high-performance resettable fuse that can be used with resistances down to 0.6 Ohm.

The Dingo, being a fixed-length mod, is a bit more challenging for sub-ohm use. Removing the spring requires bottom cap disc adjustment which will usually not be sufficient to close the circuit without a Bug or other spacer. A very good solution for springless sub-ohm vaping with the Dingo is to use the Bug in combination with a T2 extension and a 18500 battery. Otherwise, instead of going springless, the spring must be kept in check and if necessary stretched to ensure firm contact with the battery.

Uni-tube designs, such as the Guppy and the Lab, consist of fewer parts and connections than any other top-firing device, which makes them better for use with lower resistances (less fixed contacts = fewer contacts to worry about).

Although the Guppy performs magnificently in terms of efficiency, current-generation 16340 batteries are not suitable for sub-ohm use due to their discharge rate limitations and low capacity, as discussed previously. Right now, the limitations of 16340 batteries make the Guppy suitable for use with resistances in the 1.1+ Ohm range. Black AW ICR 16340s have the nominal capacity of their 18350 IMR counterparts and can maintain a relatively high voltage during their discharge, but are capped by their ~3A limit. Red AW IMR batteries have a higher current limit (~4A), but have a lower nominal capacity. It’s a toss between the two, but none of them is suitable for sub-ohm use. At the moment, the tiny size and efficiency of the Guppy make it ideal only for use with 510d-type rebuildable atomizers, which work best in the 1.1+ Ohm range. With battery chemistries constantly improving, this might soon change, though.

The Lab, which is designed for use with 18500 batteries, is one of the simplest top-firing devices and perhaps the most manageable for sub-ohm use, due to its super-efficient uni-tube design. The best high-drain 18500s are perfectly suitable for low-resistance vaping, while the mod itself completes the circuit between the battery and the atomizer using the smallest possible number of parts. Experienced vapers can configure the Lab to use a spring-less battery-to-mod contact, which can also be converted to include a replaceable, sub-ohm capable fuse (the same as the one used in the Bug) sandwiched between the battery and the cap.

Although the simpler top-firing mods like the Lab and most good bottom-firing designs are easier for sub-ohm use, more complex mech mods like the Roller are equally capable when proper handling and monitoring practices are used. Most of the subjects discussed here are common knowledge among the most experienced or tech savvy vapers, but the increasing recent adoption of mech vaping has shifted the focus from the technical aspects of high-end mech vaping to its more cultural merits. Sub-ohm vaping is not necessarily risky; however, it does push the limits of our equipment. The tips discussed here, which apply to all mech mods & atomizers, will hopefully make the sub-ohm experience a bit more accessible and rewarding.

Tags: .

Start New Discussion.