PG VG ratio for sub-ohm vaping, wicking and coils

For direct-lung vaping, the advice often fits on one line, go for 30/70. Then the same bottle gurgles in one tank, crackles dry in another and leaves a puddle under a third. The PG VG ratio alone does not explain those differences, which come from the meeting between a thicker or thinner e-liquid and a coil designed for a certain flow. That meeting is what needs understanding before you choose.

PG VG ratio and sub-ohm coils, the direct answer

In sub-ohm vaping, a high-VG e-liquid is the norm because low-resistance coils are designed for it, with wide wicking ports and loosely packed cotton. That does not mean there is one ideal PG VG ratio in absolute terms.

A 30/70 floods a coil whose ports let in more liquid than it vaporises, and starves one that vaporises more than it receives. The coil’s design, the wattage you set and even the temperature of the e-liquid decide the outcome as much as the two numbers on the label.

Why sub-ohm vaping calls for high-VG e-liquid

The pairing of sub-ohm coils with vegetable glycerine is not a bottle fashion. It follows from the way a low-resistance coil uses e-liquid, and from the way it is built to take it in.

A coil that vaporises a lot, and fast

Set to several tens of watts, a sub-ohm coil sends far more energy into its cotton than a mouth-to-lung coil, and vaporises that much more e-liquid with each puff. To avoid running dry, its cotton has to be refed quickly, hence wide wicking ports.

Wide ports let everything through

Those same ports let a thin liquid in too easily. With a base high in propylene glycol, the cotton soaks up more than the coil can vaporise, and the excess ends up in the chimney or leaks out of the airflow.

Thicker glycerine slows that flow and restores the balance between what goes in and what leaves as vapour.

Denser vapour

Vegetable glycerine produces more visible, denser vapour, which is what direct-lung vaping is after. It is the best-known argument of the three, and yet the least technical.

Thickness and capillary action, what happens in the cotton

A coil’s cotton fills mainly by capillary action, with liquid climbing between the fibres on its own, while gravity only brings it to the ports. The speed of that climb is the real story behind the ratio, and it follows a simple law.

The Lucas-Washburn law

This law of physics describes how a liquid penetrates a porous material. For the same cotton and the same distance to cover, all else being equal, the time needed rises in proportion to the liquid’s viscosity. A liquid twice as viscous takes twice as long to reach the heart of the wick.

That is why a high-VG e-liquid needs short, wide paths to the coil, and a longer wait after filling.

Temperature changes everything

The viscosity of glycerine drops very quickly as it warms. According to standard heat-transfer property tables, its kinematic viscosity falls by more than half between 20 and 30 °C, and to less than a fifth between 20 and 40 °C.

The same 30/70 therefore behaves differently on a cold table, in a pocket or in a tank warmed by a run of puffs. Its ratio has not changed, its thickness has.

When a 30/70 floods or starves a coil

The two most common sub-ohm problems share one cause, an imbalance between what the coil receives and what it vaporises. The table sets them side by side.

Point	Flooded coil	Starved coil
Signs	Gurgling, spitting, liquid in the chimney or under the airflow	Dry or burnt taste, vapour fading at the end of the puff
Imbalance	The cotton takes in more than it vaporises	The cotton vaporises more than it takes in
Coil side	Very wide ports, wattage at the bottom of the range	Narrow ports, dense cotton, wattage above the range
E-liquid side	Warm liquid, so thinner	Cold liquid, so thicker, or very high VG
Fix	Raise the wattage within the recommended range, keep the tank away from heat	Lower the wattage, space out puffs, pick a slightly thinner e-liquid

The most misleading case is the rebuildable deck, where you set the density of the cotton yourself. A tightly packed wick that worked with a 50/50 can starve the coil as soon as you move to a 20/80, without the wattage changing at all.

Cotton, coils and upkeep with high-VG e-liquid

Once you have found the right pairing, three habits keep it in balance. They apply to ready-made coils as much as to your own builds.

Prime and wait

On a new coil, put a few drops on the cotton visible through the ports, fill the tank and then wait. Geekvape, for example, asks users of the tank supplied with its T200 mod to wait five minutes after filling before the first puff, and a thick e-liquid gains nothing from cutting that pause short.

Pick the coil for the liquid

The wide-ported coils of a sub-ohm tank such as the Smok TFV12 Prince are designed for thick e-liquids and high wattages. On a rebuildable deck, the same logic means looser wicks and cotton tails that keep the juice channels clear.

Rinse more often

E-liquids heavy in glycerine and sweet flavourings leave a film on the coil wraps and the tank walls. Rinsing the removable parts in warm water, then drying them completely, limits that residue before it starts to slow the wicking.

FAQ

Five practical questions, alongside what is covered above.

Can a 50/50 e-liquid work in a sub-ohm tank?

On some coils, yes, especially those with moderate ports run towards the top of their range. On the most open coils the risk of flooding goes up, and a first try with the tank half full limits the mess.

Does the PG VG ratio change the wattage to use?

Wattage is set first by the range printed for the coil. A very high-VG e-liquid copes better with the top of that range, and a thinner one is happier towards the bottom.

Does a bottle thickened by the cold go back to normal?

Yes, glycerine regains its flow as it comes back to room temperature. Letting the bottle warm up before filling avoids cotton that drinks poorly.

Is a high-VG e-liquid suited to an RDA?

It is its most natural home, since you dose the liquid straight onto the cotton. Its slowness to spread is offset by feeding the wick tails first.

How can you tell whether a coil takes very thick e-liquids?

The maker’s product page sometimes says so. Failing that, the size of the wicking ports visible on the coil body is a good clue, the widest being designed for high-VG e-liquids.

Matching your PG VG ratio to the coil, not to a rule

The PG VG ratio is never chosen on its own. It is chosen with the coil that will take it, the wattage that coil runs at and the temperature you vape in.

Above all, remember the mechanism, cotton drinking at the speed the liquid’s viscosity allows. It explains the gurgles as well as the dry hits, and it guides all direct-lung kit, from the RDA to the high-wattage mod.

More clouds

La Voluterie writes for adults only. Most vaping products contain nicotine, an addictive substance, and nicotine vaping products may not be sold to anyone under 18 in the UK. This is editorial content, not advice to buy anything.