Ceramic Coils - Any Good For Dimitri?

Many thanks to Thundercastle for his input into this report

Some pre-manufactured coils are inaccurately described as 'ceramic' simply because they incorporate cosmetic or other sundry ceramic parts. But here we refer to 'ceramic coils' owing to the hard porous ceramic wick material (commonly pressed into a tubular bead).  This bead normally has a coiled wire heater semi-embedded into its inner diameter (see pic). 

Intro

The most common pre-manufactured coils instead incorporate cotton, which is wrapped in one or more layers around a mesh or coiled wire heater. Cotton has effective capilliary action, which helps maintain ample supply of juice to replenish that lost from vaporisation.

However, cotton scorches at just over 200°C which is not far in excess of d-juice boiling point (pure PG 188°C @1atm), and potentially unpleasant dry hits will almost certainly result if any part of the wick runs dry.

Ceramic coil types use hard sintered ceramic wicks in the place of cotton, which are reported to be far more resistant to heat damage. 

The common non-ceramic mesh/cotton coil design

Ceramic wicks

Sintered ceramic wicks are commonly used in cartridges, providing impressive performance in such a tiny atomiser. 

For larger tanks, ceramic coils are often simply scaled-up versions of cart atomisers, the Vaporesso EUC ceramic and GT CCELL2 are two examples of these.

But do they work as well in their larger size?  In my experience the answer for these two (and almost certainly others of a similar design) is a definite no.

The showstopping problem is capilliary action in the wick appears woefully insufficient to feed the heating coil at the required rate to maintain adequate vapour

Not to scale. Left is a tiny ceramic atomiser from a Ccell TH2 cartridge, and right is a much larger partially dismantled 0.3Ω Vaporesso EUC ceramic. Note the amazing similarity of the designs

Vaping Tests With Pure PG

For these I used two coils, a Vaporesso GT Ccell 0.3Ω installed in an NRG tank, plus a Vapore EUC Ccell 0.3Ω in a Veco tank.  Both have SS316L wire. Although these coils outwardly appear different, the cores are visually identical and performed indistinguishably in my tests.

The first sign of trouble came when attempting to set up temperature control. My usual basic setup for TC is the following 

If coil and wick are working correctly, the temperature of the coil should remain limited at a maximum of very near the boiling point of the juice. Therefore, providing the temperature is set above the juice's boiling point, it should never be reached irrespective of no-toke or toke rate. So the mod simply drives the user-set Wattage limit until the fire button is released.  

This principle is shown on the Arctic Fox graph below using a Kylin M tank filled with pure PG, Grus 100 mod set to TC-SS, 220°C and 30W limit (a ~7 second toke was used for the tests). Green line is the Wattage, red is temperature in °C.

The Kylin M showing exactly how the graph should appear during a toke.  Coil temp rise is limited to the PG boiling point, with extra power going into vapour production, Wattage remains at set max value throughout fire cycle in mod's attempt to reach user-set value of 220°C.  Set temperature will only be reached if wick started to dry out (dry hit protection)

However, when testing the ceramic coils in the exact same conditions, the temperature continued rising past the PG boiling point and quickly reached the set 220°C, which caused the mod to throttle back the Wattage asshown on the graph below.

The 0.3Ω GT coil shown (although both ceramic coil types performed identically). From the start of the toke, temperature quickly rose above the PG boiling point to the 220°C set value, where the mod responded by throttling back the Wattage from its 30W limit to around 15W

The graph shape is indicative of juice starvation after the first second or so of the toke.  From this it appears there is insuficient juice in contact with the coil to hold it's temperature at the boiling point.

I tried Thundercastle's tip of speeding the flow of juice through the coil.  This "Thundercastle Suck" involves creating a suction in the air passages by almost totally closing the tank's air intake vents and using significantly greater toking suction force to restore normal airflow.   The hope was that the vaccuum on the inner part of the coil creates a greater flow of juice through the wick. Using this technique, a slight improvement resulted, but nowhere near enough to restore performance to normal levels throughout the entire fire cycle

Same test conditions as previously apart from with vents nearly closed but using much harder suction on the toking to restore normal airflow. Temperature still rose to 220°C but at a slower rate, and the Wattage throttling occured later and reduced at a more gradual pace.

Vaping Tests With Dimitri Juice

Of course, I was keen to test out the cermic coils with dimitri juice.  But the above tests were carried out with pure PG, which is expected to be considerably less viscous (and therefore flow faster through the wick) than a dimitri/PG even without any added VG. In fact Thundercastle reported that in his experience, weaker mixes of 1:6 performed better than the more ususal 1:4 or stronger, probably owing to this issue.

I decided to test the 0.3Ω EUC ceramic coil with 1:4 dimitri/PG mix (no VG) and a starting power of 25W. Upon vaping, problems were immediately apparent as follows.

I tested the "Thundercastle Suck" (closed air vent/hard sucking) trick mentioned above but this did not seem to help much either.  Owing to the terrible performance and total lack of confidence in these coils I decided to abandon the remainder of the planned tests.

My Conclusions