Vaping looks so much like smoking. Putting something to your mouth, inhaling, and exhaling a cloud. This leads many people to jump to the erroneous conclusion that the physical processes involved might be similar, too. Even scientists trying to measure emissions fall for it. Like all those finding the Freaking Formaldehyde.
The fundamental difference
Draw harder, more Oxygen fuels the combustion, more smoke is generated.
The amount of vapor generated just depends on the energy used to heat the liquid. Draw too slow, the vapor gets too hot. Draw too hard and the vapor just gets diluted with more ambient air and some of the energy is diverted to heating the air.
How smokers deal with it
Most smokers don’t consciously think about it. When they switch to vaping they simply adjust their behavior to optimize the experience. After the first irritation it is rather easy learning to adapt. Trial and error. Sensual feedback.
Smoking machines, what can go wrong?
Automated smoking of cigarettes: Not much.
The physical process of combustion is pretty simple and tolerates a lot of variation in the parameters while still producing similar results. There are not many influences of variables except for the amount of oxygen drawn through into the fire to produce the smoke. You have a very wide range where the combustions provides the expected results.
Automation of vaping: A lot!
While the components are deceptively few and simple, the resulting physical process is anything but. It’s a complex dynamic process with lots of interactions. Even slight variations can have huge effects. You need effective sensors to dynamically regulate it. Like the human taste buds and the attached neuronal network. The innocent approach to select some parameters that seem reasonable and go, just doesn’t work reliably.
Simple components, complex process
The physics behind vaping might be complex enough for several doctoral theses. So I’ll rather leave the intricate details to real physicists.
It begins with the warm up phase. How long it takes largely depends on the type and amount of metal in the coil and of course the energy input. During this phase the vaper (me) usually starts drawing softly.
The main part is a dynamic balance where the temperature should be fairly constant. The vaper has some regulating influence on this by varying the strength of drawing. Here all elements interact. The details differ depending on the type of atomizer. I try to describe the action inside a currently typical bottom coil tank atomizer.
The low pressure created by the vaper drawing makes a stream of external air flow over the coil. It also sucks a bit of liquids from the tank into the wick. And a lowered pressure slightly reduces the evaporation temperature of the liquid.
The coil evaporates the liquid touching it. The vapor expands abruptly, taking some droplets of liquid with it.There must always be enough liquid at the coil to absorb the energy.
This vapor/mist gets carried away by the passing air stream thus cooling off. The air flow also get heated a bit thereby cooling the coil.
The drying wick and the low pressure transport more cool liquid to the coil.
Influence of some variables:
Energy: The amount of energy is directly proportional to the amount of liquid that may be vaporized. All energy that isn’t absorbed by evaporating liquid or heating the airstream will increase the temperature of the coil.
Strength of drawing: The total volume is not important. The important factor is volume per time unit. Drawing harder
- increases the air stream.
- more cool air absorbs energy from the coil: less energy remains to evaporate liquid.
- the lower pressure enables the liquid to evaporate at a lower temperature.
- the density of the vapor is reciprocal to the amount of air.
What could go wrong?
Swamping: When there is way too much liquid, the coil acts like an immersion heater. Vapor bubbles develop directly at the coil, but get absorbed by the surrounding liquid, before much can escape. Thus slowly heating all of the liquid. Only when the liquid gets nears the boiling point more vapor can escape.
Stalling: When the air stream is too weak to remove the vapor from the coil, this vapor hinders cooler liquid from reaching the coil. Since the energy from the coil can’t be absorbed fast enough by the already hot vapor, it heats the coil. The increased pressure also hinders the flow of liquid from the tank and through the wick. When the vapor has dissipated enough, liquid will meet the coil again. This time the coil is way to hot -> Dry puff.
Dry puff: When not enough liquid gets to the coil to absorb the energy, the coil heats up. This usually happens when too much power is applied and the wicking can’t transport the liquid fast enough. Or simply when the tank runs dry. First the vapor get uncomfortably hot and the taste changes to unpleasant. Then chemical reactions start and the taste turns excruciating. Far worse than smoking.
What is important?
When you want to test emissons, the only important part is the atomizer. The battery is not. You can use any calibrated laboratory power supply with a standard connector that mimics a regular battery. That way you can control this variable much more accurately than with a consumer product.
By now there are many 4th generation power supply devices that calculate the coil temperature and shut down the power when a set limit is exceeded. This avoids dry puff conditions. Of course this calculation depends on the coil material and the absolute numbers are not very reliable. This doesn’t matter in a consumer product, since the user regulates the limit with sensory feedback. Unfortunately this doesn’t work with the most common coil material: Kanthal. This alloy is designed to have a rather constant resistance over a wide range of temperatures.
For reliable measurement a similar process could be used. But it should be calibrated. Measure the temperature at the dry coil and the current resistance. This will give you an accurate curve. This could be problematic with already wicked cores, since you might burn the wicking and thus render the atomizer useless.
But even without calibration to calculate the real temperature, measuring the current resistance is useful. Inside the normal operation (power) range of the atomizer temperature and thus the resistance will plateau as a dynamic equilibrium is reached and all the extra energy is absorbed by the evaporation process. You need to apply enough power to reach this. Below this level the atomizer will be not produce much vapor (swamping). If you apply too much power the temperature will rise again and you are entering the dreaded dry puff zone. Anything you measure outside this range may be interesting but not applicable to realistic condition.