Helpful Techie Stuff So as we lay awake at night contemplating the behaviours of varying tank sizes and shapes it may be helpful to consider a couple of known scientific aspects of liquids and gases.
First, gases (air included) are compressible and expandable. This includes air in vacuum ( low pressure) environments. In our case the air in our tanks is in a constant state of vacuum but the amount of vacuum (in/hg) varies slightly dependent upon whether a bubble of air has just risen to the top of the tank or not. Slightly more vacuum (causing less juice flow) if it has been a while since one has risen; slightly less vacuum (causing more juice flow) if one has just risen. The overall level of vacuum is controlled (regulated) by wicking density and juice viscosity (thickness). The vacuum applied to the top of the fluid in our tanks is the same in/hg per sq.in. of surface area regardless of diameter and has no greater or lesser effect on juice flow based on diameter of the tank.
Liquids are non compressible or expandable by positive or vacuum pressure changes. The downward force provided by liquids is a result of gravitational forces acting upon the mass of the fluid. Fluid in a column will provide a downward force proportional to it's mass and height. Not it's diameter. In the case of water: water in a column will provide a downward force of .43psi (pounds per square inch) per foot of height. It doesn't matter if the column is 1/2" diameter or 100 ft. diameter; it will remain the same.
So if diameter has no impact on juice flow based on physics then one may wonder why I included diameter in my earlier post where I stated that "I like tanks with a greater dia/height ratio? When we fill our tanks we stop at the point that the juice could run down the airtube. This leaves a small amount of air in the tank. That's a good thing because without some air up there above the fluid our tanks would stop flowing juice altogether. Why? Because without air there is no vacuum; just a solid column of non expandable fluid. An air bubble would never rise. In fact the more air we have in the tank vs. juice the more consistent vacuum we have because each air bubble that rises adds a smaller percentage of air to the overall air in the tank. This translates to more consistent juice flow. Because bdc clearos have shorter air tubes (taller wickheads) you will always have ample air above the juice after refill. Even more so with larger diameter tanks.
The following is my understanding of the dynamics controlling juice flow in tanks.
The low pressure air (vacuum) above the juice is regulated by a valve (the wick) and the viscosity (thickness) of the juice and remains fairly constant throughout the tank level. However, this low pressure air also acts an air spring and the spring tension (ability to resist expansion) of this air spring is controlled by the amount of air vs. juice in the tank. Greater volumes of air are easier to expand than smaller volumes. The level of vacuum AND this air spring tension is what dictates how much draw effort is required to pull juice from the tank. So as the ratio of air vs. juice increases so does our abilty to overcome the normalized vacuum by stretching the air spring with normal draw effort. This is why as tank juice level drops we can experience flooding at or near the bottom of the tank if not wicked heavy enough. As it is, this is the condition that I wick for, the bottom of the tank, not the top.
Summary of sorts:
The looser you wick; the lower the vacuum (because air can more easily migrate through the wick and into the tank) and allows more juice flow.
The thinner the juice; the lower the vacuum (because air can more easily migrate through the wick and into the tank) and allows more juice flow.
The more air vs. juice in the tank; the less vacuum change per risen air bubble occurs allowing for greater consistency in juice flow.
The more air vs. juice in the tank the easier it is to pull juice from the tank with a given draw effort because the "air spring" weakens.
Vacuum levels in the tank do not change much with changes in fluid levels of tank.
Wicking density should be adequate to stave off flooding at or near empty tank levels.
With a lesser air vs. juice condition of a full or near full tank you can experience dry hits or burny taste because the air spring is quite strong and resists our efforts to draw juice from the tank. A quick short reverse puff into the device will enable a bubble to rise into the tank and releave the high vacuum condition. Juice will then start flowing into the wick more easily. Once the tank reaches a lower level reverse puffs are usually not needed.
Hope this helps.
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