I never quite experimenting and designing. Over my time spent modding carts and such I have come to a few definitive facts regarding cart mods. They all have drawbacks, you just need to know which fits you best.
I am going to start with a basic principal of fluidics, surface tension.
Surface tension and vacuum are primary reasons you can put a straw in a liquid, cover the exposed end of the straw, and lift the straw out of the liquid while keeping it full. When you remove your finger, you lose vacuum, thus the liquid falls back out of the straw.
Creating a sealing area of the liquid within a cart is key to maximize fluid delivery. We use absorptive materials within a cart to hold fluid. The more absorptive they are, typically, they have a higher retention coefficient. This means the more the soak in, the less they will let go.
To make a good cart, we have to look at surface tension. Inserting any of these materials does two things. They help support the weight of the fluid, and they reduce surface area exposed which strengthens retention.
Lets take a 2 liter bottle, fill it with water, then submerse it in water. Flip it over (top down) and lift it out. The bottle stays full until the top comes out of the water, it then chugs and comes out. As water comes out, air needs to enter to negate the vacuum.
A straw will hold the water, but the bottle will not. This is due to the ratio of weight to surface area of exposed liquid which effects surface tension.
Due to this principal, we only need to increase surface tension so a full cart (liquid, no filler) so it does not dump the fluid freely. A thin piece of foam material at the opening of the cart will help hold weight of the fluid and reduce surface volume, thus increasing surface tension. The less absorptive the material, the more it will release. Under the best circumstances, this is what we want.
When using the blue foam plug method (see ehiem filter mod) and no spring (just a plug), you can fill your cart with juice and cap it with this plug. Turning it over results in no loss of fluid.
When you push this cart onto the atomizer, the bridge enters the cart. The bridge is a low absorbing material, much life the blue foam, the blue foam is just more conforming to shape. The blue foam and bridge effectively become one unit, the liquid disperses through it until an equilibrium in saturation is met. Since a solid area of fluid across the mouth of the cart has never been broken (fluid in the cart replenishes the fluid which move to absorb into the bridge) a vacuum within the cart is kept. The surface tension of the fluid allows the bridging material (metal mesh and foam) to fully saturate without leaking (with assistance of the vacuum).
When fluid consumption gets to the point where this continuous fluid area across the mouth of the cart is broken you will inhale straight liquid fumes for a hit or two. This is easily identified by a hit which is very strong in flavor and kind of hits you in the sinus area. The vacuum in the cart got stronger and stronger. Air could not replace the fluid draining from the cart, but fumes from the liquid were leaching into that vacuum void space. This is what you inhale as soon as liquid continuity is lost across the cart mouth. It may reseal and break again for a hit or two, this is why you may get a few hits like this. When you first notice this happening, tilt the PV up and blow into it lightly. This will help evacuate fumes from the cart.
The PTB (lipton pyramid tea bag) mod works on this same principal, but is less reliable because the blue foam is more willing to shape fit the opening across the cart when the bridge is inserted. Cutting the PTB material too short, you are just extending the size of the bridge. Depending on how saturated the metal mesh bridge is upon insertion, it may flood. Cutting the PTB material too long and it wedges between the metal bridge and cart wall, filling the void and working like the blue foam plug. Even when the PTB work like the blue foam plug, it is displacing more space in the cart which could be used by fluid. Inserting a straw is very similar when doing it with PTB and the blue foam. Too long it seals, but slightly reduces surface tension allowing easier flooding, too short with a good seal it does nothing, too short with a non-existent seal it will contribute to flooding.
The straw mod will only assist with a high absorbing material which does not rely as much on this seal, but distribution properties of the material itself. Materials like wool, or polyfill, are more effective when air can displace behind it and assist in relieving any potential vacuum. These materials have a much higher absorptive coefficient than the metal mesh bridge, which means they will always retain higher concentrations of liquids than the mesh bridge. This is why you have to top it off repeatedly.
The blue foam plug mod works the most efficient. Variations have drawbacks and advantages. The spring (horizontal or vertical) increases pressure placed on the metal mesh bridge. This bridge inserts much further into the cart than most may realize. Increasing pressure against this bridge will compress the openings and decrease a usable porous surface area of the bridge. Although, the spring holds the blue foam higher in the cart and allows for more liquid capacity (this is offset by the spring itself though to some degree).
The blue foam plug has some disadvantages also. It increases nicotine delivery, increases throat hit, and effects vapor (sometimes more, sometimes less, depending on the liquid itself) at a cost. The blue foam plug mod will use a significant amount more fluid. The mesh bridge remains at 100% saturation while fluid remains in the cart cavity (same with the wicking material in some 510 atomizers). This allows the coil to burn as much fluid as it is capable while actuated (turned on). This does allow for shorter hits and longer battery life also though. If you vape because you just love to vape, you will use more juice (although delivery of nic is higher and you can drop to lower concentrations). If you vape for the pure nicotine, this will due it more efficiently and you will most likely vape less.
The best tips on using the blue foam plug is:
When inserting the full cart, push it straight on.
Do NOT turn the cart once it is pushed on. Doing so will twist the foam/filter material and break that liquid seal causing the entire contents of the cart to dump into the atomizer.
When you get the first fume hit, refill your cart. Do not overfill, the blue foam material should not be completely saturated when you re-install the cart.
If you wait until the cart is dry, and the bridge is dry, a lot of liquid will be pulled from a fresh cart immediately to saturate the bridge. Also, running the coil dry will accelerate deposit of junk burnt to the coil (then you got nasty taste which will require cleaning).
The blue filter material I use is found in the aquarium section of most pet stores. It is made by Marineland. I use the filter sleeves size "U" for the H.O.T. magnum pump.
When cutting a plug, use the standard (stock thickness) depth and cut 1.5 times the width and length of the cart opening. Pinch it across the narrow section, insert one side, bow it over and insert second side, then push it down flush (the atomizer bridge will push it to its final resting position).
Now that I use more juice (and just ran out again!), I am constructing carts that hold more juice. Any 510 materials (batteries, atomizers, carts)and juice that can be contributed for experimentation purposes are welcome. I am currently working on a 510 which uses direct poly to the coil and a liquid reservoir, basically a KR808D cartomizer design with a more robust coil.
