I've written an article on the more common
juice delivery
devices in my blog for beginners. I describe each (including the cartomizer and cartotanks) and provide a video on how to fill them. You can find that here:
http://www.e-cigarette-forum.com/forum/blogs/baditude/3230-1-proper-terminology-carto-tank-what-primer-juice-delivery-attachments.html
Other articles in the blog that might be useful to you can be found here:
E-Cigarette Forum - Baditude - Blogs
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To answer your specific questions, because the cartomizer is a metal tube it is nearly impossible to see how much juice is in them at any given point. People who use them learn from experience when the cartomizer needs to be refilled - the flavor will suddenly begin to taste burnt.
Do not continue vaping once this happens, as the filler material can actually burn. Once this happens the cartomizer is ruined, and you will never be able to remove the burned taste. The carto will need to be discarded.
Most people will "top off" their cartos frequently to avoid the above from happening. Adding several drops or more until the top filler looks like a slushy drink does the trick. Once you take a few vapes on a full carto, the top filler will always "look dry". This is normal and not necessarily an indication that the entire carto is dry. As long as the bottom of the carto has juice (where the heating coil is) there is little danger of the carto becoming burnt.
A cartomizer can be refilled multiple times. Once the flavor begins to taste poor or the draw becomes stiff, it is time to replace the carto with a new one within the tank.
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Many people who have used cartomizers eventually move on to using
cartotanks. This allows much larger capacity of juice to be used than a cartomizer used alone. It also allows you to view the level of juice in the tank as these are generally clear to be able to know when more juice needs to be added. It's also believed that using a carto in a tank prolongs its usefulness.
Cartotanks can be refilled multiple times, also. Although not necessary, many people top off the carto when refilling their tanks. And as when used solo, when the carto itself begins to taste poor or have a stiff pull, a new pre-punched, prefilled carto needs to replace the old one.
Above is a cut away of a
"punched cartomizer".
(click to enlarge) This simply indicates the metal casing has a hole or holes in it to allow juice from a tank to enter the cartomizer with each pull during vaping. The arrows in the diagram designate the flow of either juice or air within the carto.
Tank replacement cartos have laser-drilled holes, ready to be filled and installed in a cartotank. These are also called "pre-punched". Regular unpunched cartos can also be used in a tank, but the user must use a carto punch tool to make the holes themselves.
laser-drilled "pre-punched" cartomizer.
tool or hand-punched cartomizer.
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Cartomizers can be single (one heating coil) or dual-coiled (two heating coils). I generally advise people begin with single coils.
They also come in different resistance, or ohms. Low resistance (1.5 - 2.0 ohm) will give a warmer vape, but may also cause some delicate flavors to taste burnt. Standard resistance (2.1 - 2.5 ohm) will provide a cooler vape. Higher resistance cartos are also available, but they are for variable voltage/variable wattage battery devices.
