I always grumble a little bit when I have to use a pen that uses cartridge converters to handle bottled ink. The problem is that the capacity of these converters is so small. They are a pathetic bit of technology, when one is used to piston-filler pens. I wish someone -- perhaps Nathan Tardif at Noodlers -- would make refillable, sealable cartridges for pens that take standard, international-cartridges. (Uh... and also for Platinum, Parker and Pilot format cartridges.)
What I'd want is something like Tardif's "308" refillable cartridge that Noodler's provides for its Ahab and Neponset pens, but in an international-cartridge format. That is,
- made of strong, solid clear plastic;
- threaded for a cap that can securely seal a full cartridge when it is not being used inside a pen; and
- available in a range of lengths from very short to very long, in increments of 1 mm.
Such a cartridge would roughly get you double the capacity of standard piston cartridge converters, because all of the pen's barrel length would be used to store ink; none would be needed for the piston mechanism. You could buy one in a size that would use as much barrel length as your particular pen provided -- short for a Pilot Elite 95s, long for a Sailor King of Pen. (This is especially critical for short pens -- they desperately need the extra capacity.)
But why buy just one? Buy five, fill them with a syringe or an eye-dropper in your office, and carry them around in a screw-top plastic cylinder for safety. Now your ink problem is completely solved, when you are out & about.
For extra points, line the interior of the cartridge with a thin layer of teflon (better yet, aluminium magnesium boride). Now you will have no capillary forces keeping ink stuck at the rear of the pen; it will all flow downhill into the feed. You can afford the extra expense of doing so, because these cartridges aren't disposable. So this will be a much better ink-reservoir system than either standard converters or cartridges. A thin layer of Teflon will suffice, because the interior of the cartridge is not a high-wear surface -- and a thin layer is good, because you want the walls of the cartridge to be as transparent or translucent as possible, so the ink level of the cartridge can be visually seen.
(In a similar vein, I wish that makers of piston-fillers, like Pelikan, Montblanc, TWSBI and Sailor, would teflon-line the reservoirs of their pens. Not only would it improve ink flow, but it would improve the efficacy of flushing the pen with water to clean it, e.g., when changing inks.)
I would pay $20/cartridge for such a thing. And I'd buy a bunch of them. They'd completely alter the useability of cartridge/converter pens for me.
P.S. Alternatively, how about a piston-filler converter with a detachable pushrod/plunger? To refill the pen, you screw the rod into the back of the piston, do the refill, unscrew and detach the rod, then recap the pen. Make the piston long for stability in the converter barrel, and have the rod screw into the back of the longer piston. Simple and not particularly high-tech, but you ought to get close to a 2x multiplier in filler capacity over the current designs.