Pens with pressurised refills are often called 'space pens', but that makes them sound like impractical novelties, of no real use to most of us. Actually, they make great pens for those who need to take notes when out and about, as they can write on damp paper, and continue to work in very cold conditions that can cause trouble for most pens. They can work when used tip-up, too, so they can continue to write when the only flat surface you have available is a wall.
Many of the situations where they'd be perfect are also situations where you wouldn't want to take an expensive pen, and most 'space pens' cost more than you'd want to risk losing. Fortunately, Uni-ball have a fantastic alternative - the PowerTank. It's pressurised, with all the advantages of other pressurised pens, but is also cheap enough that you don't have to worry too much about losing it. They're practical to use, too, with a nice rubber grip and retractable tip making them easier to use with gloves, or just very cold hands!
If you don't lose your PowerTank, they are refillable. And if that wasn't enough, with our current promotion, you get a free refill for every PowerTank pen you buy - making them even cheaper to run, and adding a bit of incentive to hang on to the pen!
How Does It Work?
The PowerTank works in the same way as other pressurised pens and refills - this diagram from Spacetec shows the idea:
A small pocket of pressurised gas sits at the top end of the refill, and keeps the ink under pressure, pushing it from the back towards the tip. The ink is thixotropic, so it's qutie solid when it sits still, but becomes more fluid when it's moved. Left alone, it's sold enough at the tip that it won't leak out. As soon as a little force is applied to the ball, it becomes more liquid, and the pressure from behind it pushes ink out around the ball.