The original Apollo Bio-filter was intended to be fitted between a regulator first stage and the medium-pressure hose of a second stage. The idea was to filter out harmful contaminants that might be in the air compressed into a diver's tank. My reservations included the fact that the filter needed moistening. I also thought that the chances of unwanted bacteria growing on a moist pad in a tropical climate might outweigh the advantages of the micro-filter. It also seemed that the unit would provide a too-convenient handle for the ill-informed to use when lifting a tank. You can still buy the Bio-filter, but the newer Apollo Bio-Micro suffers from none of these defects. It uses an electrostatic filter that functions when dry and is said to be good for around 50 hours of use. It also has an activated carbon layer to filter out unwanted flavours, as one might get with an 'oily' fill. The new unit is only about the size of an integrated-gas computer's transmitter, and people are naturally wary about using anything of that sort as a tank-handle. Not only that, but it incorporates an impact-absorbing mechanism at the joint with the first-stage port, so it should not come to too much harm if a tank falls over. The unit splits in two, a plastic wrench (provided) allowing you to unscrew one half from the other. There is also a hexagonal wrench to ensure a snug fit into the regulator port, via the inside of the filter-holder. The two-part filter is contained in a little plastic cartridge, and a spare is included with the initial purchase. We do see questions posted on Internet diving forums regarding old air, along the lines of: 'I had my tank filled over a year ago. Will the air have gone bad?' Most of the air in our atmosphere is very old - millions of years old, in fact. It doesn't go off, but it can get contaminated. A properly run compressor should deliver clean, dry air to the tank being filled, especially important with nitrox. But this is not always the case. Should any moisture pass into a steel tank, for instance, it might cause it to rust inside, and inhaling fine particles of rust will do you no good at all. It's the same with any pollen or other dust. If you have reason to suspect the quality of the gas on offer, yet find yourself at a remote destination with little choice, one of these little filters could be just the thing you need. The Apollo Bio-Micro filter is available to fit either standard 3/8in ports or larger-bore 1/2in ones. It costs ?50.