Oceanic Atom 2.0 & Transmitter - O rings - BSAC Scuba Diving Forums
An O Ring, is an O Ring, is an O Ring. Nowadays no manufacturer of any device at all is going to have special size O Rings made (too expensive when Lsd are in question) and, as long as the thing's of the appropriate size, it really doesn't matter what it's made from, although the shore... If you really want replacements, take (or send) the current ones to your favourite specialist supplier of these things and they'll, ever so carefully, measure them and supply you with new ones.
The reason you are usually stuck with 15 or so is simple: it is increasingly difficult to faithfully receive a signal as the distance between the transmitter and receiver increases. A similar problem plagues transmission of signals along undersea