There are two sizes of cap, one for all Gaeltec transducers with multiple sensors and a smaller version suitable for single sensor transducers. Both are backward compatible with Gaeltec transducers manufactured since the year 2000.
The screw-on caps are used in conjunction with a red silicone rubber "O" ring. In the case of multi sensor transducers, the ring is fitted onto the transducer plug and for single sensor transducers, the ring is fitted into the cap itself.
With both types it is important to ensure that the red ring is properly in place prior to immersion. Replacement "O" rings can be obtained from Gaeltec if required.