If you are screwing the transmitter directly into a first stage HP port (or a HP stand off adapter or HP pivot swivel that are simply extending the port) then the Apeks, Mares, and Shearwater transmitters do not require an air-spool. However, the Garmin, Halcyon, and Suunto transmitters require inserting the flow-restrictor that was included with the transmitter and then screwing the assembly in to the HP port (or standoff, or pivot swivel.) Finally, snug up the transmitter with a thin wrench, but do not overtighten... the O-ring at the base of the threads on the transmitter is what makes the seal.

If the transmitter will be attached to the swivel end of a high-pressure hose then you will need use an air-spool. The Garmin and Sunnto transmitters require special unique air-spools that include flow restrictors (different from their flow restrictors used with first stage ports) that must be used in place of a standard air spool. The Apeks, Mares, Halcyon, and Shearwater transmitters do not include an air-spool and you will need to supply a standard air-spool you purchased separately with the HP hose. In this case the standard air-spool may be inserted into the transmitter and then screwed in to the hose swivel and snugged up with a wrench. Take care not to nick the o-rings during assembly; a few people also prefer to add a very small amount of oxygen-compatible lubricant to the o-rings. Regardless, when the transmitter is snugged up to the hose swivel, the transmitter may (or may not) turn freely (or at all) on the end of the hose.