Scubapro MK21 First Stage RegulatorThe MK21 is SCUBAPRO's most compact balanced piston first stage. Smaller and lighter than the MK25, it uses a smaller piston but it delivers the same level of breathing performance, no matter how hard you're breathing, how deep you're diving or what your pressure gauge reads. The MK21 also provides exceptional resistance to freezing due to the thermal exchange produced by the external fins on its polished chrome brass body. The MK21's innovative