In addition to BJD69's points, the HH has a completely different layout.
Setpoint Controller:
- APD: two wired together
- HH: one setpoint controller, one separate pO2 monitor
-- left setpoint and dive computer
-- right handset pO2 readout for all three cells
Batteries:
- APD: one compartment for both batteries in loop, depending on age unsealed at ambient pressure (and shared in Vision)
- HH: separate batteries sealed at 1 ata outside the loop inside each handset
Alarm:
- APD: buzzer, implodes around 180 msw flooding the loop
- HH: vibrating alarm (DIVA)
Cables:
- APD: wires at ambient pressure inside hoses (Classic, anyway)
- HH: sealed waterblock cables
Displays:
- APD: Either twin handsets, no HUD (Classic) or single handset and HUD (Vision)
- HH: Twin Handsets and HUD with DIVA
Deco Software:
- APD: Either not available (Classic) or optional (Vision)
- HH: Standard (nitrox, trimix used to be but may change with O2ptima deliveries)
Price:
- APD: Upgrade to Vision incl. deco around $4,000 ($4,500 with scrubber gauge)
- HH: $3,000
Original Head (and cannister if scrubber gauge is ordered):
- APD: wants it back (they add the rest in new parts and resell the CCR at a lower price)
- HH: you keep it and thus have a backup