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Scubaroo once bubbled...
I dive the Vytec with AI transmitter, and keep a brass SPG on the second HP port. If the battery in your transmitter ever dies, you still have analog SPG, and more likely, if your computer loses the AI signal, you still have the backup SPG. In a DIR/hogarthian rig, the SPG is on a 24" hose clipped off to your left side on a waist D-ring, and you don't even notice it during the dive.