Let say your using an AL80 filled to 3000psi (full)
Lets assume you dive to 60ft and this dive lasts you ~45 minutes when your tank hits 500psi
You used up about 66.67 cf of air in 45 min or 1.48cf/minute!
A spare air is 1.7cf and if you need to use it your likely stressed.
The point is I wouldn't expect that 1.7 cf spare air to last you more than 1 minute of stressed breathing.
Now factor in a deeper depth and the time your spare air will last will go down even further, If that doesn't convince her, its a waste of $200, not sure anything will..Get out the credit card
