The Dive Otter - Spare Air

A Spare Air™ is nothing more than a low cost pre-configured Pony Bottle, so we can use Rock Bottom Gas calculations to determine if the Spare Air™ is a viable tool for scuba diving.

To determine the gas we need for an emergency direct ascent we can use the CAT calculation for one person under duress:
(1.5 RMV * Average Depth * Time of Ascent) = Cuft needed

Which Pony Bottle Do I Want?

Tank/Bottle	Price	Capacity	Max CAT Depth
Spare Air™ 300	$400	3 cuft	Unusable
Spare Air™ 600	$550	6 cuft	20ft
Standard AL13	$850	13 cuft	45ft
Standard AL19	$900	19 cuft	55ft
Standard AL30	$950	30 cuft	85ft
Traditional AL40	$1,000	40 cuft	100ft
Standard AL80	$1,000	80 cuft	>130ft

Max Time at Depth

But what about the idea that having a Spare Air™ could give you the minutes you need to reach a buddy, who presumably has more air. We can do that calculation using a variation of CAT as well:

cuft / (1.5 RMV * Depth) = Time

Depth	ata	DCR	M300 Time	M600 Time	AL19 Time	AL40 Time
20ft	1.6	2.4	75 sec	150 sec	8 min	16 min
30ft	1.9	2.9	63 sec	126 sec	6 min	13 min
40ft	2.2	3.3	55 sec	109 sec	5 min	12 min
50ft	2.5	3.8	48 sec	96 sec	5 min	10 min
60ft	2.8	4.2	43 sec	86 sec	4 min	9 min
70ft	3.1	4.7	39 sec	77 sec	4 min	8 min
80ft	3.4	5.1	35 sec	71 sec	3 min	8 min
90ft	3.7	5.6	32 sec	65 sec	3 min	7 min
100ft	4.0	6.0	30 sec	60 sec	3 min	6 min

* DCR = Depth Consumption Rate : RMV at depth

Final Opinion

I just do not see how a tank smaller than 19cuft is viable as an emergency air supply. And given that Spare Air™'s largest tank they sell is 6 cuft...I can not in good conscious recommend one.