Found the post. Note my calculations assumed a LOT more diving per year, but the math is easy to change to plug in your own numbers:
How much your own tank costs or saves depends entirely on how much you dive. If you are a traveling vacation diver diving once or twice a year, it makes little sense to buy a tank. If you're going to be diving locally, regularly, however, there are true savings in owning your own tank.
here's some basic math.
Assumptions:
tank - $120 (Al.80)
fill card - $4/fill
rental - $8/tank
Number of dives:
2x / week average = 104 dives per year
1x / week average = 52 dives per year
Cost to rent 104 tanks/year: $832
Cost to fill your own tank 104 times/year: $416
Cost to rent 52/year: $416
Cost to fill your own tank 52/year: $208
Cost of owning a tank, without dives:
$120 for tank
$10 Annual Vis Inspection
$5 Hydro inspection (every 5 years, $25)
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$135 for the first year, $15/year after that.
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Total cost of 104 dives per year in a rented tank:
$832 (104 x $8 = $832)
Total cost of 104 dives (and fills) per year in your own tank:
$551 (104 x $4 = $416 + $135 = $551)
Total cost of 104 dives (and fills) after first year to fill your own tank:
$431 (104 x $4 = $416 + $15 = $431)
So at around 100 dives per year, owning your own tank saves you $281 the first year, and $401/yr for the rest of the life of the tank.
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Total cost of 52 dives per year in a rented tank:
$416 (52 x $8)
Total cost of 52 dives (and fills) per year in your own tank:
$343 (52 x $4 = 208 + $135 = $343)
Total cost of 52 dives (and fills) after first year to fill your own tank:
$223 (52 x $4 = 208 + $15 = $223)
So at around 50 dives per year, owning your own tank saves you $73 the first year, and $193/yr for the rest of the life of the tank.
Clearly, the numbers balance differently for steel tanks, but they bring other advantages for some divers that can't necessarily be quantified the same manner. And at the 100 dive/year range, even steel prices can be compensated for... especially if you buy used tanks and don't pay full retail.
But for someone diving enough, the financial advantages are all in favor of owning your own tanks.
The added convenience, for most people: Priceless.
Slam dunk, if you ask me...