I see Ian's point here: it's not a depreciation / amortisation question, it's more like calculating the cost of a membership.
There's fixed points of cost; 2 x o2 cleans, 1 visual+o2, 1 hydro+o2. Taken over a *long* period, it actually doesn't make much difference.
Example cost of ownership over 6 years:
* If the cylinder is bought 'new' this is one full set of tests.
* If the cylinder is 1 year old it's still one full set of tests although you do have to do them 1 year earlier.
So there's a cost of money which you're spending a year earlier, but the interest on ~£50 in a year is bugger all.
Well 16 quid will go a long way to paying for your next 2k diving holidays ffs
http://www.infomine.com/investment/m.../aluminum/all/ ). Good luck picking a falling trend out of that and I also don't know where you got your 2012 exchange rate from ... Brexit has managed to make Us$ purchases more expensive though (by virtue of £ weakening). But the major cost of tank production is not raw materials (about ~£16 per Al80 using your figures) but rather the energy used to produce - which is why tanks are not made in the UK as our electricity is too costly.
As for 20%, 60% 33% - I don't know any tank brand following that model - or was this just another mum and pop witch hunt?
If I were to take Ian's side of the debate, then a better argument would be to say that a 20 year old cylinder with a new test is worth about £75, and so you've lost about £80 over 20 years, meaning that the depreciation is around £2 / year, and the 'lost' test is worth around £1. It's a little bit disingenuous though as most of that loss happens in the first few years....
PS - When someone joins Hellfins halfway through the year they only pay 6 months subs (both BSAC and Hellfins). I'm not sure I'd get away with charging people the full amount!
Also depreciation is accountant's pretend money - all that really counts is what you actually pay in the stuff you can exchange for beer so a short test is a real cost as you will have 50 fewer beer tokens sooner. BTW I knew an accountant type who ran a fully-fledged household budget on a PC that went down to individual cost centres like "dog" - he was never able to give me a reply as to what happened if the dog overspent one month...
Last edited by Energy58; 20-11-2017 at 10:01 AM.