Fwd: Re: [PATCH] Fix up SAC calculations for ATM/bar confusion

Dirk Hohndel dirk at hohndel.org
Mon Feb 25 17:10:47 PST 2013


Jukka <jukka.lind at kolumbus.fi> writes:

> On 25.02.2013 23:04, Dirk Hohndel wrote:
>> Chris Lewis <chrislewis915 at gmail.com> writes:
>>
>>>> As an example one real dive: Double7/300, pressure from 260 to 10 bar.
>>>> Subsurface says I used 3500 litres but Suunto Dive Manager 3176,4 litres.
>>>> It's a 9 % error.
>> But that assumes that the Suunto DM number is correct.
>> And I have absolutely no reason to believe that it is any better than
>> ours...
> Actually Suunto's model is far from perfect ! After each full 50 bar 
> theres a flat. The bigger pressure, clearer flat. See 300 and 310 bar.
>
> 140    139.581
> 150    149.551
> 160    155.642
> 170    165.37
> 180    175.097
> 190    184.825
> 200    194.553
> 210    198.113
> 220    207.547
> 230    216.981
> 240    226.415
> 250    235.849
> 260    237.01
> 270    246.126
> 280    255.242
> 290    264.357
> 300    273.473
> 310    273.369
> 320    282.187
> 330    291.005
> 340    299.824
> 350    308.642
> This information collected via testing DM3.1.0 each point at a time.

Just for kicks and giggles, here is the comparison with our numbers -
simply calculating gas_volume(1_L_Tank, pressure) for the given values.
Second column is Suunto, third colum is Linus' function

140  139.581  138.203
150  149.551  148.075
160  155.642  157.946
170  165.37   167.818
180  175.097  177.690
190  184.825  187.561
200  194.553  197.433
210  198.113  204.068
220  207.547  210.777
230  216.981  217.562
240  226.415  224.422
250  235.849  231.356
260  237.01   238.366
270  246.126  245.451
280  255.242  252.611
290  264.357  259.846
300  273.473  267.155
310  273.369  274.540
320  282.187  282.000
330  291.005  289.535
340  299.824  297.145
350  308.642  304.830

>> So let's assume we take whatever temperature reading we have at the time
>> during the dive into account, plus the non-linearity of compression. And
>> the mixture of gas in the tank. Is that all it takes to get the correct
>> volume of gas inside the tank?
> Even a simple correction factor would give a better result.

I certainly think that this is way better than what we had...

/D


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