Effect of gas density while deep diving

[scubadada]

131 feet on air has a gas density of 6g/L. It is also 6g/L at 28%

 

[wetb4igetinthewater]

131 feet on air has a gas density of 6g/L.

131 feet = 39.9 meters. At an ATA of 4.9, air with a density at 1 ATA of 1.293 gives you 6.5 g/L, beyond the absolute max recommended of 6.2 (http://alertdiver.com/Gas-Density).

This whole discussion is making me rethink how soon I use helium, as I figure continuing a normalization of deviance is going to catch up to me. Spending a bit on helium is cheap when considering I have a wife and daughter now.

 

[scubadada]

131 feet = 39.9 meters. At an ATA of 4.9, air with a density at 1 ATA of 1.293 gives you 6.5 g/L, beyond the absolute max recommended of 6.2 (http://alertdiver.com/Gas-Density).

This whole discussion is making me rethink how soon I use helium, as I figure continuing a normalization of deviance is going to catch up to me. Spending a bit on helium is cheap when considering I have a wife and daughter now.

No problem, MultiDeco gave me 6.0 for each

 

[wetb4igetinthewater]

No problem, MultiDeco gave me 6.0 for each
View attachment 547373View attachment 547374

They rounded down the gas density from 1.293 down to 1.2. For something like this, I would hope that they'd round up to 1.3 instead.

 

[scubadada]

They rounded down the gas density from 1.293 down to 1.2. For something like this, I would hope that they'd round up to 1.3 instead.

Tell it to Ross

 

[wetb4igetinthewater]

Tell it to Ross

Just did.

 

[jgttrey]

They rounded down the gas density from 1.293 down to 1.2. For something like this, I would hope that they'd round up to 1.3 instead.

That's nuts. Thanks for pointing out. It's got to be more than just rounding off the 1.293 for air to 1.2 because MultiDeco seems to get this wrong for other mixes, not just air.

For example, a 16/50 mix at 250' yields a gas density of 5.9 in MultiDeco but 6.37 using the gas density calculator spreadsheet.

I feel like I'm missing something here....

 

[wetb4igetinthewater]

That's nuts. Thanks for pointing out. It's got to be more than just rounding off the 1.293 for air to 1.2 because MultiDeco seems to get this wrong for other mixes, not just air.

For example, a 16/50 mix at 250' yields a gas density of 5.9 in MultiDeco but 6.37 using the gas density calculator spreadsheet.

I feel like I'm missing something here....

I sent a message with a link to this thread, so hopefully Ross will read this thread.

 

[scubadada]

Ross is banned from SB, not sure he still reads it

 

[wetb4igetinthewater]

Ross's response (he mentioned being banned, but was kind enough to respond):


The sample equation is off. 39.9m is not 4.9 ATA - that is divers
rounding at work. 39.9m of salt water, at sea level, is 4.96 ATA =
(39.9/10.078) + 1. This slight variation in ATA values used, is a
common error in "diver rounded" calculations.

Gas density is easily seen at sites like this:
Gases - Densities

The Alert Diver values agree with those above, but they list the STP
values at 0° C (32F). However, our program uses the 20°C values, which
are more realistic for human consumption. If there was a 30°C set of
values available, we would use those instead.

Air @ 20°C = 1.205 g/l/ATA
1.205 * 4.96 = 5.97 == 6.0.

The water vapor content and the gas temperature at the restriction
point, is debatable, and they both affect the calculation.

There is no rounding of data in our program, but different
interpretations of the problem and a gasses density gives a slightly
different answer.


Please post this in full.


Regards
--
Ross Hemingway
HHS Software
Kingston ON Canada
HHS Software Corp.