Gas density

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mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Gas density

Post by mobu »

Hi

How does OSTC calculate gas density for a given respired gas at a given depth?
What air and the individual components of gas mixes density values does the software use to calculate it?
Does it take temperature into account?

Thanks.
mobu

OSTC plus #16088
heinrichsweikamp
Posts: 4370
Joined: Sunday 13. May 2007, 18:07

Re: Gas density

Post by heinrichsweikamp »

Hi,

It's simple:

int_O_gas_density = (unsigned int)( GAS_DENSITY_He_FACTOR * ppHe + GAS_DENSITY_N2_FACTOR * ppN2 + GAS_DENSITY_O2_FACTOR * ppO2 );

We use the following numbers:

#define GAS_DENSITY_He_FACTOR 17.64 // in multiples of 0.01 grams per liter*bar
#define GAS_DENSITY_N2_FACTOR 123.46 // in multiples of 0.01 grams per liter*bar
#define GAS_DENSITY_O2_FACTOR 141.02 // in multiples of 0.01 grams per liter*bar

These are constants at physical standard conditions - temperature compensation does not make much sense in the expected temperature regions.

regards,
Matthias
mobu
Posts: 58
Joined: Tuesday 27. March 2018, 20:01

Re: Gas density

Post by mobu »

Ok.

My question comes from the following doubt.
Data that I use (there is a little difference in the numbers with yours):

- Standar conditions. 0º C and 101,325 kPa
He 0,179 g/L
N2 1,251 g/L
O2 1,428 g/L

- Normal conditions. 25º C and 101,325 kPa
He 0,166 g/L
N2 1,165 g/L
O2 1,331 g/L

Obviously, in general, in deep dives the water temperature is neither 0º nor 25º. In my case they are usually around 11º
But these data help me to see that there is a difference relevant enough for a gas to be or not within the recommended limits, as far as its density is concerned.

So, I understand, the density value that I get in the OSTC in the simulation or during the botton time is a higher value than the real one. But I have no way of knowing which is the real one.

Regards
mobu

OSTC plus #16088
Ralph
Posts: 708
Joined: Saturday 24. June 2017, 11:31

Re: Gas density

Post by Ralph »

Hi,

our values are based on an upcomming ISO standard for gas density considerations in diving. They are also based on 1.0000 bar to be able to directly multiply them with the ppN2, ... pressures. Water temperature is not the relevant factor, as the gas leaves the 1st stage at negative (°C) temperatures and then gets warmed up on its way down the trachea. On exhalation it will be much warmer as it has been heated up by the body heat, but the hack is to judge the work of breathing and here inhalation is the dominant factor. In SCR & CCR the gas will be warmer, reducing the density-induced work of breathing, but on the other hand the loop hoses, check valves and scrubber add flow resistance, probably sapping up the temp savings by far.

BR
Ralph
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