Calculating Gas Density

[Capt Jim Wyatt]

MultiDeco has gas density calculations as a tool with 2 different temperature settings

When you look at the two different temperatures does the density vary significantly?

 

[Deep Wreck Diver]

I'd say that's significant.

Of course I have no knowledge of the maths that is being used to generate these results

 

[lostsheep]

Of course I have no knowledge of the maths that is being used to generate these results


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I suspect they are using the ideal gas law with:

[(PxV) / T ] being constant. T being in Kelvin (or Rankine)

 

[Capt Jim Wyatt]

Without any temperature adjustments I get 4.23 g/L.

I guess this is another example of things we do and believe that aren't always exactly accurate such as with OTUs and CNS%. But it is a guideline that is better than nothing.

 

[Deep Wreck Diver]

Absolutely, lots of people have dived with GD well above the recommended levels and survived. We all now understand that isn't a best practice though!

 

[Mobulai]

When you look at the two different temperatures does the density vary significantly?

Well it depends on the need to be exact to the nitty gritty and if there’s a need to account for extra adiabatic cooling

If I wanna think for example the Turbulence and need for a ReNr the Air/Gas density through a second stage in an OC dive, diving 21degC water/ambient might mean the air coming out is at 10-0 C so I’d lean towards a higher gas density; if I’m thinking about a warm humid gas in a rebreather loop I’d leave it at the lower destiny (or not concern myself much with it)
For these kind of very special cases the first online calculator I find generally just does the trick

Gas Density Calculator

This gas density calculator helps you find the type of gas based on the mass and volume entered.

www.omnicalculator.com

This one needs molecular mass but a quick search also helps out with that

 

[MultiDeco]

The difference in results come from that most of the published information, is done with a gas temperature of 0° C (Standard Temp and pressure). This includes the DAN web page, BSAC tables, and various published dive regulator studies. But humans don't breath / exhale gas at 0° C. Our airway rapidly heats the incoming air. So that testing and study information has somewhat missed the point of the diver's situation.

For our programs we offer both calculations at both 0° C, and a temperature of 20° C which is closer to a normal diver conditions. Further the calculations are done using gasses and actual water pressure / density values (ATA), and not diver rounding approximations.

We use the values of density from here:
Density of Gases Data

The programs MOD screen has both density values shown to see the differences.

 

[ginti]

I'd say that's significant.

Of course I have no knowledge of the maths that is being used to generate these results


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It's less than 10%. There are also a huge amount of other factors that play a role (precision of measurement instruments for example), but overall I would consider that not significant for the purpose of estimating WOB

PS the point is that usually in scuba we uses simplified formulas like Boyle's one. In reality, pressure P, volume V and temperature T are linked as P*V=konst*T, where the constant depends on several factors (number of moles considered, size of the gas molecules, etc. - if I remember correctly from high school ) ) - but it's an overkill for rough estimations. T is expressed in Kelvin and a variation of 30°K (equivalent to 30°C) only leads to a roughly 10% variation, hence for quick estimates I would be comfortable in neglecting it

Nerd mode off - double check what I say, not a thermodynamic expert at all here

 

[Taldan]

Isn't temperature a factor in the equation?

Insignificant.

I was curious how much of a difference temperature could make, so I calculated it using Gay-Lussac's Law. With a lower-bound of 33F (open circuit ice diving), and an upper-bound of 98F (body temp), we get a density variance of ~13%. A more realistic upper bound of 80-85F gives us a variance of ~10%

Larger than I expected it to be, but still insignificant

 

[bvanant]

The ideal gas low works (sort of) but its assumptions fail at higher pressure, but probably not enough to make a significant WOB difference. To be careful (like for a graduate thermodynamics of gases course) you can use the Van der Waals equations (PV is always larger than the ideal gas law predicts). As for temp, a 10% increase in T (K) means a change from 293 K to 322 K (20C to 50C)
Bill