Tech gas blender

Trimix - Heliox - Nitrox

Tools

Gas weight

 

This option allows to calculate the actual weight of the content of the scuba tanks, according to the internal volume of the tank, the pressure, the temperature of the gas when you read the pressure, the molar mass and the density of the gas.

The field “Total weight” considers the addition of the checked tanks only.

Best mix

 

Typical basic tool. If you press “Depth range”, the calculated best mix will be sent to the tool “END and PO2”

END & PO2

 

A typical utility, with practical approach: set the END and both PO2 minimum and maximum, enter the gas and the range of depth to check: press Calc and look at the grid …

What if analysis

 

Much more than a “Top off” option, this section is dedicated to the What if Analysis: enter a sequence of gas filling (up to three gases on top of the current mix) and the program will calculate the resulting final mixture, according to Ideal and Real Gas.

 

It is a useful teaching tool: analysis of  mistakes, temperature effects, Real Gas vs Ideal etc.

 

Example: these are the results of the following Heliox 50 filling (isotherm 20 °C):

 

1. empty bottle (residual Heliox 50, zero bar gauge)

2. filling to 100 bar g. with Oxygen

3. filling to 200 bar g. with Helium

 

The thermodynamics deviation from the Ideal Gas are correctly managed by the software when set in "Real Gas" mode

 

Pressure and temperature

 

This tool allows to calculate one unknown factor out of four: pressure and temperature, in hot and ambient status.

 

For example, if you measure the drop in pressure from hot to cool, assuming you allowed the mix to cool to ambient temperature, the program will calculate the temperature of the gas when it was in hot status.

 

Since it is unrealistic to measure the internal temperature of the gas , this tool has questionable practical relevance, but it is useful to understand the possible origin of mistakes due to temperature management.

 

Try it with the "What if Analysis" to become aware of the importance of this issue in gas blending.

 

 

Gas prices – nominal vs actual

 

In this section you can check the difference between the nominal and the actual contents of the storage cylinders.

 

This difference affects the actual cost;  for instance, in Italy the invoice cost refers to cubic meters according to the Ideal Gas. It is a simple calculation, volume by pressure: 40 liters at 200 bar is 8 cubic meters of gas.

 

In the USA, in big industries,  Helium is shipped as compressed gas, but the equivalent uncompressed volume at the base conditions of 14.7 psia and 70° F is used for accounting and billing purposes. You can find the used equation in these calculations (for Helium only) under Tools, Graphs, EOS comparison, US Bureau of Mines.

 

Example:

 

Invoice price - the calculation is performed according to the Ideal Gas assumption.  

Helium invoice 15 euros/m3

Supply tank with internal volume 40 Liters at 200 bar = 8 cubic meters = 120 euros.

Actual cost  – the calculation is performed according to the gas compressibility:  Helium at 200 bar and 20 °C = 7.5308184 mol/L; with supply tank internal volume 40 Liters = 301.232736 mol. At ambient pressure (1.01325 bar and 20 °C) this amount of gas is 7,249.52 of uncompressed Liters of Helium instead of nominal 8,000.

This means 120 euros/7.25 = 16.55 euros/uncompressed cubic meter  (+10.35 %).

The opposite is  for  Oxygen: at 200 bar gauge the cylinder contains 5% more than the nominal calculation.

If you want to take into consideration these cost impacts, enter the new numbers in "Settings" - "Gas Prices".

 

Rebreather utilities (CCR)

 

The goal is to determine the breathed gas in a Closed Circuit Rebreather.

 

The CCR maintains a constant Oxygen partial pressure, with the fraction fluctuating as depth changes.

 

Given a diluent,  the CCR will keep the PO2 constant in the loop whatever the depth; the Helium and Nitrogen fractions depend on the diluent, depth and Oxygen PO2 and hence, Oxygen fraction.

 

In this section you can check the composition of the breathed gas in each portion of the dive.

 

These tools are useful to become aware of the chancing loop gas as the depth changes; this also allows you to evaluate the open circuit bailout strategy.

Gas Loop Graph: the graph shows the fraction of the gas in the loop of a closed circuit rebreather, assuming that the Oxygen fraction is exactly reproducing the Oxygen partial pressure you set as SetPoint and according to the set diluent gas.

 

Gas Loop Data: this is similar to the topic above, with numbers instead of graphs.

In this window you can also change the SetPoint during the dive.

This feature shows the breathed mixture for each portion of the dive, so you can check your open circuit bailout strategy accordingly.

Real Gas - EOS Comparison

 

In this section you can see the functions results of some EOS (Equation of State), including the Tech Gas Blender equation.

A picture is worth a thousand words !

 

Z Factors: the graph shows the compressibility factors of each pure gas and mixtures, here you can also see the interaction effects between the different gases.

Try to select three gases:

1. Helium

2. Nitrogen

3. “Custom gas” : 50 % Helium and 50 % Nitrogen; although this is not a breathable mix, it is interesting as a case study: the two inert gases together.

The line of the mixture is outside the two curves of the single gas instead of the internal theoretical weighted average calculated with  Kay's rule.

Here you can see the origin of mistakes utilizing the wrong mixing rule in gas blending, in particular with high fractions of Helium and high pressures.

 

EOS Comparison: the most important Equations of State compared. (The equation "US Bureau of Mines" is available for Helium only).

 
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