Q ?

How can I quickly adjust an only Oxygen fraction in a Trimix without worrying about the exact composition of Helium and Nitrogen?  
Can I do this with Tech Gas Blender ?

A

Yes, with the Advanced Top Off feature: if you want to reduce the fraction of Oxygen,  set Helium (or a mixture with a lower Oxygen fraction than the current mix) as added gas.  
If you want to increase the fraction, you will need to do the opposite: top-off with Oxygen or with a mix with a higher fraction of Oxygen.
Example: you have a 52/21 at 205 bar and you want to adjust the Oxygen: Gas Blender, Advanced Top-Off,  press step 1 bar with Air and ... at 220 bar you have an almost perfect 50/20.

Q ?

When I use the Continuous Flow method, do I have to use the Real Gas model to get the best accuracy ?

A

Not if the current mix has the same composition as the final one, otherwise, you do.

Q ?

Why doesn't the van der Waals  work ?
 

A

Because it is not an equation designed to handle an  Oxygen-Helium-Nitrogen mixture, especially at these pressures.
There are many other excellent equations, created for ethane, propane, liquefied petroleum gas and other nice products but not for technical diving ... and even these, although of great scientific value, are not suitable …

Q ?

What is the order of magnitude of the errors introduced using the Ideal Gas model ?

A

First of all, it depends on whether you are preparing Nitrox or Helium-based mixtures.
In the case of Nitrox, the deviation is not so high: with EAN 50 at 200 bar with an empty cylinder, you will have about 1.4 points of deviation.
It is different when using Helium: preparing a 10/70 with Ideal Gas, you will get a difference of almost three points on Helium (starting from an empty cylinder), almost four if Helium is the first gas sequence.
The biggest mistakes are made by applying Fudge Factors in conditions other than those in which they work (the concept of rule extrapolation) and managing the temperature in an inappropriate manner. 

Q ?

What is the order of magnitude of the deviations if you do not hold the temperature isotherm ?

A

By filling with progressive steps without interruption, estimating the drops in pressure when cold, you can easily introduce differences of 4-5 percentage points for some gases. 
You can see these effects with the feature "pressure and temperature" and "What If: Ideal Gas and Real Gases" in the Tools menu.

Q ?

Note the following example which is a real case: after having prepared a 40 CF 50/20 at 220 bar, Oxygen and Helium were added, then the mixture  was left to cool to room temperature.
Then the last step: Air was added up to 229 bar (measure hot) with a modest Bauer junior (6 cm/h).
Once the mix  reached the ambient temperature, the pressure  dropped to 207 bar, which means that the temperature of the mix at the time of measurement of 229 bar was about 54 ° C.

Estimating the actual pressure drop is difficult. You should, as with Fudge Factors, find the dimensions of the adjustments  and use them only under the same conditions: same mixture, same pressures, same compressor/booster etc..
Since this is virtually impossible, there are two alternatives: either you resign yourself to a much higher uncertainty which takes you further away  from the conditions tested, or if you pursue the maximum accuracy, use Tech Gas Blender in Real Gas mode, keeping the temperature isotherm.
If you then move “on the flight” from Helium to Air by simply switching a 3-way valve without purging the lines and the compressor/booster, then you accept to sacrifice accuracy in favor of practicality, introducing an additional uncertainty factor.
In these cases, if you have not tested the procedures under the same conditions, do not be surprised in finding 7/8 points of deviation from the desired composition.

Q ?

I understand that temperature is a crucial factor. Do I have to measure the temperature of the gas to set the isotherm?

A

No, there is no need to measure the temperature of the gas: if you prepare a procedure with an isotherm at 20 ° C and the mix is instead at 30 ° C, the difference is insignificant, not detectable with our instrumentation, provided that the procedures are being followed correctly.
Instead, what does have a strong impact on the final result is measuring the pressure of the mixture at different temperatures in the different steps, without giving it time to drop to room temperature.

Q

I understand that the errors in analyzing the initial mixtures are  more significant, the higher the pressure is. Helium, however, is expensive ... How frequently is it advisable to fully discharge the initial mix for security ? How often ?

A

It is impossible to give an exact answer.
Theoretically NEVER, if all the analyses are perfect.
EVERY TIME in case the analyses contain significant errors.
So, it depends on the accuracy in the analyzing process.
We must also bear in mind the risk of cumulative error and the assumption that the instruments commonly used in technical communities do not allow very high accuracies.
Completely draining the tank every three to four refills can be a reasonable trade-off, but it depends on the consideration just made.

Q ?

What is the mole ?

A

The Mole is a unit of measurement of the International System (SI) and it expresses an amount of a chemical substance, atoms or molecules, expressed in number of elementary units: 6,023 * 10 ^ 23
This number in scientific notation expresses a value of six hundred thousand billions of billions.
This should not complicate the view of things: just consider the moles as a certain amount of particles of Oxygen or Nitrogen or Helium, knowing that a 10/70 does not mean 10% of the pressure of Oxygen and so on, but 10% of particles of Oxygen, 70% of particles of Helium and 20% Nitrogen.
What you need to know is all here, the program does the rest, enabling you to measure the moles with the pressure.

Q ?

What is the symbol mol/Liter on the blending instruction screen ?

A

It is the concentration, i.e., how many moles of each gas will be found for each liter of internal volume of a cylinder, given the pressure and temperature set.
Also in this case, the use of this information is not to complicate things. The program has the opposite ambition; we have therefore chosen to show this info to highlight how the amounts of gas are not related with direct proportionality with the pressure

Q ?

What is the symbol gr/Liter?

A

It is the density, i.e. the mass of gas per liter of internal volume of the cylinder, given pressure and temperature indicated.
The molecular weight of Oxygen is 31.9988 grams, i.e. the mass of one mole of this gas corresponds to 31.998 gr.
The molecular weight of Helium is 4.0026 gr., while that of Nitrogen is 28.0134 grams.
The molecular weight is equal to the sum of the atomic weights (atomic mass) that constitute the molecule; O2 and N are diatomic, He is monoatomic.
This means that, in case of a Trimix 10/70 at 200 bar and 20 ° C, you read a tot. of 85.177 gr. / liter, so an 80 CF contains 11.1 x 85.177 = 945.46 grams of the mixture. You can check the weight of a set of cylinders configuration in Tools, Gas Weights.

Q ?

What is the accuracy of the software in the case of mixtures?

A

Because of the countless combinations of molar composition, pressure and temperature, it is practically impossible to say with certainty what the degree of accuracy is, but you can get an idea of the magnitude.
First of all, as a gas predominates over the others, the effects of the molecular forces decrease, so you tend to get closer to the  accuracy of the data of pure gases reported in the various papers published; the uncertainties in density are: Helium 0.1% - Oxygen 0.1% -  Nitrogen 0.02 %.
To get an idea, however, we can refer to the report of investigation "Equation of State for Helium-Nitrogen Mixtures from 133.15 ° to 748.15 ° K with Pressures to 300 atmospheres" . In this text,  a maximum deviation equation of 0.0004   has been reported in more than 2,500 comparisons with experimental data
The models used by the program are considered to be the latest and most accurate available.

Q ?

I tried the feature “What if” in Tools for educational purposes. If I add 100 bar Oxygen then 100 bar Helium, why don't I get 50% Heliox with the Ideal Gas ?

A

If the cylinder is empty, zero bar gauge, it does not mean that it does not contain gas.
The software calculates, correctly, the gas present inside the cylinder at zero pressure gauge, so you have to indicate this properly.
If you want to see a 50 Heliox with Ideal Gas, you have to set as initial gas 50% Oxygen and 50% Helium at zero bar gauge, then up to 100 bar with Oxygen, then up to 200 bar with Helium.

Q ?

Is there a difference between uncertainty, precision and accuracy ?

A

Yes, these words are often confused.
Any measurement has an uncertainty which indicates its reliability, the degree of uncertainty is due to precision and accuracy.
Precision means getting a similar result every time you try, indicating how reproducible the measurement  is. 
Accuracy reflects how close a measurement is to a known, actual value.

Q ?

How long do these mixtures take to homogenize completely?

A

How long is still a controversial issue. 
The molecular speed is highly dependent on their specific atom/molecular physical properties and characteristics.
At the pressures used in technical diving, molecular diffusion is very low, Helium has a  very different density from Oxygen and Nitrogen and it takes time.
Note the reference in the U.S. NAVY Diving Manual chapter 16 - Breathing Gas Mixing Procedures: “allow the cylinders to stand for at least six hours to permit the gases to mix homogeneously, or if equipment is available, roll the cylinder for at least one hour” .
This recommendation has been criticized by some technical communities, but is still present in the latest revision of the manual. This fact, together with my personal experiences, leads me to be prudent in questioning this point.
Perhaps the misunderstanding is due to the fact that, as is often the case,  the differences in equipment available between individuals and instead organizations, like the U.S. Navy are not taken into account: they probably have equipment that allows a true agitation of the internal  content even if it has high density, impossible to be obtained manually.
A reasonable time would be 2- 3 hours for a Nitrox, whereas some Trimix (ex two Trimix mixed)  can take more than 12 hours to reach a homogenous state.

Q ?

It is a software with many settings, configurable in depth. Where should I start from ?

A

The first time you launch the application, the basic settings appear on a screen which are loaded with some assumptions according to your operating system and that you have to check:

-    Language: EN /IT

-    Pressure bar / psi
-    Temperature: ° C / ° F
-    Costs Oxygen and Helium
-    Currency: U $ / Eur / other
-    Depth: meters / feet
-    Volume: Liters / cubic feet
-    Weight: grams / lb

This screen can also be launched afterwards from the Settings menu, “Check Main Settings”.
Then you could complete your preferences:
Settings, Personal cylinders: delete all those you feel that you may never have to use and add those that are missing. It is important to enter their internal volume if you want accurate information about the amount of mixture contained and its cost.
In any case, you can return to the default settings by pressing the default button.

Do the same thing with your mixtures in Settings, Favorite mixtures.
These two settings allow you to optimize the workflow, limiting the options of choice to those you actually want.

If you use the  Continuous Flow method then check your safety limit in Settings, “Max Oxygen % in Cont. Flow”.

If you want the program to look for alternatives in case you need  to vent the initial mix, set the values that you think are acceptable and compatible within the safety limits as the final pressure of the cylinders in Settings, “Current mix Management”.
Then check the values of the warnings that appear in the filling instructions screen under Settings, “Set Warnings limits”.

Finally, if you are interested in printing, check the print options in Settings, “Print Options”.