Problem with Baltic Blender, wrong calculation

[Sevenrider860]

Starting mix is 20.0/34.0, 164 bar.
Desired mix is 21/35, 167.5 bar

3.5 Bar and 1% difference in gas percentage? That is what you are taking all this time and effort to mix to? You are dealing pressures that likely are close to the full scale accuracy of your pressure gauge and within the range of error of your sensors.

 

[Agro]

Of course it does not matter in this special case. I took these numbers because they are easy to discuss and don't have a third filling gas (air) involved. We can also discuss:
Start: 20.0/34 164 bar
Desired: 21/35 270 bar
Add O2 up to 173.3 bar
Add He up to 214.1 bar
Add Air up to 271 bar
Everything at 14 celsius

Here we have a difference of 1 bar only (270/271). Can this meen I have to fill up to 271 hot to get 270 cold? I can not imagine. I think temp. difference should create more pressure difference

 

[Sevenrider860]

Of course it does not matter in this special case. I took these numbers because they are easy to discuss and don't have a third filling gas (air) involved. We can also discuss:
Start: 20.0/34 164 bar
Desired: 21/35 270 bar
Add O2 up to 173.3 bar
Add He up to 214.1 bar
Add Air up to 271 bar
Everything at 14 celsius

Here we have a difference of 1 bar only (270/271). Can this meen I have to fill up to 271 hot to get 270 cold? I can not imagine. I think temp. difference should create more pressure difference

If you want 270 bar of breathable gas, what should your gauge pressure read? Have you noticed your final pressure is always close to 1 bar higher than your desired pressure?

 

[Agro]

I want to understand what Baltic Blender is calcualtion. At the moment I don't.

Why to we have this 1 bar difference?

 

[inquis]

Looks like a bug to me, likely caused by roundoff error. I wouldn't worry about it. The numbers are close enough to correct, and TBH the heating will change things still further. Not to mention gauge error/resolution.

 

[Sevenrider860]

I want to understand what Baltic Blender is calcualtion. At the moment I don't.

Why to we have this 1 bar difference?

If you put your pressure gauge on an empty cylinder and it reads 0 bar, that is the gauge pressure. The pressure in the tank is about 1 bar. If I tell the program I want 200 bar of useable gas at 21 degrees, then I add the gas until I read 201 bar on the pressure gauge at 21 degrees. In reality, nobody even tries to get to that this level of accuracy in either temperature or pressure.

Baltic Blender uses the GERG-2004 real gas model. Those are not calculations I want to do on a piece of scratch paper, but the ideal gas model also gets you some what close.

If you really want to understand the calculations behind the App, download load the article on the gas model used. I have provided the link below.

https://www.gerg.eu/wp-content/uploads/2019/10/TM15.pdf

There may be some bugs in the App, I don't know. But my mixes have always come out very close to what is being calculated as long as you wait for the temperatures to stabilize at each step of adding the gas.

 

[Agro]

as long as you wait for the temperatures to stabilize at each step of adding the gas.

That's my question. Do I have to wait for temp to stabilize? Does the app give pressure at high (what is high?) or low temp?

 

[Agro]

The pressure in the tank is about 1 bar. If I tell the program I want 200 bar of useable gas at 21 degrees, then I add the gas until I read 201 bar on the pressure gauge at 21 degrees.

I don't think "my" 1 bar difference is caused by this. I do not always get this 1 bar, sometimes output and target pressure are the same, zero bar difference.

 

[Agro]

If you really want to understand the calculations behind the App,

I don't want and I am sure I can not. I want to understand how temp goes into calculation and what is about this 1 bar difference.

 

[Agro]

Looks like a bug to me,

I can hardly imagine. But if it is a bug, then it is a bug.