Creating the Proper Mix Using Partial Pressure Blending

[ScubaWithTurk]

After reading this thread in the Basic Scuba section, I thought it would be nice to have all of us blenders discuss our own best practices for partial pressure blending.

The +/- 1% is what divers are looking for and from my experience, this is not always the easiest to achieve due to many variables. Having said that, I usually get mine within 3% but also have had some weird mixes from time to time.

We always fully drain the cylinder before mixing, When shooting for 32% we fill with O2 to 28.2 bar. This is done slowly (.1 bar per second) Once I hit 28.2, then we immediately fill with air from the compressor to 210 bar. This then sits 4-6 hours before we top up which usually is about 5-10 bar.

This is how I was taught but I have a feeling there are some better practices that can be used here. For the record, we use a digital gauge on the O2 whip. I know for a fact this is where things be off a bit. If you hit your mark dead nuts, then shut everything down, purge the line and then reopen to check, the readings tend to drop. What you thought was 28.2 is now 27.8 (just an example) This leads me to slightly overfill in order to compensate for the small drop.

What best practices do you use and what tips or tricks do you have for blenders?

 

[MaxBottomtime]

Technical Diving and Recreational - Nitrox Calculator

 

[ScubaWithTurk]

Technical Diving and Recreational - Nitrox Calculator

I have two blending apps on my phone, one that is in metric (all I use here and much easier I might add) and one that does both. However having the numbers is only part of the blending process. Fill speed of the O2, cool down time, do you fill the O2 and let it cool first or immediately fill with air, let that cool (and for how long) and then top up? These are a few things to consider that all can change your mix.

I love partial pressure blending and want to see the process others use and any tips and tricks they have to offer up.

 

[Vicko]

Usually i'm doing the mixing in the evening. I run all the O2 trough a 300 bar booster in hopes nothing blows up. Fill the tank when hot, then recheck the mix in a hour and see if it needs re-mixing.
When filling 3l tanks with low o2 mixes I accept that in 10 of them i'll mess one up by 3%-4%, so I fill them extremely slow and try to keep them cool.
I like using the multi deco tools as it also gives me test pressures and partial pressures at every stage of mixing.

 

[NAM001]

After reading this thread in the Basic Scuba section, I thought it would be nice to have all of us blenders discuss our own best practices for partial pressure blending.

The +/- 1% is what divers are looking for and from my experience, this is not always the easiest to achieve due to many variables. Having said that, I usually get mine within 3% but also have had some weird mixes from time to time.

We always fully drain the cylinder before mixing, When shooting for 32% we fill with O2 to 28.2 bar. This is done slowly (.1 bar per second) Once I hit 28.2, then we immediately fill with air from the compressor to 210 bar. This then sits 4-6 hours before we top up which usually is about 5-10 bar.

This is how I was taught but I have a feeling there are some better practices that can be used here. For the record, we use a digital gauge on the O2 whip. I know for a fact this is where things be off a bit. If you hit your mark dead nuts, then shut everything down, purge the line and then reopen to check, the readings tend to drop. What you thought was 28.2 is now 27.8 (just an example) This leads me to slightly overfill in order to compensate for the small drop.

What best practices do you use and what tips or tricks do you have for blenders?

I see this aspect you describe quite often. when you put in the O2 you need to let it sit and cool down. For instance if I want a 40% mix it is about 1/4 O2 and air to top off. starting from an MT tank i put in 750 lbs O2. after it sits a while the O2 psi drops into the mid upper 600's depending on how fast the fill was. I then have to top off with O2 to get back to around 750 again. The fill has to be really slow not to see this happen. I am not sure it can be done.

 

[markmud]

Technical Diving and Recreational - Nitrox Calculator

Hi Max and OP,

When I was a tankerman, we corrected all blending products for temperature and specific gravity. Does the calculator do that? We never had problems that are being described here. Once corrected, everything was simple math and very accurate.

Interesting thread...I am waiting to learn more...

m

 

[Wookie]

When I PP fill, which I will continue to do until I get my new membrane up and running, I measure what is still in the cylinder. I use the Trimix tools app to calculate my new mix and pressure. I tell the app that I’m filling 3442 cylinders to 3700, and actually fill them to 4000 smoking hot. I tell it I’m filling 3000 cylinders to 3300 and then fill them to 3700 smoking hot. I end up within .2 to .4% every time at exactly the correct fill pressure (3500 or 3100).

 

[RayfromTX]

Love me some Trimix tools.

 

[Storker]

Get a sufficiently accurate scale, and nearly every problem is solved. Just blend on gas mass.

The price of a sufficiently accurate scale may be somewhat prohibitive, though...

 

[Gareth J]

Blending by p.p.

The biggest enemy is heat.

Point 1.
Getting O2 hot is not a good thing - especially if there is any fuel or source of ignition.

Point 2.
A hot gas expands. This distorts your P.P. measurement.

Blend slowly. If you have time, leave the gas too cool.
When doing Trimix for myself, I'll add the helium in the evening, double check it in the morning and top it up if required.
Then add the O2.
Go to work, when I get back in the evening, I'll double check the O2, and top off if required. If I'm being pedantic, I'll check the mix with an O2 analyser, to confirm I haven't messed anything up - much easier to correct, or recalculate at this point. After confirming the O2, I'll add the air, then recheck again in the morning.
Granted, that's when I have time. It's the way I was taught in the days when Helium analysers where too expensive for me to own one.

The other thing to remember is that the 'ideal gas laws' fall down as the pressure gets higher. So if you have software that compensates and uses 'real gas laws', that makes Trimix mixing much more accurate.

As was said in the other thread. When blending Nitrox, most blenders mix rich. It's easy to make a mix leaner. Making it richer is much more problematic, especially if you don't have a haskel.