Trimix CF blending question

Using the blender that Scott made for me I was able to mix 32% nitrox today. It was very straightforward.

When I get the He meter and start mixing Trimix I will be mixing 14/45 or so.

<b>As I flow 45% He into the blender the rest of the flow will consist of 13.5% O2 (.209 X .65)and 41.5% N2...is that a correct assumption.</b>

In order for me to get a 14/45 mix I will have to flow an additional 0.5% O2 into the blender.

That is the way I have calculated doing fills on empty bottles.

If I bring the second compressor online that pumps pure air is it possible to maintain a 14/45 mix using the blender on only one compressor??

The compressor with the blender is 10 cfm, the other compressor is 12 cfm.

I have a toll free number if Uncle Pug or Omar care to be bothered to call me....1-877-886-6621.

Jim Wyatt
www.reef-divers.com

Math error----45% He

.55 X .209 = 11.5% O2 from ambient air

Must flow 2.5% additional into blender...


sorry for the confusion....

Jim

I use large scale Dwyer ball type flow gauges on the output of the supply regulators of my O2 & He cylinders. I have tape on the side of these gauges that I have marked for the various mixes.

On initial set up to blend nitrox I spent some time blending and analyzing and marking the tape. Now to blend EAN32 I just set the ball on that mark... I still analyze every once in a while during the fill... but the mark gets me where I need to start.

When blending EAN32 I am only adding 11% O2 but when blending TX30/30 I need to add more O2 to get the desire mix and when blending TX21/40 I still need to add a little O2 but not nearly so much.

So for trimixing I spent some extra time in the beginning blending, analyzing and marking tape to established where the balls need to be for the various mixes.... but then I still analyze every so often during a fill.

I am not sure what you mean by bringing another compressor online... do you mean tied into the same manifold system adding air to the mix??? If that is what you mean then the answer to your question is no.... not unless you were running very high FO2 and FHE on the first compressor... very impractical. Do you need additional capacity??? Perhaps and additional blender for the second compressor.

What are you trying to accomplish Jim?

I am just trying to get more volume going by having both compressors pumping at the same time.

Your answer has helped a lot. Wife demands a night out for dinner...but hope to continue this discussion tomorrow.


Jim

Originally posted by Capt Jim Wyatt
Your answer has helped a lot. Wife demands a night out for dinner...but hope to continue this discussion tomorrow.

Click to expand...

I provide the help and she gets the dinner

Jim,

I suggest that you take things a step at a time. That way you will get a feel of what happens when you make a change (one at a time).

You will really need to get a flow meter (variable area rotameter) for the helium and oxygen output to the blender. Remember that when you are using a meter marked for air the helium will have a flow about 2.5 times greater.

I have the flow amounts written in marker on the wall next to the manifold. I think that putting tape on the meter is a better idea.

What would happen if you put a trimix into one side of a set of doubles and air in the other side (isolator open)? That is what would happen when you add air into the blend from a second compressor.

I also heard that your analyzer was sent to Scott Wednesday. I expect that you will see it soon.

I will give you a call Sunday evening. I was out all day and will be taking the boat out early tomorrow.

omar

I will appreciate your call!!

Jim

I get more like 3% - actually, 3.2.

Your method assumes everything that is not He is Air but actually it isn't since there's some extra O2 in it, which gets in effect counted twice.

It's more accurate to figure the amount of O2 in the air from the nitrogen %:

(20.9/79.1) x 41 = 10.8

14 - 10.8 = 3.2

Doublecheck:

10.8+3.2+45+41= 100%

Not that this really make much difference for your purposes, since you're getting an He analyzer. But we got these calculations on the brain since we've been playing around with a little $5 premixer which premixes just the He and O2 before they go into the big mixer. It's got its own O2 analzyer - the idea being that you measure the He that way, and continuous mix trimix without needing a He analyzer.



It takes a little back and forth fiddling to get the percentages on both analyzers right, but it gets pretty easy with a little practice. It's probably not as good as having a genuine He analyzer on the output, but a lot cheaper for the homemixer - if I was a shop I'd go for the He analyzer, if only out of liability concerns (and probably go for a Nitrox Stick for the same reason) but for our purposes it seems to work fine.

Originally posted by Capt Jim Wyatt
Math error----45% He

.55 X .209 = 11.5% O2 from ambient air

Must flow 2.5% additional into blender...


sorry for the confusion....

Jim

Click to expand...

It sounds like you are mixing up heliox and than adding that to the blender. Correct?

In my opinion there is too much back and forth with the o2 addition that way. Since you are using 2 oxygen analyzers why not mix the EANx and check with Analyzer 1. Than add the helium to the mix and check the new oxygen content with analyzer 2.

For instance a 20/40 Tmx, would have a EANx of 33.3% o2 prior to adding any helium. Dial that in then add helium until your O2 is at 20%.

I have checked this method with a helium analyzer and a NIST traceable flow meter, it gets the helium to less than +/- 0.5%.

Mixing Trimix with a single oxygen analyzer can also be done this way if you are confident of what you are doing.

The easy way to do this is to have a small inline static blender with an o2 analyzer at the output for the EANX going right into the same type of mixer where the helium is added with the second analyzer at the output.

This is the same type of setup that Ross Cowell was working on a few years ago. The introduction of a low cost helium analyzer that could be used for diving I think stopped him from bringing it to market. Nevertheless, it still worked wonderfully.

Another point is some oxygen cells will experience interference with high helium mixes such as a heliox. This will result in errors up to 3% for the o2 output.

omar

Yes, that's what we are doing, mixing heliox and shooting it into the nitrox mixer.

We're still experimenting, but thought we'd try the O2/He premixer first because we figured it would be most useful if we could come up with a simple add-on for existing nitrox mixers rather than a whole new mixer. If one is going to be adding the O2 to the air first then the He in it essentially requires two mixers plumbed together with an O2 analzyer in between. And since this requires two sets of mixing baffles, intake resistance becomes a concern. An He/O2 premixer, on the other hand, can be incredibly simple, and resistance is no problem since both gases are still under (slight) pressure - our current one is just a bit of pipe with a tee on each end, and some home made baffles patterned roughly after those used in static mixers in between.

A two stage mixer that adds the air first has certain advantages, as you point out, that may win out in the end - the heliox premixer requires a little back and forth fiddling to get the flows right. Whether they are worth the extra complexity is not clear to us yet - the little bit of back and forth necessary with our current premixer doesn't seem very burdensome.

Re the Graham's Law/ 3% error on helium mixes, that got kicked around a few years ago on one of the techdiving forums and no one seemed to be having any trouble with it. As I recall it is only a problem with sensors having capillary diffusion barriers, rather than the membrane barriers used on virtually all the sensors used in diving analyzers & RBs. I've got some info on it somewhere I can dig up if anyone is really interested. Good thing to remember, though, when using scavenged medical or environmental monitoring gear.

Originally posted by omar
It sounds like you are mixing up heliox and than adding that to the blender. Correct?

In my opinion there is too much back and forth with the o2 addition that way. Since you are using 2 oxygen analyzers why not mix the EANx and check with Analyzer 1. Than add the helium to the mix and check the new oxygen content with analyzer 2.

For instance a 20/40 Tmx, would have a EANx of 33.3% o2 prior to adding any helium. Dial that in then add helium until your O2 is at 20%.

I have checked this method with a helium analyzer and a NIST traceable flow meter, it gets the helium to less than +/- 0.5%.

Mixing Trimix with a single oxygen analyzer can also be done this way if you are confident of what you are doing.

The easy way to do this is to have a small inline static blender with an o2 analyzer at the output for the EANX going right into the same type of mixer where the helium is added with the second analyzer at the output.

This is the same type of setup that Ross Cowell was working on a few years ago. The introduction of a low cost helium analyzer that could be used for diving I think stopped him from bringing it to market. Nevertheless, it still worked wonderfully.

Another point is some oxygen cells will experience interference with high helium mixes such as a heliox. This will result in errors up to 3% for the o2 output.

omar

Click to expand...