Calculating Trimix Best Mix?

[stuartv]

I am in the middle of getting my TDI Helitrox cert. I had a classroom session last night where we covered how to calculate the Best Mix (for Trimix).

Prior to taking this class, I had done a lot of my own reading and had understood END to mean Equivalent Narcotic Depth, and to be calculated based solely on the FHe. IOW, treating O2 and N2 as equally narcotic.

Last night's class and the TDI Extended Range Diving and Trimix student manual explain it differently. The book refers to "equivalent narcotic depth" but flat out tells you that you are calculating it using the EAD (Equivalent Air Depth) formula. I.e. it is based solely on the FN2, not the FHe.

I did some searching on here and found old posts stating that IANTD and NOAA (and maybe others?) say to calculate END based on the sum of ppO2 and ppN2. But, I am looking at my copy of the NOAA Diving Manual (5th ed., p.8-7 - 8-8). It is actually quite conflicted.

In 8.3.1.2.2, it says "the END is normally set a [sic] PN2 of 4.0 ata or less." It then works through an example calculation based on a dive to 230fsw where it calculates the FHe based on limiting just the ppN2 to 2.6 bar, which is equivalent to air at 75fsw. This is exactly the same process that is taught in the TDI book.

But, then in 8.3.1.2.4, it says "it may be appropriate to include the oxygen fraction in the END calculation when using trimixes."

And then it gives Table 8.3 which gives Helium selection based upon Equivalent Air Narcotic Depth. I.e. pre-calculated FHe for various dive depths and desired equivalent narcotic depths, where it is calculated using the combined ppO2 and ppN2.

I note that the FHe determined when using equivalent nitrogen-only depth of 75 fsw, for a dive to 230fsw yielded 50%. And Table 8.3 gives 55. So, surprisingly (to me!), the actual difference you come up with seems to be relatively small.


So, I gather that there is not really a compelling argument to calculate FHe in a trimix blend one way or the other? Whether you set a target ppN2 and calculate from there or whether you set a target for (ppO2 + ppN2) doesn't make enough of a difference to drive the industry to adopting a universal standard, the way it has for calculating a Nitrox Best Mix? Or is there and the industry just hasn't "gotten there" yet?

 

[tbone1004]

that was a bit of a ramble, but this is what I believe regarding trimix. Keep in mind I haven't read the TDI limits and don't bother with EAD calculations personally.

When doing best mix, you start with O2 and determine your fO2 based on your personal pO2 limits. O2 kills you, N2 makes you drunk, so O2 trumps N2 and is the start of the calculation. After determining your personal pO2 limits, you get to determine your personal pN2. That pN2 can be set based on whatever you believe your limits to be. Do you sent it to 3.2? 3.6? That depends on what you believe your personal limits to be.

I believe that O2 contributes to narcosis, and I can feel narcosis coming on when in a cave at 100ft diving EAN32 if I'm kicking hard. pO2 of 1.3, pN2 of 2.7, combined pressure of 4.0. So for me that means by total pressures if given a choice are not going to exceed 4.0. That can get very expensive when on OC, so if you are on a DPV or aren't going to be kicking hard for long, then you may choose to increase that limit to save money. If on CCR, you are using less dil, so your dil may be set to a combined pressure of 4.0 for WoB and narcosis, but your bailout may have a combined pressure that is 4.6 to save on helium. May not, but it may.

When I go CCR next year, I will not exceed a total pressure of 4 for dil, and will keep N2 to a minimum. Some people are less susceptible to narcosis than I am, so they may be able to get total pressures closing in on 5 *air to 130ft* and will push their pN2's to 3.6 or 4.0. All depends on your personal limits and belief's.

Similar to bp/w's, primary donate, pO2 limits, and decompression algorithms, you are unlikely to see a definitive consensus across the industry on how to determine best mix for trimix. For me? dive standard gasses. No O2 cleaning required, minimal boosting required, easier to deal with, and "close enough".

 

[Diver0001]

You don't calculate the FHe. You just work out what % of the gas it needs to be.

You have to start by looking at your target depth. You set the ppO2 and ppN2 target pressures based on (usually) 1.4 for oxygen and let's say 3 for N2 (this can vary from diver to diver), many divers use 4.

The calculation looks at the target depth in ATA and it's a simple matter to calculate the %O2 and %N2 in the gas. The gap is your %He

Example:

Say we're diving to 70m (8ATA) and you set your oxygen pressure parameter to 1.4 and your N2 parameter to 3.

1.4/8 = 17.5% (O2 setpoint divided by ATA at target depth).
3/8 is 37.5% (N2 setpoint divided by ATA at target depth)

Add that together and you have 55%. The other 45% is therefore the % of He you will need in the mix.

So that's what you order at the filling station. 18/45 or something close to that.

So try the calculation again using a setpoint for N2 of 4 instead of 3 and see what you get.
R..

 

[Doppler]

I just use Dalton's Law... I have two constants: a set limit for partial pressure of oxygen (let's say 1.3 bar) and a set limit for partial pressure of nitrogen (let's say 3.1 bar, about the same as breathing air at 30 metres or 100 feet); and one variable the partial pressure of helium.

Therefore, if the pressure at depth is more than 3.1 + 1.3 = 4.4 (which is 34 metres), I'll add helium to fill the vacant partial pressure.

Simple example: A dive to 65 metres... ambient pressure is 7.5 bar. 7.5 - 4.4 = 3.1 bar... that's vacant partial pressure... gotta fill it with He. Expressed as a percentage, 3.1/7.5 = 41%... I would probably use that as a starting point anyway.

 

[Doppler]

Of course, that's an OC calculation. With CCR I usually fill the diluent bottle with 10/50 and let the unit mix for me.

 

[Kevrumbo]

The current & easiest strategy to follow is to use GUE or UTD Standard Gases, select the optimum bottom mix with MOD ppO2 of 1.4 ATA and average working ppO2 of 1.2 ATA; an END of 30m/100' (4 ATA) or less; and gas density of 5 g/L or less to mitigate Work-of Breathing -all at the planned average operating depth:

FKD - Standard Mixes

Gas density guidelines

At 40:00 . . .Dynamic Airway Compression, CO2 Retention, Work of Breathing & Gas Density:

 

[kensuf]

I just use Dalton's Law... I have two constants: a set limit for partial pressure of oxygen (let's say 1.3 bar) and a set limit for partial pressure of nitrogen (let's say 3.1 bar, about the same as breathing air at 30 metres or 100 feet); and one variable the partial pressure of helium.

This.

Based on the Meyer-Overton theory, the non-metabolized portion of Oxygen should be more narcotic than a comparable dose of Nitrogen. Because some of the O2 is metabolized, it's OK to just treat O2 and N2 as equally narcotic.

My personal limits put my combined PO2 and PN2 at 4.0 in an overhead environment, with 1.2 being the PO2 and 2.8 being the PN2. Deep reefs in the Caribbean, and my acceptable PN2 tolerance goes up, but the upper limit still puts me around 130' END.

At the end of the day, a little less O2 and a little more He is never a bad thing.

So if you're going to 200' in a cave, the best mix would be roughly 17/45 (P=7.0; PO2=1.2; PN2=2.8; PHe=3.0).

 

[kensuf]

I'd like to suggest reading this:

http://www.tech-ccr.com/pdf/study-lounge-ccr/Required Reading/Narcosis Meyer-Overton.pdf

 

[cool_hardware52]

GUE "Standard Mixes" are worth understanding.

Hard to see a He dive that isn't well served by one of these mixes.

Keep in mind that unless you are in Cave Country with access to accurate preblended gas or doing your own blending you will almost always end up with mixes that are off by at least several % points on O2 and He. Worrying about selecting the "perfect" mix is a waste of time if you end up with 23/38 or 17/53. As long as you calc the MOD based on the actual O2 and there is "enough" He chances are you are close enough.

At the end of the day, a little less O2 and a little more He is never a bad thing.

Agreed. When I blend I always plan any errors to leave me short on O2 and long on He.


Tobin

 

[Remy B.]

What Diver0001 said and Doppler is what I follow

1.4 PPO
3 PPN

That give you the gas mix, you can pkay with Multi-Deco then you will know how much He and O2 to add to the mix in order to keep the PP where you want.

Of course you can go with more conservative PP but that will led to more He