Trimix diving calculations made easy

Trimix is a breathing gas consisting of oxygen, helium and nitrogen. The notation for trimix is usually Tx 10/70 – where the first number is the oxygen percentage, and the second being the helium percentage (nitrogen then makes up the remaining gas). So Tx 10/70 would be 10% O2, 70% He, 20% N2

The first thing we need to understand when looking at trimix calculations is the Equivalent Narcotic Depth (END). This along with the max partial pressure of oxygen (usually 1.4) will allow you to find the best mix for a given depth. Using the END is a way of estimating the narcotic effect of a breathing mixture, such as heliox or trimix. The equivalent narcotic depth of a trimix is the depth at which you would experience the same narcotic effects when breathing air.

For example, Tx 20/40 being used at 60m has an END of 32m (i.e. breathing this mixture at 60m has the same narcotic effect as breathing air at 32m)

Calculating the END of a mixture

END = (depth + 10) × (fraction of O2+ N2) –10

Eg. For the above mix, Tx 20/40 at 60m,

END = (60+10) × (0.2+0.4) –10

END = 32m

Choosing the best mixture for a particular depth

1. Work out the Oxygen percentage based on the a max PPO2 (1.3 / 1.4)

2. Use the END to work out the Nitrogen percentage in the mix

3. Given the %O2 and the %N2 you can work out the %He

Example

You are planning a dive to 75m and want an END of 35m

1. From the nitrox calculations, you should be able to calculate the percentage of oxygen in a mix at a particular depth, using a max PPO2 of 1.4.

So for a PPO2 of 1.4 at 75m (8.5bar), your oxygen percentage would be
1.4/8.5 × 100 = 16% O2

2. Manipulating the formula above,

END = (depth + 10) × (fraction of O2+ N2) –10
35 = (75+10) × (0.16 + fraction of N2) –10
F N2 = 0.37 i.e. 37% N2

3. So we have 16% O2, 37% N2, giving us 47% He

So our mix is Tx16/47

WARNING:  You can’t breathe mixtures of less than 16% oxygen (18% including a safety margin) on or near the surface as it doesn’t contain enough oxygen to support respiration.  In this case you would need to use a travel gas to get you from the surface to a depth where the PPO2 is at a high enough level to breathe.

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What Equivalent Narcotic Depth do you use 

This will vary from person to person, and will also depend on the conditions. You should pick a depth at which you are not affected by narcosis. For dives in clear, warm condition you can use a fairly deep equivalent depth, while in dark, cold, low visibility or in overhead environments a shallower END should be used.

Standard Mixes

Although, you can tailor your gas mix for a specific depth, through the years technical divers have used a fairly standard set of gases for certain depth ranges:

35m-45m    – Tx 21/35
45m-60m    – Tx 18/45
60m-75m    – Tx 15/55
75m-120m – Tx 10/70

These examples are just to show how easy the calculations are, and can in no way replace a trimix course.