Posted by MHK on October 10, 2000 at 09:53:29:
Since the trimix thread is heating up I thought it would be beneficial to introduce a few terms that are often times confused...
When we talk about trimix we generally refer to a couple of types of trimx..
1) TRIOX or HYPERoxic trimix- These type of trimixes involve a mix with three different gases. The norm is Oxygen ( 02 ), Helium ( He ), and Nitrogen ( N2 ). When we say HYPERoxic or TRIOX we are referring to a mix that raises the 02 level. The standard breathing component has 21% oxygen.
2) NORMOXIC refers to a mix that adds helium to the mix but leaves the level of Oxygen at 21%. Hence the suffix NORM..
3) TRIMIX generally refers to any mix that has three gases but is more aptly describing a trimix that lowers the 02 level and generally has a high He content. These mixes are generally used when you are planning a dive to greater than 200'.
Any of the above will be described numerically as follows:
20/25 ( For example ).
The 20 refers to the 02 content
The 25 refers to the Helium content
and the Nitrogen content is generally omitted..
When you are planning a dive using Trimix you are trying to balance a couple of things when you are planning your mix.
The oxygen component is the driving force behind the limiting factors. As you all know oxygen becomes toxic when breathed deeper than the maximun operating depth ( MOD ). For example, using a P02 of 1.6, a mix that has the normnal oxygen levels ( ie; 21% ) has an MOD of 218'.. Thus you wouldn't plan a dive deeper than 218' without lowering the 02 content.
The Helium component is the compnent that provides for the clearer head. We calculate this using a Equivalent Narcotic Depth ( END ). You figure out at what depth you want to calculate the clear head and then you calculate the He content accordingly.
We recommend using a max P02 of 1.4 and a END of 100'.
Hopefully this will clear up some of the confusing issues surrounding Trimixes..
Regards