VPM Trimix Decompression


Copyright 1999-2001 by Eric Maiken

 

 

Background

The multigas version of the Varying Permeability Model was used to calculate an alternative ascent profile for the 20 min trimix computer "dive" to 293 feet, analyzed in the winter 1998 issue of Immersed by Erik Baker, using his Gradient Factor Method. The profile below was calculated using the MultiGas VPM program . 

For comparison purposes, this calculation was for a bottom gas of 13/50 trimix, with ascent on trimix and 36%, 50%, and 100% Nitrox.

One of the main features of a VPM-based ascent are the deep stops required for decompression profiles. The following ascent starts at 33 feet/min to the first stop at 200 feet, which is far deeper than the stops required by Buhlmann-based programs. On the other hand, the following 70-minute schedule is greatly accelerated compared to conventional calculations.

 

The ascent schedule can only be called "radical." So, don't do it unless you want to be a crash test dummy....

 

Click Image To Enlarge

A plot of inert gas partial pressures for 16 compartments, with 1 - 600 minute nitrogen half-times is shown above, with the fastest compartments in the figure background. PPNITROGEN curves are shown as fine gray lines, PPHELIUM are shown as black, and the PPTOTAL are shown as the rainbow colored lines.

 

Click Image To Enlarge

A plot of inert gas partial tensions (T) of the 16 compartments at each of the ascent stages (denoted by the regions between the blue vertical grids). The diagonal black line tracks absolute ambient pressure. The ascent is controlled by the compartments above the ambient line (ie: with positive gradients), with the faster halftimes limiting the deep portion and slow halftime compartments limiting the shallow portion of the decompression. The allowed range of gradients are shown at each stage as the narrow black and gray lines.

Ascent Schedule for the Dive :

Depth/Run-time: 293 feet/ 20 min
Bottom Mix: 13% Oxygen, 50% Helium
 Ascent Mixes: 36%, 50%, and 100% O2 Nitrox

Decompression:

Depth
(feet)

Run-time
(min)

 %O2:%N2
0 0 21: 79
100 1 13: 37
200 2 13: 37
293 20 13: 37
260 21 13: 37
227 22 13: 37
200 23 13: 37
190 24 13: 37
180 25 13: 37
170 26 13: 37
160 27 13: 37
150 29 13: 37
140 31 13: 37
130 34 13: 37
120 37 13: 37
110 38 36: 64
100 40 36: 64
90 42 36: 64
80 44 36: 64
70 47 36: 64
60 50 50: 50
50 54 50: 50
40 60 50: 50
30 68 50: 50
20 90 100:0
0 90+ 21:79


The Calculations were done in the Impermeable regime of the VPM, with the settings: PIMPERMEABLE = 8.0 ata, r0He= 1.0 mm, r0N2= 1.2 mm.