On 05/10/2014 18:45, Robert C. Helling wrote:
Willem,
In 1) the tissues are the grey bars moving up, the fastest on the left and the
slowest on the right. In 2) the faster tissues are greener while the slower
ones are bluer.
The scale of the y-axis is twofold (as partial pressure mbars don’t mean much
to people): If the partial pressure of the tissue is below the ambient
pressure, the scale is the ambient pressure (the ambient pressure being the end
of the green area in 1) and the grey line in 2). In 1), the inert gas pressure
of the current breathing gas is indicated by the lower black line (below
ambient pressure and thus in the green area). While you dive at constant depth,
all tissues approach that line exponentially in time.
For partial pressures above ambient pressure, the scale for the over pressure
is the maximally allowed overpressure according to vanilla Buehlmann (the
beginning of the red area in 1) and not explicitly shown in 2).
If you use gradient factors (other than 100/100), you only allow a depth dependent
fraction of the allowed overpressure compared to Buehlmann (that is the percentage
of the gradient factor). The applicable overpressure according to the gradient
factor at the current depth is the upper black line in 1) and the thick black line
in 2). If GFlow < GFhigh (as it is supposed to be) you will see this black line
going up as you ascend.
As said above, at constant depth the tissues approach the inert gas pressure of
the inhaled gas. As everything is plotted relative to ambient pressure (or
M-value respectively), the tissue loadings go up on ascent while they go down
on descent.
In deco (or when the planner takes care of the ascend), you ascend until the
first tissue touches the gradient factor line. Then you wait until that tissue
pressure has gone done so it is still below that line after ascending to the
next stop depth.
Does that help?
Best
Robert
Robert,
Attached is a bar graph from a dive using trimix 27/28. After 16 seconds
the depth was 4 m fresh water, represented in the attached bar graph.
The total inhaled gas pressure is 1.4 * 1.023 = 1.417 bar for fresh
water. The partial pressure of the nitrogen in the inhaled gas is 0.45 *
1.417 = 0.638. So, if the total ambient gas pressure is represented by
the hight of the green bar and the bottom black line represents the
equilibrium partial pressure of nitrogen at the moment of and at the
conditions of measurement, I would have expected the black line to be
just below the middle of the green part of the bar graph. The current
position of the black line suggest that it perhaps indicates the
equilibrium partial pressure for all inert gases (He + N2) combined??
Similarly, the dark green portion of the bar graph seems to indicate the
present combined partial pressures for He + N2. But the bar graph surely
only pertains to the nitrogen fraction because offgassing for nitrogen
and for He follow different patterns, He requiring much longer
offgassing times?? I suspect the gradient factors apply only to the
nitrogen partial pressure?
Your comments would be very valuable.
Kind regards,
willem
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