PERDIX SHEARWATER - 7.2. GTR calculations

Operating Instructions
Page 17
RevA
7.2. GTR calculations
Gas Time Remaining (GTR) is the time in minutes that can be
spent at the current depth until a direct ascent to the surface
at a rate of 33 feet/min (10 m/min) would result in surfacing
with the reserve pressure� This is calculated using the current
SAC value�
Safety stops and decompression stops are not considered by
the GTR calculations�
To calculate GTR, start with the known tank pressure, P
tank
�
The remaining gas pressure, P
remaining
, is determined by
subtracting o󰀨 the reserve pressure and the pressure used for
the ascent�
P
remaining
= P
tank
- P
reserve
- P
ascent
, all tank pressures in [PSI] or [Bar]
Knowing P
remaining
, divide this by the SAC adjusted to the
current ambient pressure to get GTR in minutes�
GTR = P
remaining
/ (SAC x P
amb,ATA
)
Why aren’t safety stops included?
Safety stops aren’t included to simplify the meaning of GTR,
and make it consistent across operating modes that do not
include safety stops�
Managing enough gas for a safety stop is quite simple,
especially since they require a relatively small amount of
gas� For example, consider if your SAC was 20 PSI/min (1�4
Bar/min)� At a depth of 15ft/4�5m, the pressure is 1�45 ATA�
So a 3 minute safety stop would use 20 x 1�45 x 3 = 87 PSI
(6�1 Bar) of gas� This small amount of gas is easy to factor
into the reserve pressure setting�
Why is GTR limited to one tank and no deco?
Currently, Shearwater does not believe that GTR is the
proper tool for decompression dives, especially those
involving multiple gases� This isn’t to say AI in general is
not a good t for all technical diving, but the GTR function
becomes increasingly complex to manage and understand
when multiple gases are used� For one, if multiple gases
are used, then tank sizes must be correctly entered� This
is a very easy step to forget, and will lead to incorrect GTR
values� Multiple gas diving also requires further setup of
associating each transmitter to a specic gas mixture, which
besides being another setup to forget, gets complicated with
corner cases such as having multiple tanks containing the
same mixture� Further handling other situations such as only
a sub-set of the used tanks with transmitters add complexity
and potential for user misunderstandings� Overall, the extra
complexity of menus and setup burden on the user would
result in a system prone to mistakes and accidental misuse,
and not tting with Shearwater’s design philosophies.
Gas management is an incredibly important and also
complex activity, especially for technical diving� Education,
training, and planning are critical for proper gas management
for technical dives� Shearwater feels that a convenience
feature such as GTR is not a good application of technology
in this case, as its complexity and potential for misuse would
outweigh its utility�
No compensation for ideal gas law deviations
Note that all SAC and GTR calculations assume that the
ideal gas law is valid� This is a good approximation up to
about 3000 PSI (207 Bar)� Above this pressure, the change
in gas compressibility as pressure increases becomes a
noticeable factor� This is mainly an issue for European
divers using 300 Bar cylinders� The end result is early in
the dive, when pressures are above 300 PSI/207 bar, the
SAC is over-estimated, resulting in under-estimation of
GTR (although this is the good way to err, as it is more
conservative)� As the dive progresses and pressure drops,
this problem recties itself and the numbers become more
accurate�