Page 44
Doc. 13300-REC-RevB (2022-05-02)
Recreational Modes
Operating Instructions
9.7. 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 10 m/min (33 feet/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 off 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 1�4 bar/min
(20 psi/min)� At a depth of 4�5m/15ft, the pressure is
1�45 ATA� So a 3 minute safety stop would use 1�4 x 1�45
x 3 = 6�1 bar (87 psi) of gas� This small amount of gas is
easy to factor into the reserve pressure setting�
Why is GTR limited to 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 fit for all technical diving, but the GTR
function becomes increasingly complex to manage and
understand when multiple gases are used�
Overall, the required complexity of menus and setup
burden on the user would result in a system prone to
mistakes and accidental misuse, and not fitting 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 207 bar (3000 psi)� 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 207 bar/3000 psi, 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 rectifies itself and the numbers become more
accurate�