Water Rocket Computer Model Help      
Initial Conditions

The initial conditions represent the state of the rocket at
launch. Most are straightforward but the units for the pressure
in the rocket sometimes need making clearer.

The pressure readout on pressure gauges such as bicycle pumps
or car tyre pumps display the pressure relative to atmospheric
pressure hence the units 'BarG' (Bar Gauge). A pressure reading
of 0.1 Bar on a tyre at sea level shows that the pressure inside
the tyre is 0.1 Bar higher than the pressure outside the tyre.

Likewise on the top of a mountain and this is why you use that
pressure here. For the tyre, the need is to have a difference in
pressure between the inside and the outside. For the liquid in the
rocket, that requirement is the same which is one reason why
you input that figure here but when it comes to compressing the
gas, that depends upon the amount of gas in the vessel and that
will be different in the two cases which is why the figure for
actual outside air pressure is written in in one of the sections
lower down on this page.