Where gauge and absolute pressure meet, and what a person is shown.
Every pressure inside the model is absolute -- the equation of state, the
choking ratios and the regulator all need it that way. Every pressure a person
reads or types is absolute too, in psia (ADR 0007): the slides, the datasheets
and EngineDesign all quote psia, and a screen that said psig sat 14.7 psi off
every one of them.
What crosses the wire and what is saved stays **gauge**: a knob's ``psig``, the
API's ``*_psi`` fields, run records, hookups, stand documents. That is what the
stand's DAQ logs and what every saved file already holds; changing it would
make each old record mean something else. :func:`psig` and :func:`from_psig`
are the only two places the wire meets the model. Moved here from the
feed-twin app when the marching session became library code, so a second
caller -- EngineDesign's Layer X -- converts through the same two functions
rather than its own.
Text meant for people goes through :func:`say`, which prints psia from the
model's own absolute pascals (it and :func:`psia` live in
:mod:`feedtwin.model.pressure`, so the drawing reader can use them without
importing the session; this is their address). A difference -- a bias, a
droop, a sag, a residual -- is the same size either way and is written in
plain psi.
The zero is the **standard** atmosphere, deliberately and only for gauge
readings: it is the convention the stand's DAQ and the cockpit share. A caller
that knows the absolute pressure it wants (an engine config states psia) should
convert with :func:`from_psig` of ``(p_abs - ATMOSPHERE) / PSI`` -- which is
exact -- rather than assume its site's atmosphere is this one.
1.15.0