feedtwin.model.pressure Namespace Reference

Variables
	ABSOLUTE

Detailed Description

Which pressures on a drawing are absolute, which are differences, and what a
bare "psi" means.

The model works in absolute pascals. People do not: a tank "at 500 psi" is at
500 psig, a COPV "at 4500" is at 4500 psig, a regulator "set to 500" holds 500 on
a gauge. Every shipped drawing's own notes say so ("500 psig operating
pressure", "bottle gauge", "gives 550 at the tanks"). The library used to read a
bare "psi" as absolute, so every one of those was 14.7 psi low -- the drawn dome
loader disagreed with the dome knob by an atmosphere, a delivered bottle started
an atmosphere under its own fill target, and the Hookup tab printed a psia number
labelled psig.

So on a drawing, two rules, both here and nowhere else:

* **An absolute pressure written bare is gauge.** ``psi`` on a tank's
  ``pressure`` or a regulator's ``setpoint`` reads as ``psig``; ``bar`` as
  ``barg``, ``kPa`` as ``kPag``. Write ``psia`` (``bara``, ``kPaa``) to mean
  absolute. ``atm`` is absolute by nature and stays so.
* **Except at the engine.** A chamber pressure is quoted absolute, by long
  convention and everywhere EngineDesign quotes one; on an ENGINE or INJECTOR
  symbol a bare pressure is read as absolute.
* **A difference never carries a reference.** A droop, a dome bias, a crack or
  relief set pressure is the same size in psia and psig; ``psig`` on one is a
  drawing error, refused with the parameter named, because an atmosphere
  silently added to a 50 psi bias is a 65 psi bias.

A pressure parameter in neither list is read as absolute -- what nearly every
pressure on a drawing is -- so a field pid-designer adds tomorrow still imports.
``tests/test_pressure_reference.py`` fails until it is listed here, which is what
keeps the lists complete.

How a drawing is *read* is not how a pressure is *shown*. Whatever reference a
drawing wrote, a person is shown the absolute pressure in psia (ADR 0007):
:func:`say` is the one formatter.

Variable Documentation

ABSOLUTE

feedtwin.model.pressure.ABSOLUTE

Initial value:

=  frozenset(
    {
        "pressure",  # a tank, bottle, dewar or source boundary
        "chamber_pressure",
        "setpoint",  # a regulator's outlet        "dome_pressure",        "inlet_reference",  # retired: the supply effect is measured from zero inlet        "MAWP",        "burst_pressure",        "range_max",  # an instrument's full scale
        "range_min",
    }
)

Definition at line 42 of file pressure.py.