Oxygen-clean the chamber pressure tap even on a fuel-rich engine: dead-end compression at light-up

A common shortcut: the chamber pressure transducer only sees combustion gas, and the engine runs fuel-rich (no free oxygen in the products), so it does not need oxygen cleaning. The steady-state part is true, but the start is not. Before and during ignition the chamber often holds oxygen: LOX chill boil-off vented through the injector, an oxidizer lead, and oxygen-rich torch or igniter exhaust. Torch igniters commonly run far above stoichiometric O/F, so most of their oxygen passes through unburnt. The chamber tap line is usually dead-ended (a remote transducer on a standoff). When the chamber goes from ~1 atm to a few hundred psia in a fraction of a second, the oxygen in that line is compressed against the sensor. Ideal adiabatic compression from 14.7 to 270 psia raises 293 K gas to about 673 K (T2 = T1 (P2/P1)^((g-1)/g), g = 1.4), around 400 C. That is enough to ignite oils or greases, and compression heating in dead-ended oxygen lines is a recognised ignition mechanism. Cleaning the tap line and sensor for oxygen service is cheap insurance. Worse if the tap sits near the igniter port. The same logic applies to igniter-head pressure sensors, which see oxygen directly. Water or inert-gas sensors do not need it.