Which instrument primarily measures engine pressure ratio (EPR)?

The instrument that primarily measures engine pressure ratio (EPR) is the EPR gauge. This gauge is specifically designed to provide the ratio of the pressure at the turbine inlet to the pressure at the engine's exhaust, allowing for a clear understanding of the engine's performance and efficiency. EPR is a critical parameter for turbofan engines, as it indicates how effectively the engine is converting fuel into thrust.

Monitoring EPR is essential for flight operations, as it aids pilots and engineers in assessing engine performance and optimizing fuel consumption. Accurate readings from the EPR gauge help ensure that the engines operate within safe and efficient limits, contributing to overall aircraft safety and performance.

Other instruments, like fuel flow gauges and exhaust gas temperature gauges, serve different purposes, such as measuring fuel consumption and exhaust temperatures, and do not provide the specific pressure ratio measurements required for EPR. The N1 gauge reflects the speed of the engine's low-pressure compressor, which, while important, focuses on a different aspect of engine performance.