A recent development in passive safety devices for advanced liquid-metal reactors is the installation of manometerlike core assemblies called gas enhancement modules (GEMs). Knowledge of the liquid sodium level within the GEMs is required to monitor GEM operation. A microwave, resonant cavity level measurement technique has been laboratory tested on a scale model of a GEM assembly in a nonsodium environment. The theory behind this method is discussed, and the experimental results are shown to compare well with those predicted by theoretical calculation. The resonant cavity level detector tracked extremely well over the desired 0.1524- to 1.1176-m range of operation and provided accurate, reproducible results well within the desired ±25.4-mm actual level. When tested for vibrational stability, level errors of only 0.254 mm were observed. The effects of material differences between the experimental GEM (copper) and the actual GEM (Type 304 stainless steel) are calculated. The actual GEM will have poorer resolution but still be within ±25.4-mm actual level. Temperature effects are also calculated and produce a 10.5 kHz/°C shift in resonant frequency, which could cause the indicated level to exceed the ±25.4 mm allowed if large (∼149°C) temperature changes occur.