How can you tell when a system is low on refrigerant? Running a system check can determine whether that’s the case. Consider this scenario: A service technician installs gauges and thermistors on an R-134a, closed-door, medium-temperature refrigeration system, incorporating a liquid high-side receiver and a thermostatic expansion valve (TXV) as the metering device. Both measured and calculated values are listed in Table 1, along with a detailed system analysis.