Abstract: "The Environmental Working Level Monitor (EWLM) is an instrument used to automatically monitor airborne Rn-daughter concentrations and the Working Level (WL). It is an ac powered, microprocessor-based instrument with an external inverter provided for dc operation if desired. The microprocessor's control processor unit (CPU) controls the actuation of the detector assembly and processes its output signals to yield the measurements in the proper units. The detectors are fully automated and require no manual operations once the instrument is programmed. They detect and separate the alpha emitters of RaA and RaC' as well as detecting the beta emitters of RaB and RaC. The resultant pulses from these detected radioisotopes are transmitted to the CPU. The programmed microprocessor performs the mathematical manipulations necessary to output accurate Rn-daughter concentrations and the WL. A special subroutine within the system program enables the EWLM to run a calibration procedure on command which yields calibration data. This data can then be processed in a separate program on most computers capable of BASIC programming. This calibration program results in the derivation of coefficients and beta efficiencies which provides the calibrated coefficients and beta efficiencies required by the main system program to assure proper calibration of the …