The 431C Power Meter provides power ranges of 10 ?W to 10 mW fs in 7 ranges; calibrated in dBm from -20 dBm to +10 dBm fs in 5 dB steps. Accuracy of ?1% of full scale on the 100 ?W range and above DVM output of 1.000 V (600 ohms load impedance) for full scale indication Calibration Factor Control to account for thermistor calibration factor External temperature-compensated thermistor mounts are required for operation (478A or 486A series, sold separately)