The 8502AT is an 8502A that has been altered to operate at 400 Hz ±5% and has a Thermal True RMS Converter module installed. In addition the -02 Ohms Converter 04 Calibration Memory, -05 IEEE Interface and -08 A Isolator Options are included as standard. The Thermal True RMS Converter module allows the operator to measure the true rms value of an ac signal at accuracies of 50 ppm with a maximum response time of 6 seconds. This response time compares favorably with that of existing Thermal Transfer Standard which can take up to 5 minutes to complete a measurement. Multimeter operation at 400 Hz results in readings that are 5% slower than the standard 60 Hz unit. The 5% slower readings are based on timing of 57 Hz which occurs when the input line frequencies are at 400 Hz ±5%.