I work in a Metrology Lab and perform calibrations on a wide range of power meters. I am a little concerned with some ambiguities I am seeing when I am measuring the "Power Reference" output levels of power meters utilizing the procedure called out in most of the service manuals. Per the procedure I am using a model 432B Power Meter and a model 478A Thermistor Mount, both freshly factory calibrated and certified and a calibrated 34401A DMM. I am comparing this measurement against one I make using a freshly factory calibrated and certified E4419B and several factory calibrated and certified 8480 series power sensors. I just performed these measurements on an E4418B power meter and using the method called out in the manual calculated it to be 0.981 mW which means I would need to adjust the power reference to meet spec. However, when I measure the power ref. with the E4419B and power sensor I get a reading of 1.001 mW whic is almost perfect. This is quite typical of my results on all the power meters I calibrate. Why the desparity between methods and is the alternate method I use acceptable?