Fluke Biomedical RF303 - Manual (Page 65)

A-1
Appendix A
Load Issues
Load Issues
The load resistors typically used in ESU analyzers are not ideal; they have
some reactive components that are frequency dependent. The RF303 Analyzer
derives applied power by measuring the voltage across the set load and
calculating the power (V2/R). Most other ESU analyzers on the market derive
the applied power by measuring the current flowing through a set load and
calculating the power (I2*R).
At fundamental frequencies below 500 kHz and regardless of the load setting,
the two methods of measurement are comparable. Above 500 kHz, and at the
extremes of the loads, the readings displayed by the two methods differ on
opposite sides of the expected value.
As an example, when testing the Conmed Excalibur Electrosurgical Unit in the
monopolar output with the Analyzer load set to 50 ohms, the set value on the
Conmed correlates with the displayed value on the Analyzer. When in the
bipolar mode, the Analyzer displays higher than expected values – up to 35%
higher. The same test performed on some current measuring analyzers
produces lower than expected values. This discrepancy is caused by the
difference in fundamental frequencies between the monopolar and bipolar
modes. The Conmed operates at 500 kHz in monopolar mode and 1 MHz in
bipolar mode. When comparing readings measured with a V2/R device to
those measured with an I2*R device, the total measurement difference is
usually larger than 35%, because of different methods of deriving power.
This difference does not mean that the Analyzer is malfunctioning or is in
error. Rather, it reflects the different results that the two measurement
techniques produce when the load deviates from the nominal value used in the
power calculations.
Note
Most ESU manufacturers use the current measuring technique to
calibrate production units.