> Thermocouple meters might be not so precise as a Bird 43
is, but power
> measurement is direct.
The thermocouple meter reads only one vector, current. It
does not measure voltage or phase. You have only one scalar
measurement and power is a function of the square of that
reading. That means errors are multiplied. A 10% meter
tolerance gives over 20% power error. If the load is
reactive or you do not know the exact resistance component
in a non-reactive load you can not obtain a useful reading.
A directional coupler meter reads from measuring both
current and voltage and considers phase errors between the
two. Load impedance does not have to be absolute if the
directional coupler is used correctly.
This is why a directional coupler meter has become a
preferred instrument and why measuring power with an ampere
meter has fallen from preferred use.
73 Tom
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