Directional wattmeter

Another device which may be very useful is called by various names such as a reflectance bridge, a standing wave meter, or a directional wattmeter. It can be used to measure the impedance match between a coil and its voltage source, for example, the transmitter or the decoupler, by monitoring the power reflected at an impedance mismatch. 

Directional wattmeters are not very expensive and can be used to monitor the state of the impedance match under actual operating conditions. Thus, they are extremely useful when the impedance match is susceptible to changes, as when a sample absorbs so much power from the decoupler coil that the coil becomes detuned from the heat. 

Finally, the decoupler circuit can be tuned with a directional wattmeter, after the coil temperature has stabilized under power. 

Digital wattmeters are available for both AC and DC applications for both single and multiphase measurements. Without using external components, the ranges can extend from a few milliwatts to tens of thousands of watts. Direct current wattmeters are the same as single-element AC wattmeters, but it is advisable to calibrate them for use on DC circuits. 

A wattmeter, however, automatically takes the phase angle into account and computes power directly. This provides an accurate measurement. Wattmeters are relatively inexpensive and can be installed by any electrician. Several measurement ranges (numbers of coils) can be built into one unit so that the same wattmeter can be used on pilot and plant-scale equipment. 

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