In thermal wattmeter, the series wire that is heated measures the current squared, not power. This is useful for measurement of the power to the load only if the load is resistive and the resistance does not vary because the power is I^2R. You could measure the voltage squared with a similar resistive heater/thermocouple placed across the load which would measure voltage squared. However, neither gives you any indication of the AC phase between the current and the voltage, and hence cannot be used to measure real power in a device that has inductance or capacitance (all devices have inductance and capacitance). Also, such a metering of current squared can only be calibrated accurately over a narrow range of power being measured - that is why they have different resistors. So to use such meters, you have to presume that you have no phase shift between the voltage and current (a point of residual equivocation and error). To insure this is a correct presumption, you need an oscilloscope to check. You are not getting something for nothing in this type of sensing. These are not recommended for our line of research for lack of calibrated dynamic range.
Instruments that compute power by sampling the voltage and current at high rate (PCE 830) are more accurate in general and have high dynamic range. They remain accurate as long as there are no high frequency components at or above about 5/(instrument sample period). That is what the oscilloscope is used to check, or you could use a spectrum analyzer. If you have high frequency components that can create errors, you get an instrument that samples with a shorter period. Bob Higgins On Mon, Oct 27, 2014 at 2:43 PM, Jed Rothwell <[email protected]> wrote: > Bob Higgins <[email protected]> wrote: > > This is not my understanding. Most wattmeters that are implemented as you >> describe require the wattmeter to be the entire target load. Otherwise, >> there is no error-proof way to know how much power is dissipated in the >> wattmeter in relation to that which is dissipated in the device. >> > > The resistor is in series with the load, as I said. The meter also > measures RMS voltage and amperage. The resister is small so the temperature > changes are very rapid and small. Years ago, this is how some > industrial-use meters for high power worked. > > Okay, this is called a "thermal wattmeter." You can look that up. See, for > example: > > https://www.yokogawa.com/ymi/tutorial/tm-tutorial_wt_12.htm > > Some of them have two resistance heaters, with different resistivity. > > > >> Typical wattmeter instruments have a similar deficiency as was ascribed >> as an error by McKubre - I.E. taking the average of the voltage and the >> average of the current and multiplying them to get the power at a sample >> time. >> > > All of the watt meters do that as well. As I recall, they had three > methods of measuring the power, which were used at different power levels. > > - Jed > >
