Page 476 - Instrumentation Reference Book 3E

Power measurement 459

W

2

'2
z =z =z. =I =I
I
1 2 3'1 2 3
P= w, + w2=3vIcosg
Figure 20.42 One-wattmeter method for balanced three-phase systems.

k,k2v(f)i(t)
Figure 20.4 3 Electronic wattmeter.
The error between the measured and true mean the sampling wattmeter can achieve a typical
values is given by uncertainty of 1 part in IO4 at power frequencies.
Table 20.8 gives the characteristics of an electron-
.
1 Y 12-1 IT I \
ic wattmeter providing digital display.
It can be shown (Clarke and Stockton 1982; 20.4.6 High-frequency power measurement
Rathore 1984), that the error of measurement is
given by At high frequencies average power measurement
provides the best method of measuring signal
amplitude because power flow, unlike voltage
and current, remains constant along a loss-less
transmission line. Power measurements are made
where E' indicates sunimation over those terms by measuring the thermal effects of power or by
where k . I ~ J Z is an integer, i.e.. those harmonics of the use of a square law device such as a diode
the power signal whose frequencies are integer (Hewlett-Packard 1978; Fantom 1985).
multiples of the sampling frequency. Static calorimetric techniques employ a ther-
Matouka (1982) has analyzed other sources of mally insulated load and a means for measuring
error in sampling wattmeters including amplifier, the rise in temperature caused by the absorbed rf
offset, sampled data. amplitude and time quan- power. Flow calorimeters consist of a load in
tization, and truncation errors. which absorbing liquid such as water converts
Continuous analog methods employing analog the rf power into heat, together with a circulating
multipliers are capable of providing measurement system and a means for measuring the tempera-
of power typically up to 100 kHz. The Hall-effect ture rise of the circulating liquid. Because of their
technique is capable of measurement up to the potentially high accuracy, calorimetric methods
region of several GHz and can be used in power are used as reference standards. However,
measurement in a waveguide. Using currently because of the complexity of the measurement
available componen; s with 15-bit A/D converters systems they are not easily portable.