We are afraid that we can not receive any standard calibration for small power(small current condition) below to 1W(at standby mode of electric equipment) in Japan at present time. Official Calibration Lab, JEMIC(Japanese Electric Machinery Inspection Corporation) can calibrate below to 10W only. Therefore, Yokogawa tried to show the guideline of accuracy for small power as blow. It consists of summation of Voltage error, Current error, Phase angle error and special calculation error of the power meter.

Also, all WT series can measure small current below to 2 or 3% of the range rated value as below graph. (It shows 500mA range of WT110.)

An example of linearilty error (0.5 A range)

We should pay an attention of reduction for the Instrument Loss in case of low power measurement below to approx. 0.5W. It causes of impedance of voltage input circuit. If the impedance is 1M ohm and 240 V input, it influences +0.058 W, more than 10% of target.
In such case, we should check the Wiring Method described in the Instruction Manual as below;

Recommended Wiring method for small current measurement

When current waveform is distorted and has big Crest Factor characteristic like condenser input type switching mode power supply, measuring current range should be selected properly. Otherwise, measured value is not accurate because peak part of the waveform is neglected or bigger current range is selected.
In such case, Energy Star Program decides to set proper range after measuring peak current and selecting suitable range.

Object : TV, VTR, PC Printer, Copying Machine, FAX Machine

When trend of power(or current) level is fluctuated as below figure, some power meters can not detect and measure small or rapid change of power(or current). Especially, it influences Watt-hour(Integration) measuring function. Therefore, we should check lower limit and response time of power meter before use it. Refrigerator, PC

Generally speaking, voltage and current waveforms of electric equipment are distorted recently. Therefore, broad measuring bandwidth is required.
In order to measure target within 0.1% accuracy, triangle waveform requires measuring bandwidth by 5 times to the fundamental frequency and 200 times or more for square waveform theoretically. Also measuring accuracy at the high frequency should be paid an attention.
However, frequency bandwidth depends on the bandwidth of lower frequency parameter as explained in this document. Therefore, frequency bandwidth is not first priority for distorted waveform.

Common Mode Voltage ; It locates between earth ground and target equipment and oscillated voltage is added to both Hi and Lo terminals commonly.
If impedance between Hi terminal loop(R1+Z1) and Lo terminal loop(R2+Z2) are different, current becomes different in the two loops and it occurs differential voltage between input terminals and causes measurement error.

Some equipment using intermittent operation(or control) method in order to reduce power consumption at standby mode requires long period measurement and averaging function till next oscillate as below figure.
Generally, power measuring instrument has settled measuring period and update cycle. Therefore, it should be set the longest period setting when it measure such equipment. And if it is not enough, averaging function should be set on.
It is important to set averaging period according to input waveform like PZ4000.




YOKOGAWA PZ4000

Observation period of input signal