Rebuilding an energy meter for higher measuring accuracy.
With energy meter PM230 (brand: Brennenstuhl) energy consumption of connected
appliances can be measured.
The measuring range is from 5 to 3500 Watt, the meter measures in 10 mA steps,
which at 230 V corresponds to 2.3 Watt (at Ohmic load).
When the load is below 5 Watt, the meter indicates "zero".
I wanted to use the meter also at power levels below 5 Watt, and also have a
higher accuracy of the measurements.
To reach this, the meter is rebuild as follows:
In the meter is a measuring resistor, through which the current of the load
is running.
The voltage across this resistor is proportional to the current through it.
By increasing the value of the resistor 10 times, the reading of the meter will
become 10 times higher.
The meter then can measure from 0.5 Watt, with 0.23 Watt accuracy.
You must divide the reading of power or current by 10, to get the real value.
A disadvantage is, that the rebuild meter is no longer useable for high power
levels.
The original measuring resistor is in this picture indicated by the arrow,
and has the shape of a wire bridge.
The resistor value is 0.0033 Ω.
With a normal multimeter such a low resistor value cannot be measured directly.
To determine the resistor value, I send with an adjustable power supply
exactly 1 A DC through the resistor.
And then measured the voltage across the resistor, which was 3.3 mV, from which
follows the resistor value is 3.3 mΩ
The most easy way to remove the wire bridge is, to first cut it in two, and then solder both halves out of the PCB.
Then I placed three 0.1 Ω (1 Watt) resistors
parallel, on the location of the wire bridge.
The 3 resistors in parallel have a value of 0.033 Ω, so 10 times higher then the
original resistor.
To prevent the meter and resistors from overloading when to high loads are
connected, I have placed a 1.2 A fuse in one of the electrical wires.
After closing the meter, the accurate energy meter is ready.
The meter in action. The displayed value must be divided by 10, so in this case about 1.676 Watt is consumed by the load.

The rebuild meter is not suitable for measuring load currents above 1.2 A.
Also appliances which have a high inrush current (TV's, refrigerators, electro
motors, etc.) must not be connected.
For measuring higher powers, I also have such a meter in the original state.
Measuring the cosine φ
The power (P) an appliance is consuming is equal to:
P= U x I x cosine φ
Where:
P = Power in Watt
U = Voltage in Volt
I = Current in Ampere
φ (phi) = The phase between voltage and current.
The value of cosine φ can vary between:
"1" , when voltage and current are in phase.
and
"0" when voltage and current are shifted 90°.
The energy meter PM230 needs a certain minimum current, to measure the phase
between voltage and current correctly.
When the connected appliance consumes too less current, a cosine
φ value of "1" will be taken, while in real this
might be lower.
Through this, the meter can indicate too much power at low power levels.
To determine when the cosine φ is measured
wrongly, I loaded the energy meters (both rebuild and original) with several
capacitors.
A capacitor has a cosine φ of (almost) zero.
The following table gives the results of the
measurements:
So, a cosine of 1 indicates there is a measuring error.
capacitor value (nF) 
current (A) 
cosine φ original meter 
cosine φ rebuild meter 
47  0.003  1.0  1.0 
100  0.007  1.0  0.03 
200  0.014  1.0  0.03 
470  0.034  1.0  0.03 
1000  0.072  1.0  0.03 
2000  0.145  1.0  0.03 
3000  0.217  0.03  0.03 
The rebuild meter is capable of measuring the phase correctly at much lower currents.