Three AC plug-in Power meter comparison – calibration inspection

[Kiriakos GR]

I do own since 2/2017 my plug-in AC power meter, and at 7/2020 I did receive and a second one as dead gift, which I did succeed to repair and revive.
Therefore so far I had two comparison points.
Today a friend gave me as loan his own for me to test.

AC load this consists of two 150W bulbs (halogen spot lights) = 300W the theoretical total
In praxis spotlight No1 this consumes 146.5W at 235V and 149.5W at 238.70V.
The spotlight No2 this consumes 154W u at 235V and 157W at 238.70V mains input.
Therefore at 235V input A+B= 300.5W
But the input AC from mains this having regulating average 238.70V so now A+B= 306.5W

I did my testing with extreme patience so to record all values when ACV averaging at 238.70V by measuring constantly due BRYMEN BM869 DMM.

The loan power meter will be named as PM-Green.
The by Kiriakos repaired power meter will be named as PM2% because this has 2% higher measurement at ACV.
The third Kiriakos power meter which I got new, this will be named as PM-ITTSB.

The test procedure was as follow:
All power meter to measure first the A load (150W) when voltage input was very close to 238.70V.
The test repeated with A+B load = 300.5W   
 
PM-Green delivered measurements:
150.4W, 239.4V, 0.627A, PF 1.00
309,5W, 239.8V, 1.290A, PF 1.00

PM2% delivered measurements:
150W, 244.3V, 0.609A, PF 1.00
306.0W, 243.7V, 1.255A, PF 0.98

PM-ITTSB delivered measurements:
149.2W, 238.2V (238.70V DMM), 0.622A, PF 1.00
305.2W, 237.5V, 1.284A, PF 0.99 ~ 1.00 constantly fluctuating

It is very strange for my eyes, to see PM2% to come close to PM-ITTSB at the Watts power when it voltage measurement this is at the high side, and the current measurement this be at the low side.
In simple words the watt measurement comes close due error at voltage and current measurements, because the AC current measurement this is at the low side (factory calibration adjustment).
Voltage is at the high side +2.2% but lower AC measured current this seems to compensate.
The verdict is that PM2% only it watts measurement this is closer to real world.

About PM-Green, ACV this is at the high side by 1.5V, ACA current at 150W this is 5mA away from PM-ITTSB, and at 300W the current this is 6mA away.
PM-Green Watt measurement this appears close to five Watts at the high side at 300W load and 1W at high side at the 150W test.
At PM-Green with 1.5V and 6mA at the high side the calculation
1.5V x 0.006mA = 0.009 VA it does not deliver meaningful results of why and how this deliver watts measurement at the high side.
 
To summarize, this comparison just gave me a better understanding of how they stand the meters that I have right now in my hands.
If we speak about spotted peculiarities, PM-ITTSB meter this comes first because at 300W the Power factor indication fluctuates, while at 150W this is stable at 1.00, the logical is to see PF fluctuation at low power instead of measuring higher power.
Both identical meters they demonstrated unstable or lower PF at the 300W test.
PM-Green delivered much stable and close to expected Power factor at both tests, and I say close to expected because the load was 100% resistive.

By removing PM2% meter from this equation, PM-Green along PM-ITTSB meter, they are closer to its other.
The makers of such power meter they are faceless, PM2% meter this where distributed by HAMA Germany, and their documentation speaks of Accuracy at +/- 2 %.
Therefore ACV +2% at the high side this seems within specifications to them.     

In conclusion, we have to accept that within low cost products for consumers we should not expect tight tolerances and measuring accuracy down to the last digit.
I did promise to my friend with the PM-Green a calibration report, and now it’s at public view, so all ITTSB Blog readers to gain awareness, and readjust their expectations too when they spend  12~15 EUR for these meters.       

[Kiriakos GR]

I thought that I was cheated due the fake specifications, and my two power meter does not record Min-Wattage below the 2Watt mark.

Now and the third PM-Green, my new sample  this cannot record anything either.
Lets rewrite the users manual.
Since 1.8W and higher these meters can measure watts.
Since 2.1W and higher these meters can record minimum watts.

Anything between 0.00W up to 1.6W this is thin smoke and not a measurement that you can trust.
Power Factor measurement this contain glitches, and or irregularities, and or software design flaws.
For a first time at PM-Green meter, I did see PF of numbers range of .30, actually PF 0.34
My two power meters they jump from PF 0.20 at PF 0.40 as next measuring decade, PF 0.30 decade does not exist.

If these was a serious product, a new Firmware revision this it might debug them.
But in this case we have to say ... it is of what it is. 
   
For many years I am looking for a plug-in power meter with good sensitivity at below 5W.
This is my wish-list to Santa Claus, because now I did lost hope that Chinese developers haves access at such high-end technology.
     
1.8W at 230V = PF 0.40 and 20mA of current. 
Therefore and by approximation,  these watt meters they are blind at anything below the 15mA mark of load consumption.

[overvolt]

Testing of three samples makes always the test results as 100% reliable.

This work worth a donation from our readers for the blog.
No one did such a detailed testing before.
 
 

[Andrew-88]

The problem at such meters is the slow sampling.
At stable resistive loads they seem to do the trick.