How accurate are these? Just bought one - it's one with a magnetic clip which goes around the live load cable coming out of the meter. It seems to read about 20-30% under at low usage (500W) compared to the meter, but more accurate at higher usage (2-9kW). I've tried another one which gives the same results. Could I have a faulty meter? Or do these things usually read under at low usage?
TIA
-- Andy
I'd guess so - most of these inductive or whatever coupling things tend to not be accurate at smaller currents.
Andy Pandy formulated the question :
They are basically just a clamp meter, as used for measuring current. Clamp meters are not the best accuracy and they can depend upon the type of load, but I would expect much better accuracy than 20 to 30%. Have you tried it with a resistive load like a small heater or tungsten lamps?
Just looking at eBay, seems you can buy real used electricity meters for not that much, (e.g AMPY SINGLE PHASE ELECTRIC METERS OFGEM APPROVED Item number: 350195932183 for £15 delivered). Now these things have an optical / infra-red port that talks IEC 1107.
Elektor had a project & free software published in March 2002 that can read it to a PC serial port.
That article is here at a chinese site
Now the sticking point is that the meter may have password security checks that may get in the way of doing the meter reading remotely. Or perhaps not.
Well, fool me has just flung some coppers from Paypal (to another eBay seller than listed) to find out. It will probably end up on the pile of things bought to do a project but ...
:-)
With lights as the main load (about 1.5kW) it reads about 20% under, with a fan heater as the main load (about the same, 1.5kW) it reads about 15% under. With the kettle or shower as the main load it seems pretty accurate, although the usage is obviously much higher.
-- Andy
The problem is that these devices all use split core CT's (Current Transformers). The splitting of the core will produce inaccuracies. No matter how good the joining of the core, there will always be small gaps between the two surfaces which will affect the flow of magnetic flux within the core. The reason for using them is that they are very easy to fit around existing installations.
The best solution is to have a non split CT. You would have to isolate your mains supply, disconnect the live tail, poke it through the CT and then reconnect the live tail.
It could also be a phase difference issue. Are you using the same type of load to produce the readings at 500W and 2KW-9KW?
No - 500W was mainly lights, telly, fridge, freezer etc.
2-9kW was kettle, shower.-- Andy
Sooo, making some assumptions here, lights could be CFL? TV - maybe has a switch mode power supply? Were the fridge & freezer actually running at the time?
Kettle - simple heating element & shower - electrical heating with no motor?
I'm not saying that this is definately the cause just pointing out the differences.
I would like to get a remote monitor and have wondered how accurate they are.
How do you know it's the monitor and not the supply meter that is wrong?
Mostly.
Not sure. How do I tell? It's a fairly new flat screen (probably about 2 years old). It takes a few seconds to respond when switching on.
I think so but not sure.
Yup - no motor.
OK thanks.
I don't - that's my main concern! I may have been overcharged for 10 years...but before taking it up with the electric company and having them laugh at me I thought I'd check whether it's usual for these monitors to be so far out.
As it happens they've just written to me saying the meter needs changing...
-- Andy
on 28/08/2009, Andy Pandy supposed :
All modern TV's use a a SMPSU, anything made in probably the last 15 years.
I would agree with that.
I have a CurrentCost CC128 (the newer model) this has TTL level serial data dribbling out of the back that I've only recently connected to a PC to log/plot the data.
Not really checked the accuracy, but our "rapid boil" 3kW plated kettle only adds 2kW to the useage when it's on, I'll have to find my plugin power meter and see how that compares... Having said that the overall daily use is pretty close to what the bill works out at, certainly not the gross error of 2kW v 3kW.
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