Has anybody had experience with either of these units? How they perform, accuracy wise, with small loads < 10W?
A lot of my work is in energy efficiency and I would like to have a decent instrument for measure the power consumption of AC devices. I have a few Kill-A-Watts and they are fine as far as they go, but they are pretty inaccurate. I have units that differ by more than 5%. Also, they are not very helpful for very small loads like the "vampire" loads of many standby power supplies that have relatively high crest factors.
In a prior job I had access to a lab with some beautiful power measurement equipment, including some multi-channel Yokogawa machines that could do thee (or more!) phases, work with or without CTs, etc, all with many digits of precision.
I don't have access to that gear today, but would love to have something that Didn't Suck and could still do decent measurements.
Is there anything else out there under $500?
-- dave j
PS -- I haven't tried rolling my own, but I have tried using a two channel oscilloscope with one channel connected to a CT, the other measuring voltage, and using the math function to multiply. Pretty sketchy setup, but kinda works. But scopes have 8b or less resolution, so it's not a great solution.
PPS - Anyone have any idea how to get a hold of an HPM-100A in North America? They don't even appear on E-Bay
Just bumping here.
Really, nobody here has an AC power meter they want to talk about??
I've not found anything, so I built my own. There was a really old Yokogawa on eBay a few months ago for <$200. I doubt it's still up.
Skipjack,
I'm definitely interested in how your built your own. What kinds of ADCs did you use, how fast, what precision, etc. Also, how did you calibrate it?
I made a power meter a couple of years ago using an AVR and its 12b ADCs, a CT for current and a resisitive divider and clamp for the voltage. Put the AVR ground on neutral and powered it with a battery.
It worked, but not great.
I used RMS to DC converters, and then just used a 12 bit ADC to read the DC value. This gives about 0.5% accuracy for the RMS conversion (at reasonable crest factors). Instantaneous power was measured by an analog multiplier and averaging circuit. Not the most high tech method, but it is reasonably accurate. Calibration was done by comparing the measurements with DMMs for the voltage and current measurements and by confirming that the multiplier and averager work well with controlled input signals (from a function generator). It's no Yokogawa, but it's good enough for what I use it for. Current sensing was done with a hall effect for the 20A range (I wanted to be able to see DC, so no CT) and a shunt resistor for the 3A and 0.3A ranges.
How about the Metrahit Energy?
Granted, it's a bit above your budget.
Oh, the Metrahit Energy looks perfect. I'd love to own that, as its a nice DMM in itself. But in the US it seems to be > $1200. Maybe if I hadn't already bought an 87V and 34401A.
This is frustrating. I guess because it's such a specialized need, these guys can charge a lot, because fundamentally measuring power is not rocket science.
I think I will try to see if I can get the Korean unit on Fleabay. It claims capability of doing low power measurements accurately. The clamp-on YEW is really meant for motors and other high power stuff.
Thanks for the extra leads, Dave!
A nice unit I'm sure, if you need the features, but the Metrahit Energy is way more accurate: 0.4%+20 counts vs. 1%+0.5W ... at a 5W load, that would be already a 0.55W or 11% inaccuracy, whereas the Metrahit Energy would be only 40mW or 0.8% inaccurate.
Yep, haven't looked at the specs, no idea about the Prova at low loads.
Oh, the Metrahit Energy looks perfect. I'd love to own that, as its a nice DMM in itself. But in the US it seems to be > $1200. Maybe if I hadn't already bought an 87V and 34401A.
This is frustrating. I guess because it's such a specialized need, these guys can charge a lot, because fundamentally measuring power is not rocket science.
I think I will try to see if I can get the Korean unit on Fleabay. It claims capability of doing low power measurements accurately. The clamp-on YEW is really meant for motors and other high power stuff.
The Metrahit Energy is actually cheaper in the US. Just over $900.
https://www.valuetronics.com/detail/New-gossen-metrawatt-metrahit-energy-m249a.cfm#.UxEb6mBfr4YThere is a cheaper alternative still, if you are patient, the discontinued Metrahit 29S comes up once in a while on eBay Germany, for about half of that price.
the discontinued Metrahit 29S comes up once in a while on eBay Germany, for about half of that price.
Between 250 and 300 Euro
Gossen charges 133 Euro plus shipping for the DAkkS recalibration.
Ooh, the Prova 8500 also synthesizes the test voltage at whatever voltage and frequency you like, so you can make sure all your DUTs are getting 240.0 exactly, or whatever.
That is a feature I had written off because I expected it was crazy expensive. Our old power lab had a machine, I forget what kind that could do that. It was really helpful for characterizing power supplies taking out mains variability.
If I need to measure below 0.2 Watt I use my Gossen MetraHit Energy or an ELV EA8000
Picture shows standby power of a 230V/USB adapter
Wow, the price of the EA8000 is certainly in the range. Too bad it's 200-240V only what with me stuck here with wimpy US mains power.