Looking for a watt meter

[Puddin]

Hey guys I was wondering if anyone knew of a good reliable watt meter to test display the power of any particular appliance. The list of what I need it to do is:

Display the Volts and Amps of AC power usage (Watts being optional of course with the previous two)

Must work with US power outlets

Needs to be able to read power going both ways. (power going in and out or positive and negative)

I need to be able to record the data somehow. Either with an SD card or through USB or something that allows me to see the power usage over time.

I've been looking all over ebay and just coming up with a bunch of junk. I don't have a bunch of cash so cost is definitely a factor.

[-Velocity-]

Hey guys I was wondering if anyone knew of a good reliable watt meter to test display the power of any particular appliance. The list of what I need it to do is:

Display the Volts and Amps of AC power usage (Watts being optional of course with the previous two)

Must work with US power outlets

Needs to be able to read power going both ways. (power going in and out or positive and negative)

I need to be able to record the data somehow. Either with an SD card or through USB or something that allows me to see the power usage over time.

I've been looking all over ebay and just coming up with a bunch of junk. I don't have a bunch of cash so cost is definitely a factor.

There are data monitoring/logging devices that you could possibly set up, but they can be expensive. A key requirement- over how much of a time window are you trying to measure at one time? What is the smallest acceptable time window? Your requirements are a little vague, but an old oscilloscope could fit the bill, and wouldn't cost much if used. You could measure the voltage and current (using a current-sense resistor and a voltage probe- a lot cheaper than an actual current sensor) and export the waveforms its collects to a computer. But the time window it collects over would be short. Just make sure the o-scope can handle 120 VAC, which is actually a peak voltage of (2^0.5)*120 = ~170 V

If you have a complex load (one with a significant imaginary/reactive component) you have to know the relative phase between the voltage and the current to compute the power, so simply measuring the current alone won't do the trick.

Of course, the other key requirement is cost, how much are you willing to spend? Anyway, I don't know a whole lot about watt meters, there's really not something fairly cheap you can just buy off the shelf?

Edited March 11, 2015 by |Velocity|

[Puddin]

I do have an old O-scope and multimeters out the wazzu. I can test it in my home just fine but I am looking for a compact alternative to test elsewhere. As for how much I am willing to spend? That is a hard question. I would say upwards of $100. I would say 1 day worth of sampling data would be a good minimum. Obviously longer would be better but one day should sallow for a one time download each day of data.

[Tommygun]

I don't think you will find anything that can record under 200 or 300 dollars.

Have you looked at what Ideal Industries has to offer?

http://www.idealind.com/products/test_measurement/

[-Velocity-]

I do have an old O-scope and multimeters out the wazzu. I can test it in my home just fine but I am looking for a compact alternative to test elsewhere. As for how much I am willing to spend? That is a hard question. I would say upwards of $100. I would say 1 day worth of sampling data would be a good minimum. Obviously longer would be better but one day should sallow for a one time download each day of data.

1 day of sampling data, but at what data rate?

https://learn.adafruit.com/adafruit-data-logger-shield

^^

I did find this cheap arduino data logger this summer when I was looking for something like it for work, but it didn't fit my needs and never bought it. It would record to an SD card. You'd have to set up some hardware and probably do a little programming, like, for example, make a signal conditioning board for it so that you could feed the Aurdino's ADC a signal that was proportional to average power and that didn't turn it into smoke. You'd have to isolate your analog signal conditioning circuitry from AC, obviously, but that's not too difficult. Basically, how your signal conditioning would work would be to multiply the voltage across the AC load by the differential voltage across a current-sense resistor (in series with the load) using something like this analog voltage multiplier IC-

http://www.analog.com/media/en/technical-documentation/data-sheets/AD633.pdf

Low-pass filter the output of this IC (you could use nothing more than a single-pole LPF using a resistor and a capacitor) so that you get the average power over like the last second or two, and have your data logger log that. I haven't done a project where I need to sense AC voltages, so I'm not familiar with how to best isolate your signal conditioning circuit from AC, but I could look it up if this approach interests you, and you don't already know how.

The advantage to this approach: it's cheap. You can get Advanced Circuits to make you a two-layer PCB for only $33, and get the parts off Digi-key and Mouser. The disadvantage- obviously, it's gonna be more labor-intensive, and you need decently advanced electrical engineering skills, but if you say you have a bunch of O-scopes and probes, maybe you do.

Edited March 12, 2015 by |Velocity|

[cantab]

Sounds like rather demanding needs. I've got a simple plug-in meter knocking around somewhere, but that just records kWh and shows them on a little LCD display, there's no fancy computer connection stuff.

[vger]

Why not just run your home in a controlled setting (no refrigerators flipping on and off, for example) and then watch your kW meter. Do it once with the device on and then again with it off. Record how much energy is being used in both cases, subtract the difference, and you can figure it out.

[Tommygun]

Would something like this give you enough information?

http://www.amazon.com/P3-P4455-Power-Monitoring-Dummies/dp/B00685UBYQ/ref=sr_1_8/181-2848632-9120515?ie=UTF8&qid=1426132051&sr=8-8&keywords=kill+watt+meter

[Beowolf]

Would something like this give you enough information?

http://www.amazon.com/P3-P4455-Power-Monitoring-Dummies/dp/B00685UBYQ/ref=sr_1_8/181-2848632-9120515?ie=UTF8&qid=1426132051&sr=8-8&keywords=kill+watt+meter

I have what's probably the previous model of this device and have been happy with it.