How to automatically measure duty cycle of an AC device?
January 11, 2014 11:11 AM   Subscribe

I have a mains-powered pump which switches itself on and off automatically depending on water level. I want to monitor the duty cycle of this pump (how long it is on for, and how long it is off for) without having to manually watch the pump and use a stopwatch. Is there some device that I can use to monitor the duty cycle automatically (for example, an intelligent socket that I could plug the pump in to)? It would need to have a readout that told me the length of the last OFF and ON periods (remote monitoring via wifi would be ideal). Alternatively, what would be the easiest way to build such a device? I'd be competent to investigate Arduino or Raspberry Pi based solutions, for example.
posted by beniamino to Technology (11 answers total)
This Kill-a-Watt+XBee project might be a good place to start.

When you say "readout" do you mean you want a dedicated physical display somewhere? Or is being able to check it with your computer good enough?
posted by caaaaaam at 11:27 AM on January 11, 2014

My first thought is a clamp ammeter linked to a PC or other logging device.
posted by Monday, stony Monday at 11:29 AM on January 11, 2014

You don't need to build anything. Just use a Kill-a-Watt meter to determine the watts used in one minute while the device is running and multiply by 1440 for 24-hours. Then use the meter to determine how many watts are actually used in a 24-hour period. The ratio gives you the duty cycle.
posted by JackFlash at 11:42 AM on January 11, 2014 [1 favorite]

Directly related to JackFlash's method, use an AC-powered "hour meter". So if you check once a week and it says 24 hours, then you're at about 1/7 duty cycle.
posted by flimflam at 12:05 PM on January 11, 2014

The Kill-a-Watt meter will also do an cumulative power-on elapsed time measurement as flimflam describes as well as being useful for a lot of other stuff. Either method should give you similar results.
posted by JackFlash at 12:22 PM on January 11, 2014

Rather than hacking something together from electronic components, I'd suggest starting with something that is at least partially engineered/integrated. Some options:

Open Energy Monitor

UbiQuiti MPower

Avtech also has some IP-based environmental monitoring solutions that you can add a current sensor to. I imagine they have competition too.

These might not do exactly what you want, software-wise, but could solve the data-collection problem for you.
posted by Good Brain at 12:26 PM on January 11, 2014

Another solution is to use a smart UPS that has a USB output. My Cyberpower UPSes all output how many watts are being drawn (to the nearest 5W or so) and you can monitor the power draw every second from various Windows/Linux/Mac programs. It's a filthy overkill solution, but for $60 you could have it done. (One caveat; the UPS only reports load as Watts, not Volt-Amps. That may matter if you're measuring a motor.)
posted by Nelson at 1:08 PM on January 11, 2014

I did this once with an audio recorder that could be set to record when the sound reached a threshold. My pump was loud and in an otherwise quiet location. A new file was recorded each time it was triggered to record. I let it run for 12 hours then looked at the time stamp and length of each sound file to get the date. It was a quick a dirty way to get the job done with equipment that I already had.
posted by jmsta at 1:41 PM on January 11, 2014 [1 favorite]

Wall-clock. Paper over dial, under minute and hour.hands. Pen taped to minute hand, drawing on paper. Look at squiggles on paper.
posted by zippy at 1:51 PM on January 11, 2014

The WeMo Insight Switch outlet.
WeMo Insight Switch can monitor your electronics and will send information about the device's energy usage directly to your smartphone or tablet. Perfect to pair with space heaters, wall A/C units, TVs, washers, dryers, fans, lights and more.
posted by Nonsteroidal Anti-Inflammatory Drug at 2:53 PM on January 11, 2014

Thanks, some great and imaginative answers here. I'll start with Open Energy Monitor, I think.
posted by beniamino at 11:11 PM on January 11, 2014

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