The importance of continuous sampling

[Frogmore42]

@dBC

I would encourage you to do the same in reverse. Capture a trace from a real load that demonstrates when DM performs better than CM. At least then we’d have something in each hand as we weigh the balance of the virtues of each. At the moment it seems to be all downside in one direction, and all upside in the other. People speak of the “cost” of doing CM. The only cost I can identify is to channel bandwidth (*).

I believe this post is the one that restarted this discussion:

ESP8266-12E Devkit for Current & Voltage but only one analog input

I’d like to point out that the we referenced above does not include me. I was censored. I had posted a rebuttal to the suggestion that frequent random sampling was inaccurate and pointed out that it was lacking practical evidence. At first they changed the subject to this oversampling argument, then removed my post completely. Regarding the oversampling argument. I get that there is no significant energy in the harmonics and that capturing them does little to the prima fascie power measurem…

This section in particular:

One major design goal was to be able to use any VT and any combination of CTs in the 14 inputs without the need to reprogram. This required a different approach to phase correction. The buzz at the time was a paper that described a method that I think may still be in use in some of the Emon devices. I’m not finding fault with the method as used, but it doesn’t really lend itself to the any-to-any approach. That paper starts out by explaining that the method described is needed if resolution of the samples is not fine enough to correct phase errors by simple sample shifting.

I believe this is the reason why Bob chose the design he did. I am sure there are other ways to do it, but when I see something like this :

New emonpi install - interpretation/elimination of power offset

Hello. [32] I’ve installed an emonpi to a domestic supply. I’ve attached the CT sensor to the “red” line leaving the meter, and connected the AC-AC voltage sensor adaptor. The measured power appears to be dimensionally correct. It detects a 1kW kettle, c. 100W of lighting, and what I interpret as the fridge cycling with a 50W load. However, the whole things appears to have a negative (approx -40W) offset (see image). Can anyone advise what I may be doing wrong? I presume I can add the offs…

I don’t think you are doing anything wrong. It will be fairly hard to track down that “offset” error.

My money would be on Robert’s theory that a small uncorrected phase error between the VT and CT is pushing a highly reactive load(s) into the next quadrant, making it look like a 40W generator. If you wanted to try and determine which load, you could flip breakers and/or turn things off at the wall to see if you can make it go away. As Robert says, it’s likely to be something with a standby mode. The big culprits in my house are the induction hob, the microwave oven and the TV.

So, help me understand, because I am confused. It seems using CS as implemented by the emonPi it is okay/possible to have a 40W offset for a real world load for which the recommended solution is to just subtract 40W from the total?

I don’t know if this is a common occurrence using emonPi or not. But I do know that I would be very worried if I ever saw something like that with anything that I was using to measure power. I have seen a 7W apparent power offset with my Kill-A-Watt when nothing is plugged in. I expect it is not very accurate at low power levels, so don’t use it for that. Despite the fact the ecm1240 doesn’t work with my generator power, it never did something like that.

When the STM based design comes out, I know I will look at it and seriously consider getting one. But, I needed something now, not 3,6,9,12,24 months from now. The perfect design you can’t buy yet is much less useful than something that works and is available today.