[rrd-users] Dynamic Steps

Oliver Loch grimeton at gmx.net
Wed Apr 17 21:25:19 CEST 2013


> One 'tick' of your device is equal to 3600 Joules, which you consumed during 
> the time between the previous tick and the current tick.
> That is why you use ABSOLUTE, see the documentation on what this does.
> For example, you get one tick after 10 seconds. That means you used 3600 
> joules in 10 seconds (which is an average of 360W during 10 seconds). When 
> you update your database, RRDtool computes a rate of 360 and works with that 
> during the normalization and consolidation phases.
> See my site ( http://rrdtool.vandenbogaerdt.nl/ ) for some information.

Thanks a lot for pointing me into the right direction. I read the part about "Rates, normalizing and consolidating" and it all became clear :)

> Your step size is important when considering how much detail you want/need. 
> You could set it to 1.

I've set it to one and every time I get a signal I write 3600 to the database. and the average power is shown just fine. 

One problem I hit now is that there can be more than 3600 ticks per hour. Calculating the VA by the breakers and the usual current I end up with 3840 (240V*16A) and 7680VA (240V*32A). I highly doubt that the power consumption will ever hit the peak level, but levels above 3600 are possible. How should I store that information into the database? Steps smaller than one second aren't possible. Should I double the value that is written to the database? E.g. with two signals per second 7200 instead of 3600 or is there a way to represent multiple entries per second? 

Thinking about the unix timestamp I wonder if rrdtool then calculates the average of multiple values per second and just stores that average to the db?

> Your heartbeat value should be large enough to not miss updates that happen 
> to be far apart. If all consumers in the house are off, almost no power is 
> used. Your measuring device itself is the only one. Compute how long it 
> takes for it to consume one Wh, double that time just to be sure, and set 
> that as your heartbeat value.

I set the heartbeat to 14000, as the device uses 0.5w/hour which gives me 7200 seconds between ticks.



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