[rrd-users] Modifying results for GPRINT output

Alex van den Bogaerdt alex at ergens.op.het.net
Sun Sep 2 14:12:18 CEST 2007

On Sat, Sep 01, 2007 at 05:56:54PM +0100, Piers Kittel wrote:

> I have just installed a bunch of sensor stuff my friend made for me  
> to measure my electricity usage.  The sensor tells the computer when  
> my electricity meter flashes, and a number in a text file is  
> increased by one.  Every minute, the value is inserted in a RRD as a  
> counter.  One flash from the meter represents 1/800th of a kW.

This means you have an increasing number, thus COUNTER or ABSOLUTE,
which counts 1000W/800 ticks. This is 1.25W per tick.

You feed this counter to RRDtool every minute, that's 60 seconds.

The way you set it up means that the database stores ticks per second,
not kWh or similar.

Suppose you see 60 of such ticks in a minute, that's 1 tick per
second, thus 1.25W/s. This is what RRDtool should display. This
means you should multiply by 1.25, not by 1250.

If you only see one such tick per minute, that's 1/60 tick per second,
thus 1/60 * 1.25 = 1.25/60 = 0.0208333... = 20.8333... mW/s.

Seeing "m" for milli isn't bad, it's expected.  That's because you
look per second, not per hour.  Try multiplying by 3600.

Sidenote: Watt is already something per second (Joule per second).
What you are probably measuring is kWh, not kW.  See wikipedia.

I am also monitoring this usage. Unfortunately I have to do so by
hand, as of yet I have not found a way to interface this :-(

So far, the numbers look OK to me.  I am also doing some math
and compute my next bill.  The calculated bill does fall in the
same range as the expected bill, so it's probably going to be
very close.

Alex van den Bogaerdt

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