[rrd-users] CDEF effects entire timeline

[SolarMan]

Hi Jacques,
Merci, yes I have been considering something like you suggest, using
python to calculate the delta-time more accurately than using NOW in
the CDEF or storing the power in the rrd database at data capture time.
Finally I ended up improving the data capturing timing (on an Arduino)
to provide the samples nearly exactly at a one-second interval so I can
discard the division.
Remains the question why rrdgraph does what it does, since it seems so
awkward...
Thanks anyway!
Regard,
hans

On di, 2017-02-14 at 12:49 -0700, Jacques du Toit [via RRD
Mailinglists] wrote:
> Hi
> 
> If I understand you correctly….I think you’re having issues trying to
> use NOW because rrdtool will snap to the 1 second grid skewing your
> calculations ever so slightly. I don’t think it’s really designed for
> that. It would probably expect DEF like ( TMPOUT, TMPIN,
> AMOUNT_OF_WATER) and then just graph the values with something like:
> 
> CDEF:PWR=TMPOUT,TMPIN,-,AMOUNT_OF_WATER,*,0.06978,*
> 
> And then let rrdtool calculate the value per second (which is what
> you have with your 1 second step).
> 
> But I can see that unless you measure at precisely 1 second intervals
> down to the millisecond your values will be slightly erratic as the
> amount of water will vary for each 1-second RRA.
> 
> Something I would try to do is to compensate for this using python to
> store the millisecond value since the last measurement (period =
> time_now - last_measured_time). Then do something like:
> 
> DEF:TMPOUT=…
> DEF:TMPIN=…
> DEF:FLOWRT=…
> DEF:PERIOD=…
> CDEF:PWR=TMPOUT,TMPIN,-,FLOWRT,*,0.06978,*,PERIOD,1000,/,/
> 
> But it’s a bit counter rrdtool to save the “step” in an rrd in this
> way. You might actually just be better off doing all the maths in
> python and just writing the PWR as a GAUGE value in rrdtool…that’s
> probably what I’d do, but I’m lazy with RPN that way… :)
> 
> Cheers,
>   Jacques
> 
> > On 14 Feb 2017, at 20:31, SolarMan <[hidden email]> wrote:
> > 
> > I have a simple application to determine the power generated by a
> > water
> > heater. One sensor (TEMPIN) measures the in-flowing cold water
> > temperature
> > while a second sensor (TEMPOUT) does the same for the hot water
> > exiting the
> > heater. The flow rate is constant and samples are stored in an rrd
> > database.
> > Using rrdgraph I want to display the time series of the
> > temperatures plus
> > the generated power in Watt. I planned to use a CDEF to calculate
> > the power
> > by multiplying the temperature difference by the flowrate and a
> > constant and
> > divide that by the time between two successive samples. A python
> > script does
> > it all for me. Samples come in nominally - but not exactly - at a
> > one-second
> > interval, therefore the CDEF does the division using a value
> > calculated in
> > python using rrdtool.last() just before every rrdtool.update() call
> > and NOW
> > in the CDEF as you might guess. Seems not all too complex you would
> > say....
> > BUT: I noticed that the entire Power graph (PWR) jumps up and down
> > over the
> > entire time range when the time-interval (hence the division)
> > changes and
> > that is clearly not what I want.
> > Here is the CDEF (as a python snippet):           
> > "CDEF:PWR=TMPOUT,TMPIN,-," + str(flowrate) + ",*,0.06978,*,NOW," +
> > str(prevtime) + ",-,/",
> > I spent many hours reading all the available material on CDEF, VDEF
> > etc. but
> > this fenomenon apparently has never come up - or this is a feature
> > "by
> > design".
> > How can I accomplish that the power is calculated and displayed
> > truly
> > momentary per step even when the measuring interval varies?
> > Thanks everyone!
> > 
> > 
> > 
> > 
> > 
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