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! > > > > > > > > > > > > -- > > View this message in context: http://rrd-mailinglists.937164.n2.nab > > ble.com/CDEF-effects-entire-timeline-tp7583504.html > > Sent from the RRDtool Users Mailinglist mailing list archive at > > Nabble.com. > > > > _______________________________________________ > > rrd-users mailing list > > [hidden email] > > https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users > > _______________________________________________ > rrd-users mailing list > [hidden email] > https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users > > > If you reply to this email, your message will be added to the > discussion below: > http://rrd-mailinglists.937164.n2.nabble.com/CDEF-effects-entire-time > line-tp7583504p7583509.html > To unsubscribe from CDEF effects entire timeline, click here. > NAML -- View this message in context: http://rrd-mailinglists.937164.n2.nabble.com/CDEF-effects-entire-timeline-tp7583504p7583510.html Sent from the RRDtool Users Mailinglist mailing list archive at Nabble.com. _______________________________________________ rrd-users mailing list rrd-users@lists.oetiker.ch https://lists.oetiker.ch/cgi-bin/listinfo/rrd-users
