[rrd-users] rate per day graph from rate per hour DS

Simon Hobson linux at thehobsons.co.uk
Wed Jan 31 08:54:25 CET 2007

Ben Griffith wrote:

>  > I could try updating more frequently.  I think I'd have to
>>  adjust the rate multiplier accordingly.  So if I update every 15 seconds I'd
>>  have to multiply by 900 (number of seconds in an hour divided by the number
>>  of 15 second blocks in a minute) instead of 3600 to get the number of
>>  minutes the boiler was on in an hour.
>If you feel a minute to be close enough, by all means keep it this way.
>I thought about it some more and decided it would be a good idea to 
>collect at a smaller interval, and still report in minutes.  So I'll 
>probably change it to collect every 10 seconds, which means I'll 
>multiply by 600 to get the number of minutes per hour.

Does it matter THAT much ? All it means is that you will under report 
running time by an average of 1/2 minute at the END of each burn, and 
over report it by an average of 1/2 minute at the START of each burn. 
Assuming your start and stop time are random (thermostat ?) then 
these will almost certainly average out over a fairly short timescale.

Now if you have a boiler like the combi in my flat then you'd want to 
sample every second to get an accurate run-time, and even then you'd 
not get an accurate reading ! Being a combi it is sized to heat water 
to supply the taps - so it is vastly oversized for low flow rates (eg 
shower) or heating. To deal with this, most do do things - most have 
some sort of modulation so the gas valve isn't just open or shut (it 
can reduce the flame size) and if that is not enough they will cycle 
on and off quite rapidly (there is no large volume of water to buffer 
changes in temperature).

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