[rrd-users] Temperature Monitoring

Simon Hobson linux at thehobsons.co.uk
Thu Feb 16 09:00:41 CET 2012

Phil White wrote:

>To cut a very long story short, I am now of the opinion that I 
>should not set up my RRD store as
>   DS:$sensor:GAUGE:300:-50:100
>  simply because this is not the information that I am *really* 
>interested in - it is just the way that most people do it, because 
>that is the way we are used to thinking.
>I'll try to explain a bit better. I have several sensors - some 
>inside the house, some outside. When I started, the aim was to 
>monitor the temperature, and use this to automate heating. But, 
>rather than the *actual* temperature, I am actually more interested 
>in the *relative* temperature difference between the average outside 
>temperature, and inside - together with the rate of change in 
>temperature (which are probably related in some way).

I think you'll find that avg(a)-avg(b) = avg(a-b)

But if you did want to store difference, then keep the gauge data 
type, and store a-b - you'll probably want to store a or b as well if 
you want to see what the actual temperatures were.

Where it gets more interesting is if you are interested in other 
derived values - eg min or max. Here, max (a-b) is not the same as 
max(a)-max(b), so you must store a-b (and it's derivatives) if you 
want to see/use such data.

Simon Hobson

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