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Thread: Temperature reading fluctuation

  1. #1
    Automated Home Jr Member
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    Default Temperature reading fluctuation

    We have several temperature modules that generally work fine and whose readings give straight line graphs when the output is viewed through the behaviour property. I have one module though that gives a sawtooth graph. The temperature seems to oscillate by up to 0.2 of a degree between readings as if the sensor resolution is much coarser than the plotted data. There is no obvious physical reason for this, as it is in a bedroom, where the temperature might be expected to remain pretty constant as with the other temperature modules I have.

    It isn't exposed to direct sunlight and anyway I would expect most heat sources to cause a constant bias rather than cause these choppy data readings.

    Any ideas why this might be so? Do sensors normally show resolution differences and is this in tolerance or is the sensor faulty?

  2. #2
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    have you checked the polling interval, in the Properties dialogue box - maybe it's become set to a longer interval ?

  3. #3
    Automated Home Jr Member
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    I looked at that,but it is set to the default like all the others (00:05:00).

    What seems strange though is that in the Cortex main screen the value seems to change every 5 seconds (unlike all the others which change value fairly infrequently). In the space of a minute I get values between 19.99 and 20.34 for example. If the polling interval is 5 minutes, why is it changing so frequently?

  4. #4
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    modules are polled, but also shout when there's (eg) a change ...

    we've lots of sensors about the house - checking a few of them, just now, all show stepped lines with little ups & downs, all the time ...

    our heating system is not fully up & running yet, and we've no doors, yet ...

  5. #5
    Automated Home Legend Karam's Avatar
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    Just to clarify, the Cortex polling interval is generally more of a module health check. Almost invariably data is automatically transmitted by a module when it detects a signal change greater than the resolution setting. So a small resolution setting would imply that you'll get more regular readings because even small fluctuations are reported. For outside sensors we generally suggest a setting of 0.5C because the exposure to breezes is more likely to cause rapid fluctuations. But for indoors even 0.06C can be ok.

    Internally the module samples the sensor at ~ 4s intervals, so the observation that you are getting values every 5s suggests that it is seeing changes at every sample. What type of module is it? Might its mounting location be subject to breezes from behind, e.g hollow wall with some sort of draught source through wall, or colder inside cavity such that cold air mixes with warm air via the sensor body?

  6. #6
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    Yes, it is possible it is getting draughts coming from behind, through a cable duct. I'll check that.

    It is also helpful to know how the resolution setting relates to the self-reporting as well.

    thanks,
    Andrew
    Last edited by achapman; 1st May 2012 at 09:10 AM.

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