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Thread: Black Bulb Temperature Sensors

  1. #1
    Automated Home Guru Nad's Avatar
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    Default Black Bulb Temperature Sensors

    Hi all,

    It's been a while and it seems that things have gotten a bit quiet around here

    I was wondering if anyone has any experience in the use of Black Bulb Temperature Sensors to get a more accurate reading to set the HVAC settings in Cortex?

    I have multiple temperature sensors (usually 2 or 3) in a room but we find that we seem to adjust the settings more often that I would like to. The existing sensors are usually located in a DFP near the door with a couple ceiling mounted PLTs located at the opposite end of the room to the radiators. I'm not sure if this is the ideal arrangement but we normally see 3 or more degrees difference between the wall sensor and the ceiling sensors. So was wondering if we could use the Black Bulb Temperature Sensors to get a more accurate reading for what the room temperature is.

    Any thoughts and ideas would be greatly appreciated.

    Thanks,
    Nad

  2. #2
    Automated Home Legend Paul_B's Avatar
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    Hi Nad,

    I recall Chris is using some black bulbs for measuring radiant temperature. I did a quick search and this post came up with a product link - http://www.automatedhome.co.uk/vbull...ll=1#post16598

    HTH

    Paul

  3. #3
    Automated Home Legend Karam's Avatar
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    As a general ramble:

    I have no experience with black bulb sensors but I'm a little bit sceptical on a theoretical basis until someone can provide practical experience. As mentioned, I believe Chris has some but I don't know what he has found.

    From a theoretical viewpoint I think there are so many factors that could affect the feeling of temperature that measuring radiant heat at the location of a black bulb sensor may be worse in some circumstances than any better. As a starting point, I don't think either that it is about the absolute accuracy of sensors but rather how representative they are of the bulk air temperature even if it is with a (constant) offset. A ceiling sensor will almost always give a higher reading than a shoulder level one because heat rises and flows across the ceiling. This also means that the ceiling sensor will also likely be subject to more variation (non constant offset) and therefore even less representative. Shoulder height at an inner wall away from heat sources is usually a good place. Next to doorways is a natural location for modules since they are likely to be doing other things like user interface. This is not too bad a location unless your doorway leads to a room with a significantly different temperature, in which case the temperature at that location may be significantly affected by that of the other room. I know I have one in our lounge which has only a short distance through a hallway to the front door and if the front door is opened on a windy/cold day then a significant dip can be seen in the lounge. However such situations often sort themselves out in the sense that such a dip is quite short lived, the heating may try to respond but then be shut off fairly quickly so amount of heat added probably compensates for amount removed through the door anyway. This however depends a bit on your control setting e.g whether you have long run on periods, slow valves and so on. You can of course inhibit the heating control for a while around a door open event..

    Another factor is psychological. I know in my own household that there is sometimes (ha ha) disagreement about whether it feels too warm or too cold - usually amongst adults only. Sometimes it is justified by what I can see in the data, for example the temperature has indeed undershot/overshot by more than usual for one reason or another. At other times not. Since I do have multiple sensors in some areas which consistently track each other I can also tell that it is not an anomaly with the temperature controlling sensor. One other thing I have noticed, and not just in my own household, is that because there is an automation and measurement system in place it creates heightened awareness and reduced tolerance to deviation. You start to get situations where common sense fails. Its a a sunny day and warm in the house. The immediate reaction is - why is the heating on? In fact it isn't and hasn't been for the last x hours but doesn't stop the expectation that the temperature should be under total control, guess I will have to fit air conditioning next ... Then there is the partially justified version of this - It was grim earlier but now it's sunny and the room temperature has leapt up. The radiators are still feeling warm from the earlier heating stage. "Its hot in here!", feeling the radiators, "Why is the heating on?"...

    What might it be possible to do:

    Of course one can imagine using data from light level sensors or indeed external black bulb sensors to help adjust set points in a kind of pre-emptive manner. Even to use weather forecasts... These ideas always sound good for promotional blurb but when you come to practice them, unless you are very careful, you find that you open yourself up to more occasions for getting things wrong. Unfortunately if you have a system which presents you with the data as well then this becomes all too apparent.

    I think the first thing to do is try and work out what is causing the present 'failures' and see if you can do something specific for that. Obviously there is plenty of data to look at but not always so easy to correlate this to feelings or particular events.

    Anyhow the kind of tools you have:
    - Utilise min/max/average feature if multiple sensors are providing the feedback and choose included sensors wisely
    - Utilise the temperature object compensation function (see temperature compensation in Help). This is really just to help where a sensor has been put in a poor place such as an outer wall (where its reading is much more influenced by external temperature than internal)
    - Utilise a low pass filter (see logic objects) to 'slow down' twitchy sensor signals which might be leading to noisy heating control.

    More elaborate:
    - Switch between sensors at different times or under different conditions (I have implemented such a scheme on a pair of external sensors which were necessarily facing East and West)
    - investigate whether some pre-emptive signals can be used to make temporary set point adjustments or even to manipulate a temperature signal proxy (virtual sensor) which finally gets used for the controller.
    - introduce yet another sensor (yes I know we shoul really have wireless ones ...)
    - investigate the advanced controllers option, though I have to say that although we introduced these over 10 years ago I don't know of many(any?) people using them including myself I hate to admit - old plumbing and mechanics on my heating not so amenable to the necessary PWM regimes.

    Just things that came to mind, I'm sure there are others ..

  4. #4
    Automated Home Legend chris_j_hunter's Avatar
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    we’ve installed Titan BB sensors in all three of our main Rooms, wired to QAIs, in addition to multiple temperature sensors, whose readings we average & adjust using GeneralLogic and feed to single virtual sensors ... so far, we keep an eye on the BB readings, but don’t use them, mainly because limited QAI resolution leads to disappointing results ...

    when we’ve some time, we maybe will revisit this ...

    Chris

  5. #5
    Automated Home Legend chris_j_hunter's Avatar
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    comfort-wise, we’ve found radiant room fabric (walls, floor, ceiling, furniture) temperature & convective air temperature are equally significant factors affecting our comfort - our MVHR system (includes ASHP) needs its in-line post-heater (fed from the UFH arrangements) else we get air temperatures that are too cool ... we run MVHR & UFH 24/7, using multiple HVAC objects (one for the post-heater & one per floor for the UFH), with target temperatures varied during the day (cooler overnight & first-thing, warmer in the evening) and to control priorities (so they don't all call for heat at once, unless desperate) ...

    Chris
    Last edited by chris_j_hunter; 25th May 2015 at 09:30 PM.

  6. #6
    Automated Home Legend Karam's Avatar
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    Quote Originally Posted by chris_j_hunter View Post
    we’ve installed Titan BB sensors in all three of our main Rooms, wired to QAIs, in addition to multiple temperature sensors, whose readings we average & adjust using GeneralLogic and feed to single virtual sensors ... so far, we keep an eye on the BB readings, but don’t use them, mainly because limited QAI resolution leads to disappointing results ...

    when we’ve some time, we maybe will revisit this ...

    Chris
    It may still be worth while looking to see how these relate to the standard temperature sensors in the vicinity. Its true that their readings will be coarser (I think you're going to get 0.4-0.5C resolution) but should be able to see if there are marked differences. If there are, then it could mean some thought has to be given on how to use their outputs for control. For example I can imagine such a sensor placed in direct sunlight measuring quite a bit more than a shaded sensor but would you then really think to feed this 'exaggerated' value directly into the control loop? In other words does it correlate linearly to the human's feeling of warmth? I think it must be more complicated than that, for example even without radiative measuremunts humidity is another factor which really should be considered. I know that temperature/RH charts exist which try to correlate the latter two to human comfort but have not spent much time trying to determine if the level of variation of RH in a given location merits more than occasional adjustment to a user set profile.

  7. #7
    Automated Home Legend chris_j_hunter's Avatar
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    most of our PLH sensors are mounted on window-surrounds & set facing into the Rooms, so as to avoid the effects of direct sunlight etc ... the rest & all others (eg: in MFPs), including our BB sensors, are set on internal walls & built-in furniture, well away from such problems ... plus there are clip-on pipe sensors, so we can measure heat-flows etc, too ...

    was going to copy some comparison graphs, for one Room, for yesterday, but sum total of BB readings for the whole 24hrs was just three points (20.57 at 03:11:13 + 21.04 at 10:52:14 + 20.57 at 19:08:40) - as against the nearby PLH sensor which showed all the details you would expect (ups & downs, within about a 1degC temperature range - we weren’t around, but Cortex tells of useful solar-gain from our vacuum tubes, for a few hours, so there was clearly some sunshine for some of the time, too) !

    Chris

  8. #8
    Automated Home Guru Nad's Avatar
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    Some very interesting points there. My sensor arrangement is usually one near the door (DFP) and then 2 in the ceiling (PLT/ PLH) as we tend to have a very minimalist approach to interior design so there is very little on the walls. None of the sensors are ever in direct sunlight but I do find the house is very susceptible to solar gain. I have to admit, I have been quite successful with the use of the temperature compensation settings but i think this is manly due to the fact that the walls of the house has the insulating properties of a sheet of toilet paper.

    I think that Karam makes a very good point with regards to focusing on the absolute accuracy of the values. I know I've looked at the DFP and thought "it's 21.8 degrees in here, how can it still feel cold!" I think some of the issues were due to old aluminium frame windows and walls with no insulation. These will be resolved during the renovation works so should lead to a more controllable environment.

    Thanks for the responses guys, appreciated as always.

    Thanks,
    Nad

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