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Homework Help: Will house burn down enclosing camera in foam ball?

  1. May 21, 2015 #1
    1. The problem statement, all variables and given/known data
    This is not exactly homework, but is homework level so this is probably the best place to ask rather than the more learned sections. However, I'm prepared to defer to your better judgement.

    I want to put a web camera inside a foam ball. The camera is a retail product so it's passed relevant EU safety standards. My concern is will heat build up cause the foam to melt, or burn my house down by starting a fire.

    The camera runs off 5V DC consuming approximately 500mA.
    The hollow sphere is of outside diameter 300mm with a wall thickness of 20mm. I would characterise this as a thin walled vessel.
    The sphere's made of expanded polystyrene /Styrofoam.
    This is all the data I know for certain.

    What will the temperature rise be inside the sphere above ambient?

    2. Relevant equations
    As this is a safety issue, I'm not giving any equations that I have used. I do not want to influence /prejudice anyone's thinking. I'd very much welcome an independent assessment.

    3. The attempt at a solution
    I've considered the thermal conductivity of the material, the power dissipation of the camera and the sphere's mean surface area. I've also assumed that the thermal conductance through the camera's mounting to the outside is negligible.

    I've calculated a 7 degree rise inside the sphere. I deem this acceptable but what do you think?
  2. jcsd
  3. May 22, 2015 #2


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    Is the camera going in the centre of the hollow ball? If so how will the camera see out? Lens angle will determine the size of the hole needed?
  4. May 22, 2015 #3


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    Inside diameter is 260mm (0.26m).
    Area 4piR^2 = 0.21 sqm

    Thermal conductivity polystyrene (k) about 0.03 W/(mK)

    Power = k * Area * T / thickness

    T = Power * thickness / (k * area)

    = 2.5 * 0.02 / (0.03 * 0.21)

    = 8 degrees.

    What's the rated temperature range for the camera?

    If your house does burn down don't blame me!
  5. May 22, 2015 #4
    Mea culpa. It's difficult to write down all the things that aren't obvious to me.

    Yes, the camera is at the centre of the sphere and the camera is totally enclosed. The camera does not need to see out of the sphere. I realise that there will be differential heating of the sphere due to convection currents within, but I don't have the resources to model that kind of heat flow. I'm really only looking for a non dynamic steady state approximation that I hope averages out to 7ish degrees.

    The specification available to me only gives a storage temperature range (-20 to +60 degrees C). The apparatus will be operated at UK room temperature not likely to exceed 40 degrees. With a 7 degree thermal gain it should be okay I feel.
    Last edited: May 22, 2015
  6. May 22, 2015 #5


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    Foam shells are cheap? Set up a trial with a thermometer inserted through a covered hole so you can retrieve it after 5 or 6 hrs to note the reading.

    We are all curious to learn what will be revealed by filming the inside of a closed foam ball? :smile:
  7. May 23, 2015 #6


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  8. May 23, 2015 #7
    I'm hoping random noise will be revealed. This will then be used as an entropy source for a true random number generator.

    http://www.reallyreallyrandom.com if I may.
  9. May 24, 2015 #8


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    I suspect some light will get through 20mm of foam so there might be some sort of day/night cycle but that could easily be fixed with some paint.

    Pretty sure web cams will have some sort of noise filter built in. Noise is the enemy of compression techniques. The filter won't eliminate all noise but it might remove some frequencies making it uncertain how random it is.
  10. May 25, 2015 #9
    This may be getting off topic; however... Good call, but I've got that issue covered - get it? :wink:

    As for the filter, I don't think that it's possible to substantially remove the noise I'm looking for. What's a PITA is the JPEG algorithm itself. It acts simultaneously as both an ersatz low pass filter reducing the entropy available to me for extraction, and increasing pseudo entropy due to it's coding specification making entropy characterisation problematic. I'll post my findings on the reallyreallyrandom website.
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