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Relative permittivity -- help please

  1. Nov 17, 2014 #1
    So I am a 17 years old student who is interested in electronics and programming them (I am expirienced with Arduino).
    I and my friends decided that we will join a competition for all technical schools in my country. The goal of the competition is to make a project that will do something while it is flying with a balloon to the stratosphere.
    I had the idea to measure how the relative permittivity will change as the temperature, pressure and the humidity changes. But I have a few problems my teachers don't know how to measure it when you are looking on temperature and those factors. I have basic electronic knowledge and good programming skills (I know about electronic, currents and lot more because I am going to a technical school and we learned all).

    So if anyone knows how I could make that I would like to know.

    Thank you all, Filip.
  2. jcsd
  3. Nov 17, 2014 #2
    I wouldn't expect significant/measurable change in permittivity before altitudes like 50 km, when one enters ionosphere. Ionosphere permittivity depends on frequency. How high can ballon climb?
  4. Nov 17, 2014 #3


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    Well, air pressure will go down, but measuring temperature directly sounds much easier.
  5. Nov 18, 2014 #4
    The balloon will go 30 km to statosphere I found on wikipedia that the pressure at a height of 30 km is 1/1000 of watter level pressure, is there realy nothing to measure if the pressure and temperature (-60 celzius) change?

    Mfb what do you mean measuring temperature I know how to do it but how to get permiitivity out of that I know some formulas for permittivity but they don't include temperature and preasure but I found something about "Related parameters of air" and I found a formula for delta that says:

    (pressure1 / pressure0) * ((273 + t0) / (273 + t1))
  6. Nov 18, 2014 #5
    At radio frequencies, expected changes are probably in order of a promile or less. Great experimental care is required to make measurements accurately.
  7. Nov 18, 2014 #6


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    I suggest to measure the temperature instead of the electric permittivity.
  8. Nov 18, 2014 #7
    Yes but its a competition and for measuring temperature is only one tiny component needed.. I dont think that we would win a good place :D

    I got some help from a friend who is studdying electronics he sayed that I should make a plate capacitor and measure the capcivity while its flying and I could get permittivity from a simple formula :D ty all for replays
  9. Nov 19, 2014 #8
    The make it and measure it real goood:D
  10. Nov 19, 2014 #9
    Hi all one more question if I get permittivity and I measured teamperature, humidity, pressure is there any formula so I can caculate how much voltage is needed for electrical breakdown?
  11. Nov 19, 2014 #10
    Permittivity has nothing to do with breakdown voltage. Temperature and pressure does. If in uniform E-field, at temperature ϑ=20 °C and pressure p= 1013 mbar, breakdown voltage equals Vb, then, at some other temperature ϑ and pressure p, breakdown voltage is:
  12. Nov 19, 2014 #11
    First thank you zoki and sorry if I am being stupid but can you give me a example I don't see the whole picture (sorry if it is obviously). I see the formula and I see Vb' and Vb at the end I would get V'b = (some number) Vb?
  13. Nov 19, 2014 #12
    Yes. To calculate "some number" p should be in mbar, ϑ in °C.
  14. Nov 19, 2014 #13
    So I can do it like this Vb' = 3 * 106 (because Vb of air is 3MV/m) * 0.289p/(273+ϑ). Right?

    Lets say temperature in statophere is -60 celzius, and I found out thet the pressure is 1013hPa / 1000 (at the heightest point of statophere)

    Example Vb' = 3000000 * ((0.289 * 101300)/(273 - 60))
    Vb' = 3000000 * (101.3/ 213)
    Vb' = 3000000 * 0.475
    Vb' = 1426760,56 V

    P.S: What is this 0.289 number?

    Thank you for helping me!
    Last edited: Nov 19, 2014
  15. Nov 19, 2014 #14
    Seems you're confusing breakdown voltage Vb and breakdown field Eb. Relation between these two in uniform filed is Vb=EbxD , where D is distance between flat plate electrodes. Breakdown E-field of standard air is about 3 MV/m, a bit more or less depends on D. Breakdown voltage in kV of standard air you calculate as:
    Vb=24.24⋅D + 6.08⋅√D
    where D is distance measured in cm between parallel plates.
  16. Nov 19, 2014 #15
    Thank you. Can you explain me what 24.24 is and what is 6.08? I have to document all the steps of my project.
  17. Nov 19, 2014 #16


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    The variation in permittivity of the air would be less than the thermal expansion / contraction of the capacitive measurement plates needed.

    The charged particle or ion density is one possible parameter that could be measured. At low altitudes, air pollution might provide most ions, while as height increases, the greater ionisation by solar UV radiation would generate significantly more ions. There would be pollution level and day / night variations in ion density.
    To measure ion concentration you would need a few hundred volts to attract and collect ions from an airflow drawn through a passage. The ion current could be converted to a voltage using a low leakage MOSFET Op-Amp in an electrometer with a high value feedback resistor.
  18. Nov 19, 2014 #17
    Sorry, but I have detailed reference only in my language. These coefficients and the equation are derived by fitting to empirical data concerning research of so called "Paschen's curve" for air. See here what is that curve or just google words "Paschen's curves", "Paschen's law"...
  19. Nov 20, 2014 #18
    Thank you all for your help I will now collect as much information as I can and try to figure out something awesome!

    You all are awesome, Filip.
  20. Nov 21, 2014 #19


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    The permittivity of the atmosphere in the ionosphere is based on the ionisation.I have a feeling that the presence of capacitor plates of a practical size and spacing and the associated circuit could affect the ionisation levels. Did you consider measuring the presence of ionising particles or radiation (UV?) at high altitude, instead? That might be a possibility and impress the judges.
  21. Nov 21, 2014 #20
    Yes sphiencentaur I am going to make a ion chamber with my team so we can meassure the radiation I saw how it schoud look like and I will read about it today. Do you have expirience with ion chambers so I can ask you if I get any problems?
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