# Relative permittivity -- help please

## Main Question or Discussion Point

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.

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I had the idea to measure how the relative permittivity will change as the temperature, pressure and the humidity changes. .
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?

mfb
Mentor
Well, air pressure will go down, but measuring temperature directly sounds much easier.

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?
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))

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?
At radio frequencies, expected changes are probably in order of a promile or less. Great experimental care is required to make measurements accurately.

mfb
Mentor
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))
I suggest to measure the temperature instead of the electric permittivity.

I suggest to measure the temperature instead of the electric permittivity.
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

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
The make it and measure it real goood:D

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?

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?
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:
V'b=Vb⋅0.289p/(273+ϑ)

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?

I see the formula and I see Vb' and Vb at the end I would get V'b = (some number) Vb?
Yes. To calculate "some number" p should be in mbar, ϑ in °C.

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!

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So I can do it like this Vb' = 3 * 106 (because Vb of air is 3MV/m) * 0.289p/(273+ϑ). Right?
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.

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.

Baluncore
2019 Award
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.

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.
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"...

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.

sophiecentaur
Gold Member
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.

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.
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?

dlgoff
Gold Member
The old standard for detecting/measuring ionizing radiation uses a Giger-Müller tube. You'll need some way to store counts/time at different altitudes?

The old standard for detecting/measuring ionizing radiation uses a Giger-Müller tube. You'll need some way to store counts/time at different altitudes?
I thought of meassuring volatage that comes from the ions by a module and saving it into a sd card. Do you have a better way?

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sophiecentaur
Gold Member
Do you have expirience with ion chambers so I can ask you if I get any problems?
'Fraid not. Not my field. But there is bound to be a PF member who can answer any specific questions.

dlgoff
Gold Member
I thought of meassuring volatage that comes from the ions by a module and saving it into a sd card. Do you have a better way?
From my experience, counting events is the usual way. Scintillation counters is one way that I'm familiar with and have used. There are chemical cocktails that will do for Liquid Scintillation counting.

Saving to an sd card sounds right to me.

Baluncore
2019 Award
Scintillation counting can get quite involved.

You can use a closed chamber and measure incident radiation.
Or you can use an open flow chamber and measure the charge of ions available per m3 of air.

What height is the balloon expected to reach?