Measuring Air Permittivity: Change in Density

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    Air Permittivity
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Discussion Overview

The discussion centers on methods for measuring the change in permittivity of air in relation to its density, particularly in the context of using ionizing pulses to gauge air velocity. Participants explore various approaches and considerations related to accuracy, experimental setup, and the relationship between pressure and permittivity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants inquire about methods for measuring the change in permittivity due to air density and the context of the measurements, including desired accuracy.
  • One participant suggests that collecting charge density from generated electrons could represent air density, but notes the complexity due to varying ionization characteristics with pressure and flow velocity.
  • Another participant mentions developing a mass air flow sensor and questions whether pressure readings could assist in calculating air density and permittivity relationships.
  • There is a discussion about the need for relative versus absolute measurements and the frequency of dielectric constant measurements.
  • A participant cites a static permittivity value for air and suggests that for certain applications, a value of 1 might suffice.
  • One participant reiterates their focus on measuring air density and questions the proportionality of collected charge from ionization to air density.

Areas of Agreement / Disagreement

Participants express various viewpoints on the methods and considerations for measuring air permittivity and density, with no clear consensus reached. The complexity of the relationship between ionization, pressure, and air flow introduces uncertainty and differing opinions.

Contextual Notes

Participants highlight the dependence of measurements on factors such as pressure, flow velocity, and the specific experimental setup, indicating that assumptions and conditions may significantly affect outcomes.

Calvin Cox
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Does anyone have a method of measuring the change in permittivity due to air density.
 
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Calvin Cox said:
Does anyone have a method of measuring the change in permittivity due to air density.

What is the context of the question? What accuracy are you trying to achieve? What have you found in researching this question so far?
 
We are using an ionising pulse to measure air velocity. If we collect generated electrons on a plate would the charge density represent air density?
 
Calvin Cox said:
We are using an ionising pulse to measure air velocity. If we collect generated electrons on a plate would the charge density represent air density?

That's a hard question. The ionization characteristics of air vary with pressure, but in a very complicated way:

http://en.wikipedia.org/wiki/Paschen_curve

And that is for stationary gasses. Having the air flowing with some velocity will make things even more complex, it would seem. Have you tried any experiments yet?
 
We are developing a mass air flow sensor so we need air velocity and density. Could we use the pressure reading to help the calculation. We can also measure the charge density in an area where the air is almost static. Can you suggest any documents that would detail the pressure/permittivity relationship.
Thanks for your help.
 
Will the measurement be relative or absolute?

Also, at what frequency do you wish to know the dielectric constant?
 
Last edited:
From Wikipedia, the static permittivity of air is 1.00058986 ± 0.00000050.
The permittivity at high frequencies would probably be closer to1.
I think you can use 1 for your purposes.
 
Just to reiterate, I am attempting to measure air density. I don't need absolute values as they will be relative to the small sample of air we are sampling. The sampling frequency only needs to be once a second. We are using ionisation to create a signal to measure velocity. My question is: if we collect charge from the ionisation pulse will that be proportionate to the air density?
 

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