Measuring Air Permittivity: Change in Density

[Calvin Cox]

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

 

[berkeman]

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?

 

[Calvin Cox]

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?

 

[berkeman]

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?

 

[Calvin Cox]

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.

 

[DragonPetter]

Will the measurement be relative or absolute?

Also, at what frequency do you wish to know the dielectric constant?

 

[Meir Achuz]

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.

 

[Calvin Cox]

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?