Measuring Air Permittivity: Change in Density

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    Air Permittivity
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SUMMARY

This discussion focuses on measuring the change in air permittivity due to variations in air density, particularly in the context of developing a mass air flow sensor. The participants explore the use of ionizing pulses to measure air velocity and the potential relationship between charge density and air density. They reference the Paschen curve to highlight the complexities of ionization in flowing gases and discuss the static permittivity of air, which is approximately 1.00058986. The consensus is that for their application, a relative measurement of permittivity is sufficient, with a sampling frequency of once per second.

PREREQUISITES
  • Understanding of ionization processes in gases
  • Familiarity with the Paschen curve and its implications
  • Knowledge of dielectric constants and permittivity
  • Experience with mass air flow sensor design
NEXT STEPS
  • Research the relationship between pressure and permittivity in gases
  • Investigate methods for measuring charge density in ionized air
  • Explore the impact of air velocity on ionization characteristics
  • Study the principles of dielectric constant measurement at different frequencies
USEFUL FOR

Engineers and researchers involved in sensor technology, particularly those focused on air quality measurement, fluid dynamics, and dielectric properties of gases.

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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