25-06-2012, 01:06 PM
Bubble Detection Sensors for Non-conducting Liquids
[attachment=25404]
Goals for Bubble Sensing Devices:
Detect small bubbles in a small fluid channel without immersion of sensor parts in the channel
Differentiate between a gas filled channel and a fluid filled channel
Be sensitive to rather small gas fractions passing down the small fluid channel
Optical Sensing – Total Int. Reflection Method
Critical angle
In geometric optics, at a refractive boundary, the critical angle is the angle of incidence above which total internal reflection occurs.
The angle of incidence is measured with respect to the normal at the refractive boundary. It is given by:
where θc is the critical angle, n2 is the refractive index of the less dense medium, and n1 is the refractive index of the denser medium. This equation is a simple application of Snell’s law where the angle of refraction = 90°.
Note: Total internal reflection only occurs when the incident ray is in the denser medium.
If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence.
For visible light traveling from glass into air (or vacuum), the critical angle is approximately 41°.
For water into air (or vacuum), the critical angle is approximately 48.6°.
For polycarbonate (n=1.59) into air (or vacuum), the critical angle is approximately 39°.
Summary and Conclusions
A wide variety of bubble detection schemes suitable for use with non-conductive
liquids have been introduced
Many of these are practical for miniature application
Microprocessor based electronic readouts for these sensing schemes
are not complex or expensive.
[attachment=25404]
Goals for Bubble Sensing Devices:
Detect small bubbles in a small fluid channel without immersion of sensor parts in the channel
Differentiate between a gas filled channel and a fluid filled channel
Be sensitive to rather small gas fractions passing down the small fluid channel
Optical Sensing – Total Int. Reflection Method
Critical angle
In geometric optics, at a refractive boundary, the critical angle is the angle of incidence above which total internal reflection occurs.
The angle of incidence is measured with respect to the normal at the refractive boundary. It is given by:
where θc is the critical angle, n2 is the refractive index of the less dense medium, and n1 is the refractive index of the denser medium. This equation is a simple application of Snell’s law where the angle of refraction = 90°.
Note: Total internal reflection only occurs when the incident ray is in the denser medium.
If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence.
For visible light traveling from glass into air (or vacuum), the critical angle is approximately 41°.
For water into air (or vacuum), the critical angle is approximately 48.6°.
For polycarbonate (n=1.59) into air (or vacuum), the critical angle is approximately 39°.
Summary and Conclusions
A wide variety of bubble detection schemes suitable for use with non-conductive
liquids have been introduced
Many of these are practical for miniature application
Microprocessor based electronic readouts for these sensing schemes
are not complex or expensive.