Because aeration can cause very serious problems, it is important to be able to accurately measure the aeration at any given time. With proper measurements one can quantify the susceptibility of one's design or component to the negative effects of aeration. Good measurement is key, the following is a discussion of the types of aeration measurement technologies that exist today and their relative advantages and disadvantages.
Gamma ray absorption
The absorption of gamma rays by a material is dependent on the density of the material. The aeration is directly proportional to the density of the two phase mixture (air and oil). During a gamma ray absorption measurement, a small bundle of collimated gamma rays created by a source, are directed through the detector on the other side. The intensity of the ray is determined on both sides of the air bubbles and the relationship between the two measurements gives a result for the aeration percentage.
The major disadvantage of this technique is that it only gives an averaged aeration percentage across a small cross-section of the bubbles and not a volumetric one. The speed of the measurement is also bounded by the nature of the radiation source. For an accurate measurement one needs a strong source or a long measurement time. If the source is not strong enough the detected signal will be weak, giving less accurate results. This can be helped by detecting the signal over a longer period, bringing with it the disadvantage that each measurement will take more time and become insensitive to smaller fluctuations of the aeration in the time. Because the source and detectors are situated on the outside of the pipe, much of the useful radiation may be absorbed depending on the thickness and properties of the container material.
The X-ray absorption technique works in a similar way to that of gamma ray absorption. The higher intensity of the x-rays solves the problems of time resolution and accuracy. This does however bring with it the extra problem of shielding and health risks. As well as this the relationship between the aeration and the absorption of x-rays is not a simple monochromatic relationship which ensures complicated calculations of the aeration percentage.
The impedance can be measured by inserting two probes into the void flow. The impedance measured in a two-phase mixture (air and oil) changes if there is liquid or vapour around the sensor. The aeration percentage is determined by measuring over a certain period to get a time averaged air bubble detection. One therefore needs to measure over a long period in order to get a reliable average. This is disadvantageous when frequent measurements are required. This technique is cheap and is excellent for local aeration percentage results but cannot be used for volume averaged aeration percentage measurements. Also, inserting probes into the flow is disruptive for the flow of the mixture of air and oil. The most obvious disadvantage is of course that it cannot be used for measurements in non-conducting fluids like oil.
Optical bubble probes
Optical measurement techniques make use of Snell’s law, which suggests that the refraction and reflection of light is dependent on the refraction index of the materials between which the refraction takes place. The refraction index is also dependent on the aeration percentage, so that by measuring the reflection and refraction of light at the end of a probe, the aeration percentage can be determined. This technique has the benefit that it can be used for conducting and non-conducting fluids. However, it is intrusive and only suitable for very local aeration percentage measurements.
The Capacitive technique provides fast sampling by using nonintrusive electrode pads around the pipe containing the mixed phase flow. An electric field is generated between the pads that fluctuates reliably in relation to the aeration concentration. This results in a powerful and accurate solution for determining aeration percentage. There are two important factors that influence the dependence of the capacitance of the sensor on the aeration percentage. The first is the distribution of the two phases within the pipe and the second is the homogeneity of the electric field between the electrode pads. With proper application, this technology is capable of measuring aeration to the nearest 1% while generating a live reading every second.