Entrained Air Measurement in Coolant Filtration and Hydraulic Systems

Steve Dougherty, Product and Process Engineering Concepts Ken Doenges, Product and Process Engineering Concepts

Product and Process Engineering Concepts (PAPEC) was formed in 1985 with the objective of developing and marketing instruments, and to provide consulting for the measurement and control of entrained air in process fluids.

The company’s expertise and subsequent patented products are used in many diversified industries; the expertise is generally applicable to any process that is impacted by entrained air and/or foam issues.

In the coolant filtration industry, as in many industries using fluids, entrained air (foam) can cause numerous problems, including pump cavitation, workplace environmental issues, and reduced coolant effectiveness with associated productivity penalties and their costs.

In hydraulic systems, entrained air, as well as dissolved air, can cause problems with actuator response due to increased elastic modulus of the oil. PAPEC products are able to measure dissolved air by measuring the entrained air that results from subjecting fluid to a high vacuum.

Whether the cause of the entrained air is improper system design and operation or poor management of coolant/lubricant composition, precise measurement of entrained air distribution throughout the system can provide valuable information for corrective action to help reduce, control or eliminate the costly machine faults and treatment of coolant systems.

Entrained air measurements can be made at various locations in a system using the Quick Air portable manually operated tester. Pulse Air online automated sensors can be added at strategic fixed positions.

Air Tester

The Quick Air (Figure 1) is a portable, manually operated tester for rapid, accurate entrained air measurements throughout process systems.

Figure 1. Quick Air Portable Manually
Operated Entrained Air Tester

The entrained air detection device can be used by technicians to conduct extensive surveys throughout the process to identify sources of air entrainment and to identify methods to control and reduce air entrainment. Conversely, unskilled operators can use the tester to measure entrained air at critical locations, then use the information to make routine operating decisions involving the use of defoamers.

The sample is collected directly from the process fluid by a piston and cylinder sampling arrangement. As the sample lever is moved, the sample piston is pulled into the cylinder and a sample of process fluid follows.

The compression piston also follows, entering the bottom of the cylinder to isolate the sample. As the sample lever is moved further, the sample piston comes up against a stop. However, the compression piston continues to move, thereby compressing the sample.

A flush-diaphragm pressure sensor in the sample piston measures the pressure, and a linear position transducer measures the movement of the compression piston. A microprocessor calculates the air content using the ideal gas law and displays the result digitally.

The test takes a matter of seconds. Measurement range (volume percent) is 0.1 to 100 percent, and accuracy is nominally ±1 percent of measurement.

Air Sensor

An automated on-line tester, called the Pulse Air (Figure 2), is a solid-state sensor that measures entrained air content directly in the process fluid. The sensor is installed at a position in the process equipment where the entrained air content has been determined to provide the greatest process impact.

Figure 2. Pulse Air Automatic On-line
Entrained Air Sensor

The 4-20 ma output from the locally mounted operator interface is used to transmit the entrained air measurement to a process controller, which is provided by the process owner or by PAPEC. The output from the process controller is used to drive an appropriate device, such as a variable-speed defoamer pump, to control the entrained air at acceptable levels.

The sensor is designed to be inserted into a process pipe or tank. Periodically, a high-speed magnetostrictive actuator moves a compression piston rapidly into the process fluid. The piston moves approximately 0.005 inch in about 100 microseconds.

Because of the inertia of the fluid in the vicinity of the end of the piston, the fluid does not have time to move away from the rapidly advancing piston, and the fluid is locally compressed at the end of the piston. The result is a pressure pulse, measured by a flush-diaphragm pressure transducer mounted in the end of the compression piston.

The magnitude of the pressure pulse is inversely related to the amount of air in the fluid: less air, greater pressure pulse; more air, smaller pressure pulse. Measurement range (volume percent) is 0.05 to 50 percent, and accuracy is nominally ±2 percent of the measurement.


The Quick Air and Pulse Air have been used successfully in many diverse industries including pulp and paper, food, pharmaceutical and hydraulic fluids.

In pulp and paper, most applications are for optimal management of defoamers. These are typically on-line closed-loop applications using the Pulse Air in conjunction with a process controller to continuously adjust variable speed defoamer pumps as necessary to maintain the entrained air at a desired target. The Quick Air is also frequently used in the industry to find and correct trouble spots.

In food and pharmaceuticals, the filling of product containers is most conveniently done by volume. Measurement of the entrained air ensures that the correct amount of fluid volumes are added to the containers. Commonly, the Quick Air is used for these applications because the Pulse Air does not meet sanitary requirements.

In hydraulics, dissolved air which can easily exceed levels of 10 percent by volume, causes the oil to become spongy. This can cause problems with actuator response, especially in critical applications such as space launch vehicles. Typically, the dissolved air is removed in high-vacuum loops which allows the dissolved air to degas and be removed as it comes out of solution. In two applications, the Pulse Air has been installed in the high-vacuum line to measure the progress of the degassing.

In coolant filtration systems, the primary use of either the Pulse Air or the Quick Air entrained air detectors is to effectively establish at what percent entrained air is in the fluid so to reduce, control or eliminate costly use of defoamers.

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