How to Choose an Entrained Air Tester

Choosing an Entrained Air Tester

You need to detect hydraulic aeration before it reduces pump output, delays actuator response or damages critical components. This guide shows you how to choose an entrained air tester based on monitoring frequency, equipment risk, sampling conditions and the type of air you need to measure. Manual instruments suit fast spot-checks and troubleshooting across several assets. Automated analyzers suit frequent monitoring of dissolved air. At PAPEC, we draw on over 40 years of experience and recommend direct compressibility measurement rather than indirect bulk-property methods.

How Entrained Air Triggers Cavitation and Component Failure

Entrained air consists of discrete bubbles suspended in hydraulic fluid. It differs from dissolved air, which remains molecularly dispersed in the oil until pressure and operating conditions force it out of solution.

Those bubbles make the fluid more compressible. Part of each pump stroke is then spent compressing air rather than moving oil. The result can be reduced pump capacity, pressure ripple and slower or inconsistent actuator response. A system may therefore lose performance before obvious surface damage appears.

The more destructive stage occurs when bubbles travel into a high-pressure zone. Their rapid collapse produces concentrated impacts near metal surfaces. Repeated collapse can pit pump port plates, valve spools and seats, eventually causing erosion and component failure. Heat generated during compression may also degrade the oil and contribute to deposits in small passages.

As a practical warning level, Power & Motion reports that a rise toward 1% to 2% entrained air signals developing gaseous cavitation risk. The acceptable limit for your equipment still depends on its lowest local pressure, duty cycle and required response accuracy.

We recommend measuring air directly before relying on noise, foam or vibration alone. Those symptoms can point to aeration, but they do not tell you how much air is present. Our detailed guide explains how to identify cavitation risk through direct air measurement.

Manual Entrained Air Testers for Spot-Checking and Maintenance Audits

A manual tester is usually the practical choice when you need periodic checks at several machines. It lets a technician collect or isolate a sample, apply controlled compression and calculate the entrained air content. This approach suits preventive maintenance rounds, post-repair verification and fault isolation.

PAPEC offers the manually operated QuickAir_OC and Squeezer for entrained-air testing. The QuickAir_OC is designed for swift measurements, while the Squeezer allows precise manual adjustment. A maintenance team can compare results at the reservoir, return line, pump inlet and downstream of a suspected restriction instead of installing a separate monitor on every asset.

Use a consistent test point and repeatable operating condition. Take the sample from an active flow path while the machine is at a stable operating temperature, then test it immediately so bubbles do not rise out of suspension. For captured samples, limit pressure loss and contact with the atmosphere. NASA’s technical report describes closed-system sampling as a way to prevent pressure loss and atmospheric contamination.

Manual testing is especially useful:

• During planned condition-monitoring rounds or quarterly maintenance audits.
• Immediately after commissioning, an oil change, line bleeding or a component rebuild.
• When comparing several points to isolate an air leak or pressure-drop problem.
• When a pump becomes noisy, output falls or actuator timing becomes inconsistent.
• When you need a portable instrument for multiple hydraulic systems.

A spot-check only represents the system at the moment and location tested. If aeration is intermittent, record machine load, oil temperature, valve state and sample location with every result. A repeatable rising trend is more useful than an isolated reading taken under unknown conditions.

Automated Analyzers for Continuous, Mission-Critical Protection

Continuous or automated monitoring becomes more valuable as the consequence of a missed event rises. Consider it for high-duty machinery, processes with rapid pressure changes and circuits where a brief aeration event could interrupt production or damage expensive equipment.

PAPEC offers the D_GR as a fully automated device for precise, real-time monitoring of dissolved air content. It uses a piston-and-cylinder process that expands a liquid sample to release dissolved air and then compresses the air-liquid mixture. A microprocessor calculates air content using thermodynamic gas laws.

An automated analyzer reduces dependence on the timing of manual rounds. It can show gradual change after a process adjustment and reveal conditions that disappear before a technician reaches the machine.

Automation does not remove the need for good test-point design or fault diagnosis. The sample still needs to represent the fluid passing through the risk area. A rising reading should trigger checks for suction-side leaks, restricted inlets, poor reservoir return design and pressure drops across valves or orifices.

Choose manual testing when events develop slowly and technicians can check the system often enough to act. Choose automated monitoring when the interval between safe operation and damaging aeration may be shorter than your inspection schedule. Many plants benefit from both: continuous monitoring on the most critical process and portable testing for confirmation and investigation elsewhere.

Addressing Dissolved Air Liberation in Low-Pressure Zones

Dissolved air may have little immediate effect while it remains in solution. The risk changes when oil flows through a constriction. Pressure can fall sharply downstream of an orifice, throttling valve or restricted suction line, causing dissolved gas to form free bubbles.

Those bubbles then travel with the oil. When they reach a high-pressure part of the circuit, they can collapse and produce cavitation damage. For this reason, an acceptable entrained-air result at one test point does not prove that dissolved air cannot be released farther downstream.

We recommend measuring both forms when you are investigating severe pressure-drop cavitation or confirming the performance of a deaeration process. PAPEC offers the Squeezer DualAir for entrained and dissolved air measurement. The automated D_GR tracks dissolved air so you can determine whether treatment has reduced it to the required operating level rather than assuming the deaerator has completed its job.

Compare measurements before and after the suspected restriction, and include pump inlet conditions where practical. If readings rise after a particular valve or flow-control block, the pressure drop may be liberating dissolved gas. If air is already high at the inlet, investigate suction plumbing and reservoir conditions. Our guide to controlling pump cavitation from entrained air covers the next diagnostic steps.

Compressibility Versus Bulk Property Measurement Methods

The measurement principle matters most when air content is low. We consider fluid compressibility the most direct way to evaluate entrained and dissolved air. A positive-displacement piston changes a known sample volume under controlled pressure. Pressure and displacement data are then used with thermodynamic gas laws to determine the gas volume.

This method measures the air content itself. It does not attempt to infer a small air concentration from changes in a much larger bulk reading. That distinction matters when the oil varies between batches or changes with age and use.

Acoustic velocity, attenuation, density, refractive index, electrical conductivity and similar bulk properties can respond to factors other than air. Fluid composition, temperature, suspended solids and chemical aging may alter their readings. Bubble-size distribution can also affect acoustic measurements. At trace concentrations, these influences can be large compared with the change caused by the air being measured.

We therefore do not recommend bulk-property methods as stand-ins for direct compressibility testing where low-level accuracy affects a maintenance decision. Mechanical piston displacement gives you a direct physical basis for comparing machines, operating states and corrective work.

Selection Checklist: Matching Specifications to Plant Operations

Start with the decision the result must support. A portable audit tool and a continuously monitored critical circuit have different requirements. Then check the instrument against the real conditions at your sample point.

1. Define the target. Decide whether you need entrained air, dissolved air or both. Entrained-air testing addresses bubbles already present. Dissolved-air measurement addresses gas that may be released in low-pressure zones.
2. Match the monitoring interval to the risk. Use manual testing for scheduled rounds and multi-asset troubleshooting. Use automated analysis where air conditions can change between rounds or failure consequences are severe.
3. Check pressure limits. Do not treat transducer maximum, proof pressure and burst pressure as interchangeable. The QuickAir_OC normally tests samples at atmospheric pressure and reaches approximately 60 psig during testing; its pressure transducer has a maximum operating rating of 100 psig. The D_GR_SM has a 100 psia normal operating limit, a 150 psia proof limit and a 300 psia burst pressure.
4. Confirm temperature suitability. The QuickAir_OC cylinder operates from 32°F to 200°F. Operation above 120°F may require pressure-transducer re-zeroing. The Squeezer DualAir is specified from 0°F to 190°F.
5. Review wetted materials and the environment. The QuickAir_OC cylinder is compatible with fluids that are not corrosive to 304 stainless steel, provided its vent line is not exposed to corrosive fluid. Its electronics enclosure is not environmentally isolated, so protect it from spray, submersion and corrosive gases.
6. Plan representative sampling. Select an active-flow point close to the suspected risk area. Avoid open containers and delays that allow bubbles to escape. Record load and temperature with each result.
7. Establish a baseline. Zero or reference the tester as required, then determine the normal result for each machine under repeatable conditions. Use trends and post-maintenance comparisons instead of relying on one universal alarm value.
8. Confirm usability. Consider test time, operator steps, data recording and access to sample points. A technically capable instrument will not help if the procedure cannot be repeated safely during normal production.

You can compare our manual and automated air measurement products once you have defined these operating requirements.

Discuss Your Hydraulic Air-Testing Application

Contact PAPEC in Eagle, ID, USA, for expert consultation on your fluid, test points, operating limits and monitoring frequency. Call us at 253-335-3665 or email steve.dougherty@papec.com to determine whether a manual tester, dual-air instrument or automated dissolved-air analyzer fits the application.