How to Check Hydraulic Oil Quality

Are you wondering how to check the quality of hydraulic oil? You’re in the right place! Ensuring the quality of hydraulic oil is crucial for the efficient operation and longevity of your hydraulic systems. 

Hydraulic oil contamination can damage systems and components, reducing performance and increasing repair costs. Identifying contamination symptoms is crucial to prevent further damage and ensure optimal operation. 

Understanding hydraulic fluid functions is important for maintaining equipment. These functions include power transmission, temperature control, lubrication, contamination prevention, and sealing.

Knowing how hydraulic fluid works can help prevent expensive repairs and downtime. Keeping equipment in good condition is crucial for avoiding costly repairs and minimizing downtime. Let’s discover the essentials of hydraulic oil quality and its role in keeping your machinery running smoothly.

Hydraulic Oil and Its Role in Hydraulic Systems

The primary function of hydraulic fluid is to act as a medium for power transmission in hydraulic systems. Additionally, it plays crucial roles in regulating temperature, preventing contamination, providing lubrication, and sealing the hydraulic system.

Hydraulic systems are crucial in our daily lives, having significant functions in automobiles, aeroplanes, industrial machines, and construction gear. To understand hydraulic fluid, we need to know what it does in the hydraulic system and the jobs it does.

Heat Regulation

One key function of hydraulic fluid is temperature regulation. The reservoir cools the fluid before pumping it through the system. As it flows around the moving parts, it cools them and absorbs heat, which it then carries back to the reservoir for dissipation.

In extremely cold conditions, operators can preheat hydraulic fluid before circulating it through the system. This helps keep the components warm, ensuring that the system remains operational even in low temperatures.

Energy Transmission

The primary role of hydraulic fluid is to transfer power. It serves as an effective medium for power transfer due to its incompressibility. When you push on the liquid in a pump, it quickly moves the force to another place, making power transfer efficient.

Hydraulic systems use fluid power to make pistons and rotary motors move, converting mechanical motion into fluid power and back again.

Hydraulic systems can lift cars and extend aircraft landing gear because the fluid is strong and cannot be compressed. Beyond its primary role in power transfer, hydraulic fluid also performs several essential secondary functions.


A key function of hydraulic fluid is lubrication. Hydraulic systems contain numerous metal components that move against each other, and without continuous lubrication, these parts would wear down rapidly.

Hydraulic fluid, a blend of lubricants such as vegetable oil, mineral oil, or synthetic lubricants, is pumped through the system to lubricate areas in constant motion, such as piston, vane, or gear pumps, and valves that are frequently opening and closing.

Contamination Prevention

Hydraulic fluid also plays a crucial role in removing contamination from the system. Despite the protection offered by seals and filters against most impurities, no mechanism or seal is perfect. 

Contaminants may comprise of external elements like dirt and water, along with tiny fragments of metal and chemicals that detach from hydraulic parts during functioning.

Over time, dirt and metal particles get into the fluid, causing more friction and wear on the moving parts, which makes things work less effectively. When the fluid moves around the system, it sticks to these bits of grime and metal and carries them out.

Sealing Function

Hydraulic fluid also helps seal hydraulic systems. These systems must keep pressure to work properly. Even though they use gaskets, valves, and other methods to prevent leaks and maintain pressure, hydraulic fluid adds another layer of sealing.

As the fluid is pushed through the system under pressure, it penetrates the cracks between components and around seals, creating an internal seal between parts. The tiny barrier formed by the surface tension in hydraulic fluid helps keep out small particles that might enter the machine through these gaps.

Signs of Contaminated Hydraulic Oil

Hydraulic oil contamination is the leading cause of damage to hydraulic systems and components. It results in reduced performance, higher maintenance and repair costs, and safety hazards. Identifying common symptoms of contamination and taking preventive measures is crucial.

If you notice any symptoms, have the oil tested and take corrective action. Regular oil analysis and maintenance can help prevent contamination and ensure your hydraulic system operates optimally. So, what symptoms of hydraulic contamination should you be on the lookout for?

Change In  Colour of Hydraulic Oil

A change in the colour of hydraulic oil can indicate contamination. If the oil appears foggy or has a different colour, it may be contaminated. Additionally, a darker colour could suggest burning or the presence of sludge, which often indicates bacterial contamination.

Increased Machinery Noise and Vibration

Increased noise and vibration might indicate hydraulic contamination. Contaminated oil doesn’t effectively lubricate machine parts, leading to increased friction and noise. If you’re familiar with your machine’s usual sounds, you’ll notice any unusual noises.

While your machine might be naturally noisy, any increase in noise and vibration beyond its usual level could signal contamination. It’s essential to watch out for changes in noise and vibration specific to your machine.

Oil Odour

The smell of oil can also indicate issues, particularly if it has a strong odour, which could signal bacterial contamination.

High Hydraulic Fluid Temperatures

Elevated fluid temperatures may indicate hydraulic contamination. When particles hinder fluid flow, preventing proper heat dissipation, temperatures can rise beyond normal levels.

Fluids are considered high when temperatures exceed 180°F (82°C). It’s important to watch out for this because the heat can harm seals and make the hydraulic fluid less effective.

Tools Needed for Testing

Numerous types of testing and analysis instruments are available for evaluating different properties and characteristics of hydraulic oil. Here are some examples of commonly used tools for detecting and analyzing hydraulic oil:

Moisture Analyzer

Used to measure the moisture content of hydraulic oil. Examples include the Karl Fischer titrator, coulometric instrument, and infrared spectrometer.

Viscosity Tester

Used to measure the viscosity of hydraulic oil, including kinematic viscosity, absolute viscosity, and relative viscosity.

Acid Value Titrator

Used to measure the acidic components in hydraulic oil, typically utilizing the acidity titration method.

Filter Performance Tester

Used to assess the filtration efficiency of hydraulic oil, including methods such as particle counting and transmittance tests.

Alkali Titrator

Used to measure the alkaline components in hydraulic oil, typically using acidity titration or differential titration methods.

How To Test For Water Contamination In Hydraulic Oil

Contaminated water in hydraulic oil can cause issues like decreased efficiency, corrosion, and wear in your hydraulic system. Here’s a simple guide on how to test for water contamination in hydraulic oil:

  1. Visual Inspection: Start with a basic visual check. Here’s what to do:
  2. Look for Cloudiness: Pure hydraulic oil is clear. If it looks milky or cloudy, it may contain water contamination.
  3. Check for Bubbles: Shake a small sample of the oil in a clear container. If you see bubbles that don’t disappear quickly, it could indicate water contamination.
  4. Crackle Test: The crackle test is a quick and easy method to detect water:
  5. Heat a Metal Plate: Use a hot plate or similar device to heat a clean metal plate to around 150°C (302°F).
  6. Drop Some Oil: Place a few drops of hydraulic oil on the heated plate.
  7. Listen and Observe: If you hear a crackling sound or see bubbles forming, there is water in the oil. The more intense the crackling, the more water is present.
  8. Calcium Hydride Test Kit: For a more precise measurement, use a calcium hydride test kit:
  9. Prepare the Kit: Follow the instructions to set up the test kit.
  10. Add Oil Sample: Place the hydraulic oil sample into the test container.
  11. Observe the Reaction: The kit will typically contain a reagent that reacts with water, producing hydrogen gas. Measure the gas produced to determine the water content.
  12. Karl Fischer Titration: For accurate and detailed analysis, consider the Karl Fischer titration method, often used in laboratories:
  13. Sample Preparation: A technician will prepare the hydraulic oil sample.
  14. The titration process involves titrating the sample with a Karl Fischer reagent.
  15. Read Results: The process measures the exact amount of water in the oil, providing precise results.
  16. On-Site Water Detection Sensors: Modern hydraulic systems might have on-site sensors:
  17. Install Sensor: Make sure you properly install the sensor in your hydraulic system.
  18. Monitor Readings: Regularly check the sensor readings to monitor water content in real-time.

How to Use a Particle Counter to Measure the Level of Contamination in Hydraulic Oil

The hydraulic fluid contamination particle counter is a device designed to measure the quality of hydraulic fluid by detecting contamination particles. There are various advanced instruments. Some people fixate on equipment, while others prefer portable options that they can carry to different locations.

Particle counters operate using three methods: light scattering, light obscuration, and direct imaging. Each technique directs light through the fluid as it flows through a detection chamber, with each method using a different approach to analyze the fluid.

Light Scattering Particle Counters

In the light scattering method, a highly focused beam of light, such as a laser, passes through the detection chamber. Particles within the chamber cause the light to scatter, increasing the light intensity. 

A sensitive sensor on the other side of the chamber detects this increase and converts it into data indicating the level of contamination.

Light Obscuration (also known as Light Extinction) Particle Counters

In the light obscuration method, an LED light passes through the chamber, and a diode measures the amount of light lost or “obscured” by the particles in the fluid.

When a particle passes through the beam, it decreases the amount of light received by the diode. This change allows for the determination of particle size and the accurate assessment of contamination levels.

Direct Imaging Particle Counters

Direct imaging particle counting utilizes a high-resolution camera to detect particles. This type of particle counter captures images of the particles within the fluid and sends them to a computer for analysis using specialized software.

This software can provide detailed data on particle size, colour, and shape, and users can store and reference this data over time. However, this approach is only practical for relatively slow flow rates.

Tips For Maintaining Hydraulic Oil Quality

Maintain Filters

Regularly clean or change hydraulic filters as per the manufacturer’s recommendations. This helps eliminate contaminants from the hydraulic fluid, safeguarding components and ensuring the system operates smoothly.

Regular Inspection

Regularly inspect all hydraulic parts, such as pumps, valves, cylinders, and hoses, for any signs of wear, corrosion, or damage. Promptly replace any worn or damaged parts to prevent potential failures.

Monitor Hydraulic Fluid Levels

Keep an eye on the hydraulic fluid levels regularly and refill as necessary to ensure everything runs smoothly. Always use the type of hydraulic fluid recommended by the manufacturer.

Check for Leaks

Regularly examine hydraulic hoses, fittings, and seals for any signs of leaks or wear. If you spot any, fix them right away to avoid losing fluid and keep the system running smoothly.

Monitor Pressure

Keep an eye on hydraulic system pressure levels using pressure gauges and adjust them as needed to meet operational needs. Too much pressure can stress components, while too little might signal a problem.


Make sure to lubricate moving components like hydraulic cylinders and motors correctly to minimize friction and wear. Use lubricants endorsed by the manufacturer for the best results.

Bottom Line

In conclusion, checking the hydraulic oil quality is essential for the efficient operation and longevity of hydraulic systems. By understanding the roles of hydraulic fluid and recognizing signs of contamination, you can prevent damage and ensure optimal performance.

Regular monitoring, maintenance, and the use of appropriate testing tools are key to preserving the integrity of hydraulic oil and avoiding costly repairs. By following these practices, you can keep your hydraulic systems running smoothly and effectively for years to come.


Q1: How can I test for water contamination in hydraulic oil?

Ans: A straightforward test for water contamination is the “spat” test: Put a few drops of oil on a hot plate heated to above boiling but below 176°C (350°F). If the oil bubbles or sizzles, it indicates an unacceptable level of water in the oil.

Q2: What are the potential consequences of using low-quality or contaminated hydraulic oil?

Ans: Contaminated hydraulic fluid can degrade and impair lubrication, affecting your GSE’s performance. It may lead to corrosion and the formation of sludge, causing more serious problems.

Q3: Why is a particle count test important for assessing hydraulic oil quality?

Ans: Particle count data is vital for proactive monitoring, showing early wear signs and ensuring hydraulic fluid cleanliness for equipment protection.

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