Effects of Temperature and Humidity on Jet Fuel Lubricity Testing

1. Overview

Jet fuel lubricity can affect the operating condition of aviation fuel pumps and other moving components exposed to friction. ASTM D5001 describes a test method using a Ball-on-Cylinder Lubricity Evaluator (BOCLE), in which the wear scar diameter on a steel ball is used as an evaluation parameter. Under the same test method and operating conditions, a larger wear scar diameter generally indicates lower fuel lubricity.

For the test conditions discussed in this article, the fuel temperature is controlled at 25 ± 1°C, while the relative humidity of the conditioned air is maintained at 10 ± 0.2%. Temperature or humidity fluctuations may influence the measured wear scar diameter and consequently affect the assessment of fuel lubricity.

Based on comparative test observations, this article discusses how temperature and humidity may affect test results and provides recommendations for controlling laboratory conditions. The results described below apply only to the samples, equipment and test conditions used and should not be regarded as guaranteed results for all jet fuels.

2. Effects of Temperature Changes

2.1 Observed Test Trend

With humidity and other test conditions kept unchanged, the sample temperature was gradually increased from 20°C to 40°C.

Under the conditions of the comparative test, the reference fuels and selected military and civil jet fuel samples showed a similar overall trend: the measured wear scar diameter increased as the fuel temperature increased.

2.2 Possible Mechanism

An increase in temperature may reduce the kinematic viscosity of jet fuel. This can weaken the lubricating film between the steel ball and rotating test cylinder, increasing direct contact between surface asperities and resulting in a larger wear scar.

At a lower temperature, the relatively higher viscosity of the fuel may support the formation of a more continuous lubricating film, thereby reducing wear under the same test conditions. The actual response may also depend on fuel composition, trace components and test-system condition.

2.3 Practical Influence

If the fuel temperature is higher than the specified 25°C test condition, the measured wear scar diameter may be larger, making the fuel appear to have lower lubricity than it would under the specified condition. This may lead to unnecessary adjustments in anti-wear additive treatment and increase fuel-blending costs.

If the test temperature is too low, the measured wear scar may be smaller, potentially masking insufficient fuel lubricity. Results obtained outside the specified temperature range should therefore not be used directly for a formal conformity assessment.

3. Effects of Air Humidity

3.1 Observed Test Trend

With the test temperature fixed at 25°C, the relative humidity of the conditioned air was increased from 10% to 50%.

In the comparative test described here, the wear scar diameter of Reference Fuel A increased from 0.85 mm to 0.93 mm, while that of Reference Fuel B increased from 0.56 mm to 0.65 mm. These observations indicated that increasing air humidity was associated with a larger measured wear scar under the stated test conditions.

3.2 Possible Mechanism

When high-humidity air is introduced into the test system, the fuel may absorb a small amount of moisture from the air. Because water is more polar than the hydrocarbon components of jet fuel, it may alter the adsorption and lubrication behaviour at the metal contact surface.

This change may interfere with the protective lubricating film and increase friction or wear. The extent of this effect can vary according to fuel composition, moisture content, equipment condition and environmental stability.

3.3 Practical Influence

If ambient humidity or conditioned-air humidity is unstable, repeated measurements on the same sample may show increased variation. This can affect test repeatability and make it more difficult to determine whether a fuel complies with the applicable lubricity requirement.

Limits such as ≤0.65 mm for certain military applications and ≤0.85 mm for certain civil applications should be treated as specification-specific examples. The applicable product standard, procurement contract or regulatory requirement should always be confirmed before a compliance decision is made.

4. Repeatability Verification

A fully automatic Ball-on-Cylinder Lubricity Analyzer manufactured by Beijing Chaoyang Gaoke Applied Technology Research Institute Co., Ltd. was used for the verification test. The instrument was configured to monitor temperature and humidity in real time.

With the temperature and humidity maintained within the specified ranges, five replicate tests were performed on each reference fuel, military jet fuel and civil jet fuel sample. Under the stated test conditions, the observed range within each sample group was below the applicable repeatability limit.

These results indicate the repeatability achieved with this particular test setup and operating procedure. Actual results may vary depending on the fuel sample, instrument condition, test consumables, calibration status, operator procedure and laboratory environment.

5. Laboratory Control Recommendations

5.1 Temperature Control

Before testing, the fuel sample should be allowed sufficient time to reach the specified temperature. The fuel temperature should be maintained at 25 ± 1°C throughout the test. Lubricity testing should not be conducted in areas subject to rapid or substantial room-temperature fluctuations.

5.2 Humidity Control

The air supplied to the test system should be properly conditioned or dehumidified, with its relative humidity maintained at 10 ± 0.2% for the test conditions described in this article.

During rainy weather or under high-humidity laboratory conditions, the air-conditioning and dehumidification system should be checked before batch testing begins. Testing should be postponed if the specified humidity range cannot be maintained.

5.3 Equipment and Data Recording

Where test records and traceability are required, equipment capable of continuously monitoring and recording temperature and humidity may be used. Recorded environmental data can support result review, quality control and subsequent investigation of abnormal test results.

5.4 Operating Procedure

Temperature and humidity parameters should be checked before each test. If either parameter is outside the specified range, the condition should be corrected and the sample retested before a formal test report is issued.

6. Conclusion

Increasing fuel temperature may reduce viscosity and weaken the lubricating film, resulting in a larger measured wear scar diameter.

Excessive air humidity may introduce trace moisture and alter lubrication behaviour at the metal contact surface, potentially increasing wear and affecting the test result.

Maintaining the specified temperature and humidity conditions is important for improving the consistency and comparability of jet fuel lubricity measurements. Laboratories should conduct testing in accordance with the current effective version of ASTM D5001 and any applicable product standards, contractual specifications or regulatory requirements.

Technical Notice: The numerical results in this article are based on the comparative tests described above and are provided for technical reference. They do not constitute a guarantee of instrument performance or a universal conclusion applicable to every fuel sample. Before publication or use in formal quality assessment, the test data, specification limits and standard requirements should be independently verified.