Freezing Point and Pour Point: Two Seemingly Similar Indicators, What Are the Fundamental Differences Behind Them?

When conducting low-temperature performance testing of oil products, pour point and freezing point are two indicators that often appear simultaneously and are easily confused. Many people in the laboratory told me that when they first started in the field, they couldn't figure out why sometimes they measured pour point and sometimes freezing point, and what criteria to use depended on what. Today, I will thoroughly clarify this issue.

Differences in measurement principles

Condensation point determination (performed according to standard GB/T 510): Place the oil sample into a test tube, cool it down at a specified rate, and tilt the test tube at 45° at every designated temperature interval for 1 minute, observing whether the oil surface moves. When the oil surface no longer moves, the temperature is the condensation point. This method determines the critical point at which the oil "completely loses its fluidity".

Pour point determination (performed according to GB/T 3535, corresponding to ASTM D97): The process involves cooling, but every time the temperature drops by 3℃, the test tube is tilted to observe whether the oil sample flows. When the sample no longer flows, the temperature at that point is added by 3℃ to obtain the final pour point result. The "+3℃" correction is necessary because the temperature interval for each inspection is 3℃, and the actual "initial non-flowing" temperature is somewhere between the observed temperature and the temperature increased by 3℃.

The relationship between the resulting numerical values

The results of the pour point and freezing point of the same oil sample are not exactly the same. The pour point is usually 3 to 6℃ higher than the freezing point. This is not due to instrumental error, but rather the difference in measurement definitions themselves - the freezing point requires the oil surface to be "completely still" within one minute after tilting, with stricter judgment criteria; the pour point can be recorded as long as the oil is observed to be non-flowing, which is slightly more lenient.

Which standard to use on which occasion

In the domestic electric power and petroleum industries, the freezing point (GB/T 510) has been used as an assessment indicator for the acceptance of transformer oil and lubricating oil for a long time in history, which is reflected in many domestic standards.

The pour point (GB/T 3535/ASTM D97) is a common standard for international petroleum product trade and imported oil products. HZNQ-1101 clearly complies with ASTM D97 and is suitable for occasions where international testing reports are required, such as the acceptance of imported lubricating oil and the inspection of exported oil products.

The difference in practical operation

The structures of the two measuring instruments are basically the same, with the main difference lying in the operational steps: for the pour point test, a wait time of 1 minute is required after each temperature tilt, and the pour point can be checked every 3℃ drop, resulting in slightly higher overall efficiency. The HZNQ-1101 supports two testing modes, allowing operators to select the corresponding steps based on the actual standard being implemented.