Questions about the analysis of diesel cold filter plugging point?
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When analyzing the cold filter plugging point of diesel, why do some samples become cloudy after adding antifreeze in the specified ratio? Sometimes they even take on a milky white color, yet it is still possible to filter them using a suction tube – even at -20°C (excluding the possibility of an error in the ratio used). After adjusting the ratio again, the situation isn’t the same; the temperature reaches -7°C, and the reproducibility is poor. According to the specifications in the standard SH/T 0248-2006 for analysis, it is best to test the sample at a temperature 5°C above its cloud point; thereafter, filtration by suction should be performed each time the temperature drops by 1°C. This process is repeated until it becomes impossible to fill the suction pipette within 60 seconds. The temperature at which filtration first starts is recorded as the cold filtration point for this test. Our sample has a cloud point of 3°C, so filtration by suction was carried out more than 20 times. Note: For item 10.1.8: if the time it takes to reach the scale mark on the pipette after the first filtration exceeds 60 seconds, the experiment is aborted, and the experiment is repeated at a slightly higher temperature ； The meaning of this standard is that as long as filtration is possible, filtration should continue for every 1°C drop in temperature, until the cold filtration point is reached. According to SH/T 0248-2006, there is no requirement to replace the sample or to reheat it; only one sample is used. The standard also does not specify whether it is necessary to take new samples for testing or to reheat them, nor does it set limits on the number of times filtration can be performed. However, the repealed standard SH/T0248-1992 specifies samples at 5–6°C above the expected cold filter point. It is stipulated that the same sample shall not be heated and melted more than 3 times.
It is normal for antifreeze agents to have the effect of lowering the cold filter point. If the same oil sample and the same antidetonator are mixed in the same ratio, the cold filter point should exhibit good repeatability. The optimal oil cooling filter temperature for the original owner should be around 0°C; a drop to -7°C is also considered normal. If the repeatability is poor, the poster should consider relevant testing procedures. The sample preparation process (the state of the oil and the antifreeze, whether it has solidified), the method and temperature used for heating the oil (there have been cases where electric heaters were used, resulting in excessively high local temperatures that could render the antifreeze ineffective), filter issues, and the stability of the filtration pressure. It is most likely the heating method and temperature of the oil. The original poster can look carefully for the reason.