The ACFTD size calibration method aims to determine the pollution level of solid particles. The definition of irregular shaped particle size is based on the longest chord diameter of the particle, with a standard particle size range of 1 μ m to 100 μ m.

Limitations of ACFTD calibration for liquid particle counters:

1. ACFTD uses an optical microscope to measure the longest chord diameter of particles, while a liquid particle counter measures the equivalent projected diameter of particles. The two have different definitions of particle size;

2. The standard particle size distribution of ACFTD was measured and statistically analyzed by several American college students during their research project in the 1960s, without providing measurement uncertainty. Due to the limited measurement conditions at that time, the human eye and optical microscope had poor resolution for small-sized particles. Therefore, the number of ACFTD small-sized segments measured and statistically analyzed is less than their actual particle size distribution. Therefore, when using ACFTD to calibrate liquid particle counters, significant errors will occur in the small-sized segments;

3. The standard particle size distribution of ACFTD is the result of dust analysis on specific batches at that time. There are differences in particle size distribution between batches, which is also an important source of poor consistency in using ACFTD to calibrate liquid particle counters in different laboratories;

4. The ACFTD contains a large number of particles smaller than 5 μ m (approximately 38%), and when measured using a liquid particle counter, there is a significant overlap error;

The most important point is that ACFTD has now ceased production.

The MTD size calibration method is also aimed at determining the pollution level of solid particles, but the definition of irregular shaped particle size uses the equivalent projected diameter of the particles, with a standard particle size range of 4 μ m~70 μ m.

MTD has significant technical advantages compared to ACFTD:

1. MTD has undergone measurement verification by NIST (National Institute of Standards and Technology), providing measurement uncertainty. Using it to calibrate liquid particle counters, it can be traced back to international units of measurement and has good traceability;

2. The production and control of MTD adopts more advanced technology, with less variability between batches and more accurate particle size distribution;

3. The physical and chemical properties of MTD are the same as those of ACFTD, but it contains fewer particles smaller than 5 μ m (with a content of about 15%~19%), which makes it easier to disperse in oil and reduce the coincidence error during particle counting.

In summary, the physical and chemical properties of ACFTD and MTD are basically the same, but there are differences in processing methods, surface geometry, weight percentages, etc., especially in the definition and distribution of particle size, which are vastly different. Compared to others, MTD adopts more advanced production, control, and measurement methods, which are guaranteed by relevant metrology departments and conform to the trend of technological development. This makes the results obtained through its calibration more accurate, and the consistency between laboratories will also be better.