The original pulp freeness measurement, as measured by either the Canadian Standard Freeness test (CSF) or the Schopper-Riegler test (SR), is to a great extent a measure of the fibrillation and fines content of pulp, and simulates the drainage rate on the wet end of a paper machine. The CSF test starts with one liter of pulp suspension at a consistency of 0.3% being poured into a drainage chamber. The bottom lid and the air valve on the drainage chamber are opened to start the drainage process. The flow of water through the screen at the bottom decreases as a fiber pad builds up in the bottom of the chamber. The total volume in milliliters of water that drains from the chamber is the freeness value of the sample.
In a manually operated freeness test, variation in how the sample is handled each time a test is made means the precision and reproducibility are not as high as in automatic tests. Another drawback of manual freeness tests is the time needed to collect samples and determine pulp consistency. It is quite common that only a few measurements per sample point can be carried out in a day, and this low frequency of freeness measurements is not suitable for controlling the process with any precision. These drawbacks are easily overcome using an online automatic freeness testing system, where the samples are handled the same way every time, both in the preparation stage and in all the tests, resulting in better accuracy and precision. Plus, automatic freeness and fiber analyzers can collect samples from multiple locations and process hundreds of measurements per day—giving much more data that can be used for tighter process control.
Freeness measurements are also used to determine whether pulp has been sufficiently developed by refining. The principal objectives of pulp refining are to:
- Increase the flexibility of the cell wall to promote increased contact area
- Fibrillate the external surface of the fiber and generate fines
This further promotes the formation of hydrogen bonds and increases the total surface area available for bonding. When more strength is desired for a certain paper grade, the specific energy of the refining process can be increased by narrowing the gap through which the fibers are forced in the refiner. Because this results in a lower freeness, the freeness measurement is often used as an indicator of the degree of refining.