The degree of pulp knocking is a comprehensive index that characterizes the degree of cutting, filamentation and brooming of pulp fibers during the pulping process, usually expressed in °SR (Schober degree). The measurement results directly affect the prediction of the physical properties of paper and the regulation of the production process. The measurement process is carried out according to standards such as ISO 5267-1 and GB/T 3332, but operational details and environmental factors often introduce errors, affecting the accuracy and repeatability of the data.
Common measurement errors
The error mainly occurs in four aspects: instrument status, operating process, sample processing and environmental conditions. These factors often interact to deviate from the true value of the final reading.
Instrument-related errors
The state of the measuring instrument is the foundation. Worn or clogged water filters can alter the water flow velocity, resulting in systematic high or low results. Temperature sensor deviation directly affects the calibration, as water temperature is a critical parameter (standard requires 20±0.5°C). Circumvention measures include regularly calibrating the water temperature with a standard thermometer and checking and replacing the filter as often as specified by the standard. Make sure the Schober meter is placed horizontally, otherwise changes in the water flow path will introduce bias.
Operational error
The inconsistency of the operation method is the main source of random error. Insufficient sampling representation, inaccurate slurry concentration determination, uneven agitation, or inconsistent pouring speed can significantly affect the results. Avoidance requires strict standardized operation: samples should be fully mixed and quickly sampled, and concentration determination should be carried out in parallel; When pouring slurry into the measuring cylinder, the fixed height and speed should be maintained; At the same time, the action of pressing the stopwatch and opening the drain valve needs to be practiced simultaneously until proficiency. If the sample itself contains more bubbles or undispersed fiber masses, it will also interfere with water filtration, so violent shaking should be avoided during pretreatment.
environmental error
Fluctuations in ambient temperature will affect the stability of the slurry water temperature and the size of the instrument components. The laboratory should keep the temperature and humidity relatively stable. In terms of calculation, the knock degree value relies on measuring the time T (seconds) required to filter out a specific volume of water, obtained by empirical formulas or lookups. Formula application errors or table lookup deviations are common oversights. For example, it is necessary to confirm that the formula used matches the instrument model and standard version. The recording time should be accurate to 0.1 seconds.
Summary of key control points
| Error category | Critical control points and evasion actions |
| Instrument status | Calibrate the water temperature sensor regularly; Check and replace the filter screen according to the standard cycle; Keep the instrument at absolute level. |
| Sample preparation | ensure that the slurry sample is uniform and representative; Accurately measure and adjust to standard concentrations; Avoid introducing air bubbles. |
| Operation process | Standardized pouring action synchronized with timing; Use a metrology verified stopwatch; Fix the same operator or cross-compare. |
| Environment & Data | control laboratory ambient temperature stability; Check the applicability of the formulas or conversion tables used in the calculations; Conduct parallel tests. |
Validity confirmation
It is recommended to perform at least two parallel tests per measurement. If the difference between the two results is outside the standard range (e.g., the tolerance is often ≤1°SR when the knock value is 20-50°SR), it should be discarded and retested. Establishing an in-house calibration program and regularly using stable slurry samples for repeatability testing is an effective way to monitor the stability of the entire measurement system.
Conclusion
The accuracy of pulp knock measurement relies on fine control of the entire process. By systematically managing instruments, standardizing operations, controlling the environment, and handling data rigorously, common errors can be significantly reduced, providing a reliable technical basis for production and research. Continuous personnel training and equipment maintenance are at the heart of ensuring measurement quality.
References
ISO 5267-1: Pulps — Determination of drainability — Part 1: Schopper-Riegler method.
GB/T 3332 Pulp - Determination of pulp beating (Schober method).
TAPPI T 227 om-19: Freeness of pulp (Canadian standard method).
Wang Zhonghou, Pulp and Paper Analysis and Testing, China Light Industry Press.