Overview
The thickness and apparent density of paper are the basic indicators for evaluating its physical properties, which directly affect the stiffness, opacity, printability and end use of paper. Thickness typically refers to the vertical distance between the two surfaces of the paper under standard pressure, measured in micrometers or millimeters. The apparent density is calculated by the quantification (mass per unit area) and thickness of the paper, reflecting the compactness of the paper structure, in grams per cubic centimeter. Accurate measurement of these two parameters is of great significance for paper production quality control, product development and performance evaluation.
How it works:
Paper thickness gauge is a precision instrument based on the principle of contact measurement. Its core components include a measuring surface, an opposing datum, and a device that applies standard pressure. When measuring, the paper specimen is placed between two planes, and the displacement generated by the measuring surface is converted into a thickness reading by a mechanical or electronic sensor at a defined static pressure. To ensure measurement accuracy, the instrument must meet the requirements of relevant standards for measurement pressure, contact area, parallelism and calibration cycle. Typical measurement pressures range from 49 to 100 kPa, and the contact area is typically around 200 square millimeters.
Assay method
The measurement process should be carried out under standard temperature and humidity conditions to avoid changes in paper size caused by environmental factors. First, a sufficient number of specimens are randomly selected from the sample, and the size should be larger than the contact surface of the thickness gauge. After cleaning the measuring surface of the instrument, perform zero point calibration. Place a single specimen flat in the measurement area, apply pressure to the specified value smoothly, and record the reading after the indicated value is stable. Multiple measurements should be taken at different locations on each specimen and averaged. The calculation of apparent density should be combined with the measured thickness and the quantification of the specimen. The calculation formula is as follows:
ρ = (G / t) × 10³
where ρ is the apparent density (g/cm³), G is the quantitative (g/m²), and t is the thickness (μm). 10³ in the formula is the unit conversion factor.
Influencing factors
The reliability of the measurement results is affected by several factors. In terms of instruments, the stability of the measurement pressure, the flatness and cleanliness of the measuring surface, and the accuracy of calibration are crucial. In terms of operation, the specimen should be free of wrinkles and damage, and avoid additional tension when placed. In terms of environment, temperature and humidity need to be strictly controlled because paper is hygroscopic. In terms of materials, the roughness, compressive elasticity and anisotropy of the paper surface will also introduce measurement variations. Therefore, strict adherence to standard operating procedures and documentation of relevant conditions are the basis for ensuring data comparability.
Relevant technical standards
The method for determining paper thickness and apparent density has been regulated by a number of international and national standards. These standards specify in detail instrument specifications, test environment, sampling methods, operation steps, and result presentation methods, providing a unified testing basis for the industry.
| Standard number | Standard name |
| ISO 534 | Paper and cardboard—determination of thickness, density and specific volume |
| GB/T 451.3 | Determination of paper and cardboard thickness |
| TAPPI T411 | Paper, cardboard, composite cardboard thickness measurement |
| GB/T 451.2 | Quantitative determination of paper and cardboard |
Summary
Using a paper thickness gauge to measure paper thickness and calculate apparent density is a standardized and important physical test. By understanding instrument principles, following standard methods, and controlling for influencing factors, accurate and reproducible data can be obtained. These data not only serve the monitoring and adjustment of the production process, but also provide the key performance parameter basis for further processing and application of paper.
References
ISO 534:2011, Paper and board — Determination of thickness, density and specific volume.
GB/T 451.3-2002, Determination of paper and cardboard thickness.
TAPPI T411 om-97, Thickness (caliper) of paper, paperboard, and combined board.
Pan Jixing. History of Chinese Papermaking Technology. Cultural Relics Publishing House.