Continuous porosity profile measurement of paper, board and nonwovens

Porosity: an important quality parameter

Porosity and air/gas permeability are important quality parameters for many paper, board and nonwoven products. Laboratory testers can be used to obtain porosity measurements from individual samples.

In many cases it would be useful to know if the porosity varies as trends in the machine direction (MD) or in different regions in the cross direction (CD). In all papermaking processes, different regions of the web, especially the edges, will always have different physical properties.

Unlike a Tapio Analyzer, online traversing QCS scanners are limited in sample spacing by the high production speeds, and because the web is constantly moving, their diagonal scan path mixes MD and CD variation, so they do not directly measure true CD profiles.

How does a porosity sensor work?

The figure below presents the basic structure of the sensor. A measurement chamber at the paper web is held at constant pressure (vacuum level depending on a standard) and the air flow rate is measured. In a Tapio Analyzer, the web can be tightened on the sensor surface by geometric placement.

Depending on the application, implementations may include:

• Placing the pressure transducer directly in the measurement chamber
• Reducing the leak rate and improving the seal with a suction ring, or by polishing the surface around the measurement chamber
• Measuring the pressure as well as the air flow, and compensating regulator drift with it

Measurement principle of a porosity sensor. The measurement chamber covered by the measured paper is held at constant pressure by an accurate vacuum pressure regulator and the resulting flow through the paper is measured to determine the porosity.

The Tapio Measurement software allows for convenient measurement and calibration of all involved signals, using pre-determined values, functions or calibration samples to map sensor voltages into the physical quantities being measured.

Benefits of Tapio porosity profiles

Tapio has been offering porosity profile sensors for almost two decades. The porosity sensors are suitable for use in Tapio Analyzers or as online sensors in suitable production lines.

Porosity is available as an option, depending on Analyzer configuration and application. With the Tapio Analysis service you can examine variations that traversing QCS scanners may smooth out, and investigate likely machine or process sources. Tapio Analyzers measure porosity and correlate it with other physical properties (e.g. caliper, basis weight, ash content), with the channels aligned in software to the same positions in the paper.

A high-resolution CD porosity profile measured from 13 true CD strips of paper cut from a parent reel. The gray lines represent the individual CD strips and the blue line is the mean profile. Fine variations like these, repeating in each CD profile, may be smoothed out by a traversing scanner's spot size and filtering.

For customers without access to a Tapio Analyzer, we provide measurement services. Simply:

1. Send us your sample (an MD roll or CD strips cut from the parent reel).
2. We prepare and measure the sample with a Tapio Analyzer.
3. You get the measurement report, the original data and an online review of the findings, including likely sources of variation.

Request Tapio Analysis