Online porosity measurement systems have been operating successfully in paper mills on the late 90´s. The technology started with high cost specialty paper grades but has spread to high-quality coated and kraft paper machines throughout the world. The concept of measuring and controlling porosity online exploded early 2000.

Laboratory testing procedures are delayed and also subject to error because of the small sample size. Measuring several hundred feet of the paper during one testing cycle, the online system averages the equivalent of thousands of sampling points compared to a single, delayed, sample from the laboratory and therefor reflect much more accurately the true porosity level at any time and just for the finished reel.

When the online porosity measurement system was initially introduced, it was not designed as a traversing system.

Since paper machine components are causing variations both in the cross-machine direction and the machine direction, identification of variations associated with these components also required cross-machine direction testing. Most variations in porosity, however, are caused by sources located before the headbox in the papermaking process. Therefore, the variation is usually distributed relatively uniformly on the web in the cross-machine direction, so a traversing system is not required.

Read more about this in PULP & PAPER MAGAZINE: Online Porosity Measurement System Offers Cost-Effective Quality Control


Porosity in Paper

Porosity in Paper, if you can measure it accurately and reliably you can control it!

There are several Online Porosity Measurement Systems on the market capable of measuring porosity in paper. Accurate on-line measurement and control of porosity or air-permeability is used in the pulp & paper industry.

Analogue porosity systems have been sold over the past 30-40 years. Some were not able to withstand the rigours and stringent requirements imposed by the paper industry and have been discarded. Others failed to maintain operator confidence due to calibration and reliability issues leaving mill managers wary of claims that ‘All problems have now been resolved znd several major supplier of scanning systems (QCS) to the paper industry have been selloing on line porosity system since early 1990s. Generally, these systems infer porosity measurements based on a series of assumptions and mathematical models.

Most Systems using an analytical platform for on-line measurement of porosity of most continuous sheet processes. It is a precision device that measures either air permeability or air resistance. In addition, these systems provides intelligence at the measurement sensor to ensure measurement reliability and accuracy required for closed loop control of refining, filler, stock flow ratios, etc

These Systems measures Porosity (Air Permeance or Air Resistance) of the continuous web of paper, glass fiber matt or any other continuous sheet or felt, using a measurement method similar to standard laboratories techniques.

The measurement system monitors the time required for a known volume of air, under a known differential pressure, to pass through a given area of the moving web. The elapsed time is converted into porosity or air permeance units used by the customer. Optionally, it measures the volume of air passed through the web using the unique TimeSlice platform supporting full web slice integration of scanning systems. The measurement sensor incorporates intelligent signal processing and its automatic calibration maintenance and self cleaning cycle will operate reliable in the manufacturing environment. The software platform provides data acquisition, analysis, presentation and reporting for real-time on-line usage for the operating staff and long term storage and grade data retrieval for management reporting and historical quality analysis.

The ability to measure sheet porosity on-line has been around for quite a few years, but many such on-line porosity systems have been discarded due to doubts about measurement accuracy and the need to reject a large proportion of the readings. Very few systems have therefore been used for closed loop control. The development of improved sheet sensor contacting technology in this category has greatly reduced the number of readings that need to be ignored and allowed closed loop control to become a reality and can now be clearly demonstrated.

Demand for these systems has continued as the high return on investment in it grows. Payback is typically measured in weeks rather than months or years. Every installation result in an increase in average porosity and a decrease in porosity spread.

Immediate benefits of a freer sheet include improved runnability, reduced draw between sections and fewer sheet breaks. Porosity related losses at grade change are generally eliminated isung these porosity measurement and control systems.

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Porosity in Geology

Porosity of a rock is a measure of its ability to hold a fluid.

The percentage of pore volume or void space, or that volume within rock that can contain fluids.

Mathematically, porosity in geology is the open space in a rock divided by the total rock volume (solid + space or holes). Porosity is normally expressed as a percentage of the total rock which is taken up by pore space.

Porosity in Geology can be a relic of deposition (primary porosity, such as space between grains that were not compacted together completely) or can develop through alteration of the rock (secondary porosity, such as when feldspar grains or fossils are preferentially dissolved from sandstone’s).

For example, a sandstone may have 8% porosity. This means 92 percent is solid rock and 8 percent is open space containing oil, gas, or water. Eight percent is about the minimum porosity that is required to make a decent oil well, though many poorer (and often non-economic) wells are completed with less porosity.

Porosity can be a relic of deposition (primary porosity, such as space between grains that were not compacted together completely) or can develop through alteration of the rock (secondary porosity, such as when feldspar grains or fossils are preferentially dissolved from sandstone’s).

Porosity can be generated by the development of fractures, in which case it is called fracture porosity. Effective porosity is the interconnected pore volume in a rock that contributes to fluid flow in a reservoir. It excludes isolated pores.

Total porosity is the total void space in the rock whether or not it contributes to fluid flow. Thus, effective porosity in geology is typically less than total porosity. Shale gas reservoirs tend to have relatively high porosity, but the alignment of platy grains such as clay makes their permeability very low.