Determination of paper air permeability

1. Fundamentals

The pressure difference method is used to place the pre-processed sample between the upper and lower measuring surfaces, and a constant pressure difference is formed on both sides of the sample.Under the action of pressure difference, the gas flows from the high pressure side through the sample to the low pressure side, and the permeability of the sample is calculated according to the area, pressure difference and flow through the sample.

1. Definition of breathability

Average air flow through paper and board per unit area, expressed in microns per pascal-second [μm/ (Pa·s)], at unit time and unit pressure difference under specified conditions.Its calculation formula is as follows;Ps = V/(delta p, t)

Where: Ps — permeability, μm/Pa·s

V — Volume of air passing through the sample during measurement time, mL

△ P — Pressure difference on both sides of the sample, kPa

T — Determine the time, s

1. Unit of breathability

The unit of paper permeability is “μm/ (Pa·s) (micron per PASCAL second)”.1μm/ (Pa·s) =1mL/ (m2·Pa·s) =1L/ (m2·kPa·s).For example, a paper with a breathability of 10μm/(Pa·s) can be interpreted as 10mL/s (10mL/s) of air flowing through 1 square meter (m2) of paper at a pressure difference of 1 Pa (Pa).Or at a pressure difference of 1 kPa, air flow through 1 square meter (m2) of paper is 10 liters per second (10L/s).

In practice, breathability is also expressed in other units, such as CU [cm3/(min·cm2)] for cigarette paper, which refers to the air flow rate (ml /min cm3/min) across 1 square centimeter (cm2) of the surface of the tested sample at a pressure of 1 kPa.In the “Gellet method” breathability meter, the breathability unit is also expressed as “Gellet second (s)”, which refers to the time taken to pass 100 mL gas in an average pressure difference of 1.23 kPa and test area of 6.42 mm2.

1. Test area and pressure difference used by breathability tester

The test area and differential pressure of the laboratory breathability meter are specified in the standard.Common areas are: 10cm2, 6.42cm2, 2cm2 and so on.The common pressure differences are 1kPa, 1.23kpa, 1.47kpa, 10kPa, etc.Theoretically, the flow rate is proportional to both area and pressure difference. For a numerical permeability sample, specifying the test area and pressure difference ensures that the flow rate is in a reasonable display range.For paper, due to the uneven and its own characteristics, in fact, the flow with area and differential pressure are not fully proportional to the relationship.So each test method standard specifies the test area and differential pressure.In special cases, such as the sample with too much or too little permeability, the test area can be appropriately reduced or increased to ensure the flow in a reasonable range of display.

5, several methods of paper permeability standards

At present, the international standard methods for measuring paper permeability are as follows:① Schopper method,② Bendtsen method,③ Shffield method, ④ Gurley method,⑤ Oken method.

Each method has different requirements, but its core is to improve the accuracy of pressure difference and read flow rate in different ways within the measurement range.”Shaub method” is to use the water level height difference to indicate the pressure difference on both sides of the test sample, in a certain time by measuring the drainage volume to calculate the flow.In this method, the difference of water level and height is used to indicate the pressure difference on both sides of the sample.It is not only inaccurate, but also limited in measuring range. This method is out of date now.Under constant pressure difference, “Bentsen method” and “Scherfeld method” switch different range flowmeters according to different permeability of the sample. The flowmeter error is about 5%, which is difficult to improve.For low permeability samples, it is simply not suitable for an accurate flowmeter.Although “Gelai method” better solve the measurement repeatability problem, but in its principle structure, the cylinder moves downward according to its own weight to form a pressure difference, and the pressure generated is affected by the falling speed and sealing fluid.There is also an error between the actual flow value and the displayed calculated value.The defects are particularly prominent for samples with higher permeability.

6, model RH-TQ100 permeability meter performance characteristics

Rh-tq100 paper permeability meter is developed and produced by Guangzhou Runhu Instrument Co., LTD.According to the different range of permeability of the sample, it is divided into different models A, B and C.Measuring range from 0.01μm/Pa·s to 3330μm/Pa·s, basically covering the whole range of paper permeability.

Rh-tq100 paper permeability tester adopts the latest mechanical and electrical technology, which greatly improves the test accuracy and accuracy.Pressure difference, the use of high precision pressure sensor, resolution of 0.001 kPa;Flow measurement calculation, the use of mechatronics technology, very accurate measurement of the gas flow through the sample, its accuracy within 1%.Much higher than the general standard of 5% error.The measurement calibration of pressure difference and flow rate is traceable.Both high permeability paper (such as filter paper, molding paper, toilet paper, etc.) and low permeability paper (such as greaseproof paper, insulating paper, coated paper, etc.), have a good measurement effect, measurement repeatability within 1%.The instrument has obtained two national patents.