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How to convert results measured by the Porozit to the new French federation standard

Introduction

Many of you asked us how do we convert results measured by the POROZIT: into cmcm2 / sec (pressure of 200 mm of water) and litre/m2/ sec (pressure of 200 mm of water).

The conversion into the above units would be a simple task if we could be sure that the airflow approximately doubles as a result of approximately doubling the suction. If due to the greater rate of airflow the flow changes turbulences can occur. This non-linear flow may decrease the volume of air flowing through the textile in a given unit of time.

Therefore, apart from the calculated results we had to build a device operated with stronger vacuum and we had to carry out comparison measurements between this device and POROZIT.

The measured results have to either support or discredit the calculated results.

In summary the determination of the new unit:

1. With calculation
2. With comparison measurements carried out with paragliding textiles
3. Conversion of cm³/cm²/ sec into Litre/m²/min

1. Calculation explained clearly:

Achievable unit cmcm² /sec (pressure of 200 mm of water)

Data of the POROZIT device:

• Average measuring vacuum – 10 mbar
• Suction surface – 38.50 cm2
• Measured air volume: – 250 cm3 Let’s presume that our POROZIT device displays 1 sec at the end of the measurement. This means that under 1 sec 250 cm3 flew through the 38.50 cm2 surface of the textile.

We however, are interested in the amount of air flowing through 1 cm2 surface of the textile. Therefore, the total amount of air volume has to be divided by the suction surface: 250cm³ / 38.5. That is 1/38.5 part of the total air volume (250 cm³) flows through 1 cm2 surface of the textile in 1 sec. In case of 10mbar vacuum: when using the POROZIT 250/38.5 = 6,5cm air is let through 1 cm2 surface in 1 sec.

What remains is the increasing of the vacuum:

• The pressure (or vacuum force) of a water column of 200mm = 19.6 mbar
• The current vacuum is 10 mbar therefore, this has to be multiplied by 1.96 so that it equals to 6mbar which equals to the pressure of a water column of 200 mm.

If we presume that the volume of air flowing through the textile increases directly proportionally with the increase of pressure we get the following equation:  So, by calculation we get this result which figures still have to be verified in practice by the measurements.

2. Practical measurement carried out with textiles of varying porosity:

We performed numerous measurements and the measurements verified the calculated results by minor differences. That is, as a result of almost double the volume of vacuum almost double the amount of air flew through the various textiles. The air flow changes directly proportionately with the change of pressure in the lower registers of pressure according to academic literature as well.  Consequently, the calculated and the measured results are very similar, so we used the average of the two methods.

As a result, the recommended conversion is the following (12.3+12.7)/2=12.5  3. Conversion of cm³/cm²/ sec to Litre/m²/min:

What is left now is the conversion to Litre/m²/min (pressure of 200 mm of water).

• So, the volume is multiplied by a thousand: 1000cm³ = 1dm³ = 1 Litre
• The surface is multiplied by ten thousand: 10000cm² = 1m²
• The time is multiplied by sixty: 60 sec= 1 min

So, the converter figure is:    So, these figures are stricter than in case of other manufacturers but in order to ensure flight safety we request that you use these conversions when using POROZIT devices.

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