Before looking at a paragliding porosimeter, let's get some boring stuff out of the way quickly now. Here's a formal definition of how porosity is measured, called porosimetry:
Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous nature, such as pore diameter, total pore volume, surface area, and bulk and absolute densities.
The technique involves the intrusion of a non-wetting liquid (usually mercury) at high pressure into a material through the use of a porosimeter. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid's surface tension.
Bleehhh. Do you have to know that to take care of your paraglider? Thankfully, no. The idea behind using a porosity-measuring device in paragliding is simpler than that. Here's a definition that is directly applicable to paragliding.
Wing porosity can be defined as the air permeability of its cloth, measured in liters per square meter per minute. The test is done under the standard pressure of a 200 mm water column, that is 20 millibars.
Mmmm that's a bit better. To put it really simply, the porosity of your paraglider canopy is how fast you can force air through the fabric. If the answer is 'like a sieve', then would you feel safe flying in it?
To get a good measure of porosity, you need an instrument called a porosimeter. But not an industrial one like the Sheffield Instrument device or the PoreMaster Mercury Intrusion device. There's really only one instrument that's widely used in paragliding, and that is the JDC Mk-1 Porosimeter. See further down for it's specs.
Actually, there's another one around, called the Speedtech Instruments MK-1 Porosimeter. But I strongly suspect it's the same unit as the JDC, just sold under a different name.
In a nutshell, as porosity increases over time, your paraglider canopy starts behaving less and less like a wing, and more and more like a parachute! Wings are supposed to maintain a pressure difference between the top and bottom surfaces as they fly through the air. So you don't want air leaking through and reducing that pressure difference, it kills performance.
A modern paraglider straight out of the factory has very low porosity. If it eventually ends up with high porosity, all those fancy DHV tests don't mean much any more. The wing was not tested in that condition!
It's ok for a round parachute to have some porosity since it's main function is to produce high drag forces to slow down descent. A bit of air leaking through makes little difference.
So porosity is quite important from a safety point of view, not just performance. Annual canopy inspections are becoming standard now, and they include porosity checks. Not much point in doing them yourself though, porosimeters aren't cheap!
The numbers obtained from a series of porosimeter checks will tell you how far your wing has degraded, and whether it should be retired from flying. You just can't tell by looking at the material. Also, these numbers are a good indicator of second hand value. The people who do the checks know how to value a paraglider like this.
It seems the industry standard for paragliding, and possibly other air sports as well, is the JDC Mk-1 made by JDC Electronic S.A., a Swiss company. I've seen prices anywhere from around $1100 through to $1500. Their simple-to-operate meter measures the time it takes for 0.25 liters (0.0088 cubic feet) of air to pass through 38.5 cm² (5.97 in²) at 4 hPa (10 mb) of pressure. The time obtained then has to be converted to the standard units of liters per square meter per minute to get a permeability score. Here's some more specs.
Some sources recommend a porosity check on a new paraglider at 2 years or after 200 hours of flight time, whichever occurs first.
Another thing to keep in mind is that darker colored canopies should be checked somewhat earlier since they absorb considerably more UV from the sun. Maybe a good reason to consider lighter colors when making that purchasing decision!
And apparently coated materials, whatever their advantages, are more likely to degrade with usage. Fraying or delaminating might also be a problem, eventually. Of course this might change in the future with advancing technology, but just keep it in mind.
Porosity tests are done very carefully and systematically by some manufacturers. I won't go into all the finer details of porosity checking here, since you are unlikely to have to do-it-yourself.
I'll just mention that the permeability score of your wing is just a number which represents the average liters per square meter per minute of airflow through the cloth. They range all the way from 0 - 20 for 'as new' cloth condition, through to 150 - 300 for 'strongly-used but still airworthy'. Anything over 300 really shouldn't be flown! At least higher than you're prepared to fall, say coasting along some little sand dunes down at the beach or something.