I bought my carbon from Greens Horticulture they sell by volume not weight so to work how much I needed I worked out the volume of the whole filter in litres and subtracted the volume of the empty centre…. Mine was roughly 15000 cm3 or cc’s so I needed 15 litres of carbon. If you are buying by weight then you need to multiply the volume in litres by 0.45 kg to get the weight in kg's.
The dead filter
One of 4 rivets holding it together
Rivet drilled out with a 4.5 mm drill bit
With the top lited off
with the carbon tipped out
I washed it out as it was dusty inside (I blow into my filter)
The carbon from Greens
Some news paper helped guide the carbon in
Filled to the top after repeated tapping of the sides to settle the carbon down
Riveting back up again although self tappers or the like would have been enough
10 mins start to finish and not as dirty as you'd expect!
Edited to add: cost me £30 for the carbon for a 200mm filter
The filter in the pic was ~£100 new 4 years ago and it's an 8 inch filter. The carbon you need is 4mm air purification activated carbon pellets (google) this is the standard industrial air purification pellets used in industry, they are little black cylinders.
A quick guide based on different diameters of duct won't work as filters are different lengths from brand to brand, but the maths isn't hard, especially if you use an online volume of a cylinder calculator. First measure the outside width / diameter of the filter in cm then measure it's height, using the calculator you will get the volume of the whole filter in cubic centimetres or cm3, this figure is the volume of the whole filter. The centre of the filter though is hollow so we don't need to buy carbon for that so we take the height or lenght we used in the first calculation and the internal diameter of the filter, which is probably the duct diameter, we then work out that volume and subtract it away from the first volume we worked out. This final volume is the volume of space filled with carbon in the original filter in cubic centimetres, we divide this by 1000 to convert it into litres.
1 litre of carbon weighs 0.45 kg so if you worked out your filter volume to be say 10 litres you need to buy 4.5 kg of carbon, a 5kg tub is less than £40, although that was a quick google and a more intensive search could reveal cheaper prices... don't buy of fleabay though as you can't tell spent carbon from fresh.
If you can't deal with the maths then pm me the dimensions and I'll tell you how much carbon you need
I am sure top end filters have a better live span because the carbon is more compressed in them, Randalizer posted a strip down of his in the thread and the carbon is partially crushed and bagged, this will give a larger surface area and therefore a longer lifespan but the same filter could still be refilled with standard industrial grade pellets and work at 100%, just maybe not for as long as it's first life
Edited by distracted, 04 December 2012 - 01:21 PM.