If you some type of carbonaceous and/or highly porous biomass based porous media, there is a good chance that it is going to float. So using standard porosity testing for soils is problematic. You can of course use gas adsorption and surface area analyzers to get volumes of various pores, but this doesn't capture the inter-particle porosity piece of overall porosity. At least I don't think so. (Please enlighten me if this is not the case.)
Has anyone got a simple, reproducible, and accurate way to measure the total porosity of "floatable" porous media such as wood chips, charcoals, crop residue, etc.?
A full adsorption and desorption isotherm will allow you to calculate not only volumes of pores, but also a pore size distribution. This would be analogous to a soil porosity measurement like the flotation media you discussed. What it won't do is measure the permeability of the sample. The density issue (float ability) isn't the only reason your measurements won't work well - the surfaces of these materials, especially the charcoals, can be hydrophobic, so forcing water into a pore space isn't going to work. You could potentially use a less polar, less dense solvent and make density corrections, but otherwise nitrogen or mercury porosimetry is a bit of a standard when it comes to assessing biomass' porosity!
I would try to use liquids less dense than water. Maybe kerosene would be a good idea? It has ben successfully applied to porosity measurements of shales (publication attached).
Article Dual liquid porosimetry: A porosity measurement technique fo...
What Jillian said about utilizing a full adsorption/desorption isotherm to determine pore volume and pore size distribution is correct, but I would substitute argon for the adsorptive gas instead of nitrogen. This is because argon has less quadrupole moment character than nitrogen does; N2 has been known to have selective adsorption, for example, with cations on mordenite surfaces in a mixture of N2/O2/Ar (http://pubs.acs.org/doi/abs/10.1021/ie050128v?journalCode=iecred). Additionally, nitrogen is diatomic and N2 is a linear molecule, which might be problematic.
Probably the best recent paper I've read about optimizing results from the BET method was written by Martijn F. De Lange, et. al., and is entitled,"Adsorptive characterization of porous solids: Error analysis guides the way" and was published in Microporous and Mesoporous Materials in 2014.
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