As far as sawdust is concerned, then for sure skeletal density will always be different from bulk density, irrespective to the way the latter is measured. As no extremely narrow pores are expected, you might measure the skeletal density by applying Boyle-Mariotte equation to your material using air instead of helium. If you are definitely not equipped for doing this, you can take the value of skeletal density taken from the literature for the same wood species.

Alain

Many thanks Alain for your insightful response.

Another way is Hg intrusion. Bulk density is surely different from skeletal density in wood! In bulk density you include the pore volumes however in skeletal density you exclude them! that is a big difference.