Hey Bertie,
Don't waste your time!
I was referring to those filters last year, being laughed at and told I was way to eager to fly.
We've used them in Africa on AS350's, and compared them to regular sand-filters/particle separators and seen a big diffrence.
But volcanic ash you will be told is nothing like sand, so it does not matter what type filters you use, the best is to stay on ground!
cheers...
by the way, the STC for the MD was received in 2009