Well, I've operated the equipment, which pretty much put me off them from the get-go. First, you're measuring something that's in suspension; by definition it's prone to random dispersion, no matter what you try. (And I tried plenty.) The analysis has significant variability when you look at the raw data. Those nice, small two-digit-three-number ISO codes cover more real estate than you might think, which gives the analysis the appearance of precision.
After I climbed off of the bench, I had occasion to wonder "Yes, I'm having particles counted (badly), but where do those particles come from?"
Unless the sample was taken in an ultra-clean particle count jar by an experienced, well trained technician using proper equipment with a procedure that include adequate flushing, then properly protected from contamination right through the testing process, you can pretty much line the birdcage with your particle count results.
You've just succeeded in making a fairly wild estimate of the number of particles in a sample that may or may not (most likely not for the purposes of this exercise) represent the system in question.
But wait there's more...
I haven't got the foggiest notion about the nature of said particles. A little iron filing or a little chunk of cellulose filter media, it's all the same to Mr Laser Particle-Counter, he mis-counts 'em all.
Yeah, when they bring out the particle count data, it's like when the band takes a break and they bring out the karaoke machine.