I am developing a SOP for soil texture analysis (mastersizer 3000 hydro EV, wet process) but I find different results each time;

I have to decide on:

-Choice of refraction and absorption index (I use 1.53 and 0.1)

Choice of dispersion (I use pre-measured ultrasound for 5 min with a frequency of 50% (max 40Khz)) but it does not allow disintegration if the agitation is low.

-Choice of agitation (2500 rpm) that breaks down the particles and the soil becomes silty to clayey.

-20% sucking

-Choice of measurement time.

Thank you for your feedback