Can't Judge a Powder
Can the 9-v conductivity meter be commercial product, or does it have to be home-made?
As long as the conductivity meter is operated by a 9 volt battery, it is allowed.
how many grams of the sample are given to the students? If they run out, can they ask for more without penalty?
It will be up to the event supervisors. The rules state that event supervisors must notify the students at the beginning of the competition if refills will be allowed or not. Since materials vary widely in density, there is no minimum or maximum number of grams to provide to the students.