Exactly. Also, it's a bit hard to determine but the container looks closed. In my mind that would provide another surface to enhance the triboelectric potential

Exactly. Also, it's a bit hard to determine but the container looks closed. In my mind that would provide another surface to enhance the triboelectric potential you mentioned. Edit: Thanks for that link. Why don't they remove the triboelectric contribution altogether by using a more sensible container for obtaining reliable results?