Thanks so much for this! I made my salinometer the night before using your way and although I didn't place in water quality at state(got thrown into the event last minute... Thats also why i was making my cheat sheet and salinometer the night before) im pretty sure i got the salinity right!The problem we have found seems to be with the clay. It can dry out, change shape slightly, not seal correctly, absorb water. What I have found to be more accurate is to use different materials that are impermeable to water. I have had much more accuracy using a pipette or plastic syringe. The bulb part at the bottom I filled completely with sand (if you do not fill it completely, the air pocket will make the salinometer tip to the sides). Then, because my pipette was not long enough to keep the top above water, I plugged it with some clay and cut a straw in half and pushed the straw onto the pipette. I then taped over the edge of where the straw connects to the plastic pipette. I then started with my 0% solution and marked the line, then I mixed a 10% solution and marked the line. Then using my ruler I made a line that was 1/2 way in-between and then just experimented with eyeballing the other 4 lines in between 0 & 5 and 5 & 10 to make them equally spaced. I have tested this salinometer with other % solutions and it seems to be fairly accurate now. Here is a link of what the plastic pipette looks like (ask your science teacher if they have some!) and I used a larger straw that seemed to fit snugly. I cut the tip of the pipette somewhat to be able to pour the sand in. Hope this is helpful.