"I have several ways of doing it but egg yokes will not begin to cook until 150 degrees and I have also found that using half of an egg shell floating ontop of the water with egg yoke inside moves the temp needed to cook to 160 degree. I tested this mostly on the indoor stove because it has been cloudy but I feel confident that it will not be a problem to use this in a solar cooker because there seems to be soo many diffrent ways it can be used. Also one egg yoke can be used to test many diffrent cooking pots and the egg yoke can be eaten after the water has reached temp so there is zero waste. Or they could simply place the entire egg yoke in the pot and when it is done they eat the whole egg yoke."
After thinking more about this the idea of another food like potato, carrot, or any local type of food that has a reaction at a certain temp would have the same purpose as the egg yoke. The item would be placed in the water and when it has the reaction then the person would know that the water is safe to drink. Maby it becomes tender or has a reaction of changing color or??? I am thinking it is similar to the early europeans who thought that only beer was safe to drink becase they did not understand that the boiling of water was what made it safe to drink. The people who are in need of a way to test water problly are not getting sick from the soups or sauce of the foods they are making because they are looking at the food that is being cooked and waiting until it is done which would be an indicator in the food of it becoming tender or a change of color.