I found this: Calabrating hydrometer-from Speckledhen. 1. You can calibrate a hydrometer by mixing in a bowl 1/4 cup of salt in 1/2 cup water. 2.Place bowl inside a zip lock baggie with your hydrometer. After at least 8 hours, your hygrometer should read 75%. 3. If it says, for example 80% you know that it is reading 5% to high and then you can make adjustments in your readings and tried on one but, I have one in the bator I wanted to leave there. Now that I have one checked and can put that one in can I use the same mix or do I need to make a new batch of salt solution?