A fellow TFF'er just asked how I obtain the temperature readings I include in my reports. As we move toward summertime stratification and the development of a thermocline, this will become increasingly important to understand.
I use the FishHawk TD device. It takes temperatures in 5-foot increments. It has a non-replaceable coin battery, but, I've had my unit for over 6 years on the original battery, using it once per week.
Here is one of the first links that popped up on a Google search: FISHHAWK TD
Holy cow! Where did you get that? That was my dads invention back in the early 70's. He made them in his garage. He still has a few. Dads gonna be 86 this year. He is gonna freak out when I show him this.