Monitor your hot tub with state of the art IOT device.
User this application with Photon and firmware from :https://github.com/zeik/hot-tub-meter/tree/master.
You can measure top and bottom temperatures during heating of your Hot tub.
This is especially useful for large Hot tubs heated with wood on your cottage.
You can build the needed hardware yourself, or contact me if you want to purchase ready solution with 3D printed case.