Both the SafeDip and the SaltDip displays have liquid crystal displays (LCD) much like today’s TV’s and cell phones. The storage of the SafeDip and SaltDip would need to follow the same lines. You do not want to leave these items in an outside storage area where it will drop below freezing (32°F). Should you leave your tester in a freezing temperature, you need to transition the unit to a safe temperature SLOWLY. Do NOT immediately take it from freezing to a hot location such as the inside of a house, or a car that with the heat running.
LCD generally recover as they warm up and will operate normally afterwards. However if you actually let the crystal freeze, you can damage or permanently alter the characteristics of an LCD display that hasn't been designed for low temperature use. Exposure to low temperatures can create bubbles or leading to permanent damage to the display.
So in cooler temperatures, just keep in mind it’s best to keep your SafeDip & SaltDip indoors.