Delta - Model HD 2301.0 -Portable Humidity Thermometer

The keyboard of the Thermohygrometer Model HD2301.0 is composed of double-function keys. The function on the key is the `main function`, while the one above the key is the `secondary. function`. When the instrument is in standard measurement mode, the main function is active. In the menu, the secondary function is enabled; press the DATA+UNIT keys together to open the. menu.