The Plus monitor records radon within the first hour of having batteries inserted, but the short term reading will not be displayed on the LCD screen until the first 24 hours have elapsed. If you wish to know the hour/hour reading prior to the initial 24hrs, you will need to plug the unit into your PC with the provided USB cable and the CRA SW.
Note: This also applies to all new tests started. The first 24 hours will not be displayed on the LCD screen. Use the CRA software if needed to obtain your readings.