The Device Disconnection Setting is what determines the time the Moasure device disconnects and turns off.
The Moasure device will generally turn off within a minute if you turn it on without starting a measurement to save your battery. You can change Moasure Device time out setting through your Settings. Below are the steps to do so:
Open Settings by tapping on the cog icon in the right upper corner
Tap on Measuring Options
Tap on Moasure Device Disonnection and change it to your desired value. You can select the following values: one Minute, two minutes and five Minutes.