The serial number can be found in two places.
1. On the retail box. Labeled 'S/N'
2. On the monitor's plastic shell, beneath the end cap
To locate the serial number on the pHin Monitor shell.
1. Remove the end cap by twisting it, as you did with when you first dropped your pHin in the water
2. Find the laser etched manufacturing information on the bottom of the shell
3. Locate the 9 digit serial number at the top of the manufacturing information, labeled "SN"
4. The serial number can also be found in the small QR code if the serial number label has been obscured. Use your mobile phone to scan the QR (exactly how depends on your phone and software version, iOS 14 shown here)