interfacing a Stratologger altimeter with an Arduino Uno
The logic level converter's low voltage reference pin should be coming from the stratologger's pins 2 and 5 (+3.3V and GND on the DATA port), since that's the source of the actual input signal.

If the stratologger didn't have its own dedicated power supply you could have connected the setup as is right now and used the +3.3V to also power the device, similar to this setup from Sparkfun's logic level converter guide.