Is it possible to use the VoltDiff function with the OBS-3+ to get an output in mV (similar to what it can be done with the OBS-3-LD&A)?
The OBS-3+ is measured using a voltage measurement. The OBS-3+ manual, Appendix B, shows an example CR1000 program. This uses a VoltSe to measure the sensor. You could easily use VoltDiff instead if you have enough channels available on your logger.
The example program converts the mV to NTU, but you could easily change this to just store the mV if you want.
The manual can be found here :-