Suspect Depth Reading with C120W/DSM30/SS60

[TideRace]

I have a C120W display connected to a DSM30 sounder module and SS60 transducer. I have noticed that the depth reading reported by the Raymarine system is always different than the depth reading reported by my Furuno 585/B164 system, although the offset is not consistent. Usually they are off by 5-6' but I have seen a difference of 12'.

The transducers are at the same depth in the hull, approximately 4' apart, fore and aft.

Referencing shallow water depths in my harbor and depth data from charts offshore, I am inclined to believe the Furuno.

1. Could this be a software (update) issue for the C120W and/or DSM30?

2. Is there a calibration function for the DSM30?

3. Is there an Operation Manual for the DSM30 (I haven't been able to find one)? There are many features and settings that I may want or need to explore or adjust but I'm not finding it intuitively obvious to do so without a manual.

Thanks for any help.