I have level data from a non-vented transducer in a water body and barometric pressure data from the same location. My elevation is 500 meters, how do I determine the true depth?
The true depth can be calculated by subtracting the actual atmospheric air pressure from absolute referenced level data.
Typically a non-vented depth transducer will have an absolute range, but this should be verified by checking the reading at zero depth, which should be around 10 mH20 or 1 bar, which would signify that the transducer does indeed have an absolute reference.
If the reading is close to zero, it will most likely be a sealed gauge reference type instead, this means that it will have a fixed zero offset that will typically correspond 1 bar, which will need to be determined precisely by subtracting the zero depth reading from the actual atmospheric air pressure and then adding the result to all non-vented depth readings, e.g. if Zero depth = -50 mbar, Atmospheric pressure = 980 mbar, then 1030 mbar should be added to all the depth data to convert it to absolute readings.
If the barometric pressure is the actual pressure at that location without any meteorological adjustment for elevation you can simply subtract that reading from the absolute referenced level reading, but be careful to ensure all calculations are carried out in the same pressure units.
If the barometric pressure was obtained from a meterological service or barometer it is most likely to be referenced to sea level and you will need to compensate it for the 500m elevation by using a barometric formula to calculate corrected pressure for elevation.