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Onset's HOBO Water Level Logger is calibrated from 10 to 30 psia for the 30' range versions. The 10 psia lower limit creates an effective altitude restriction of about 10,000 ft when the logger is used. The unit can be used above 10,000 ft, but it is outside of the calibrated range, and the accuracy specification does not apply. The logger will function, but the accuracy will get worse the farther below 10 psia the pressure drops.
As altitude increases, the nominal atmospheric pressure (psi) goes down. For each psi of altitude, that is one more psi of water that can be added to the depth measurement range. For example, if the logger was deployed at 6,700 feet, atmospheric pressure is nominally 11.3 psia. Allow an additional 0.5 psi for atmospheric variations. Therefore, the logger has 18.2 psi (30 - 11.3 - 0.5) of water depth before it exceeds its rated pressure. 18.2 psi equates to 41.9 ft of water depth. So, at 6700 ft, the logger can be safely used at depths of nearly 42 ft.
Similar calculations can be done for other depth range versions. See the data sheet for the pressure range for the version you are using.
Here are two helpful sites:
This one converts altitude to pressure:
www.grc.nasa.gov/WWW/K-12/airplane/atmosi.html
This one converts pressure to inches of water:
www.convert-me.com/en/convert/pressure