Onset's HOBO® U20-001-01 Water Level Data Logger is calibrated from 10 to 30 psia. The 10 psia lower limit creates an effective altitude restriction of about 10,000 feet when the data logger is used. The unit can be used above 10,000 feet, but it is outside of the calibrated range, and the accuracy specification does not apply. The data logger will function, but the accuracy will get worse the further below 10 psia the pressure drops.
For each psia of altitude, that is one more psi of water that can be added to the depth measurement range. For example, if the data loggers were deployed at 6,700 feet, atmospheric pressure is nominally 11.3 psia. Allow an additional 0.5 psi for atmospheric variations. Therefore, the data 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 feet of water depth. So at 6,700 feet, the data logger can be safely used at depths of nearly 42 feet.
Here are two helpful sites:
- converts altitude to pressure
- converts pressure to inches of water