HOBO U20 Water Level Logger Manual
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© 2008–2017 Onset Computer Corporation. All rights reserved. Onset, HOBO, and HOBOware are trademarks or
registered trademarks of Onset Computer Corporation. All other trademarks are the property of their respective
companies.
This product has been manufactured by Onset Computer Corporation and in compliance with Onset’s ISO 9001:2008
Quality Management System.
Patent #: 6,826,664
12315-H MAN-U20
The black acetal cap is provided to help protect the
communications window. Acetal is resistant to most solvents,
fuels, and lubricants.
The polycarbonate communications window is sealed as an
additional barrier to water and dirt entering the logger housing.
Compensating for Drift
All pressure sensors drift over time. The drift for the pressure
sensor and electronics in the HOBO Water Level logger is less
than 0.3% FS (worst case) per year. In most applications, drift is
not a significant source of error, because the offset created by
any drift is zeroed out when you take a manual reference level
measurement and use the logger software to automatically
calculate the level readings relative to the reference
measurement. In effect, you are re-zeroing the sensor each
time you apply a reference reading to the data file.
Pressure sensor drift matters only when absolute pressure
values are needed, or if there are no recent reference level or
depth measurements available. For example, if the logger is
deployed for one year and no new reference level readings are
taken during the deployment, it is possible that the sensor
could have drifted as much as 0.3% FS by the end of the
deployment.
It is possible to determine the actual amount of drift during a
deployment if a reference level is taken at the beginning and
the end of a long-term deployment. The results of applying the
two different reference levels (once at the beginning of the
data file, and again at the end of the data file) can be
compared. Any difference between the files indicates the
amount of sensor drift (assuming accurate reference levels).
Verifying Accuracy
You can check the
differential accuracy
of your loggers for
water level measurements by deploying the loggers at two
depths and comparing the difference in level readings. When
verifying the accuracy this way, be sure to allow the loggers’
temperature to stabilize at each depth. Use the logger software
to convert the readings from pressure to level. The level
readings should be taken close enough together that the
barometric pressure does not change.
You can check the
absolute pressure accuracy
of your HOBO
Water Level Logger by comparing its ambient pressure readings
to a second HOBO logger. Their readings should be within each
other’s specified accuracy. Alternatively, you can check the
pressure reading against an accurate local barometer. If you
use a non-local source of barometric information, such as the
NOAA website, adjust for altitude.
Recalibration
If you would like to have your logger’s absolute accuracy
verified against a NIST standard, or to have your logger
recalibrated, contact Onset or your place of purchase for
pricing and return arrangements.
The Battery
The battery in the HOBO Water Level Logger is a 3.6 Volt
lithium battery.
Battery Life
The battery life of the logger should be about five years or
more. Actual battery life is a function of the number of
deployments, logging interval, and operation/storage
temperature of the logger. Frequent deployments with logging
intervals of less than one minute, and continuous
storage/operation at temperatures above 35°C will result in
significantly lower battery life. For example, continuous logging
at a one-second logging interval will result in a battery life of
approximately one month.
To obtain a five-year battery life, a logging interval of one
minute or greater should be used and the logger should be
operated and stored at temperatures between 0° and 25°C (32°
and 77°F).
Voltage
The logger can report and log its battery voltage. If the battery
falls below 3.1 V, the logger will record a “bad battery” event in
the datafile. If the datafile contains “bad battery” events, or if
logged battery voltage repeatedly falls below 3.3 V, the battery
is failing and the logger should be returned to Onset for battery
replacement.
Replacing the Battery
To have your logger’s battery replaced, contact Onset or your
place of purchase for return arrangements. Do not attempt to
replace the battery yourself. Severe damage to the logger will
result if the case is opened without special tools, and the
warranty will be voided.
WARNING:
Do not cut open, incinerate, heat above
100°C (212°F), or recharge the lithium battery. The battery
may explode if the logger is exposed to extreme heat or
conditions that could damage or destroy the battery case.
Do not dispose of the logger or battery in fire. Do not
expose the contents of the battery to water. Dispose of the
battery according to local regulations for lithium batteries.
