HOBO U20 Water Level Logger Manual
1-800-LOGGERS 3
www.onsetcomp.com
Software
HOBOware Pro software is required for logger operation. Using
a reference water level, HOBOware Pro automatically converts
the pressure readings into water level readings. The software
also supports compensation for temperature, fluid density, and
barometric pressure.
Communication
For launching and reading out the Water Level logger in the
field, you can use a laptop computer with HOBOware Pro and
an Onset Optic USB Base Station (BASE-U-4), with a coupler
(COUPLER2-B) or the HOBO Waterproof Shuttle (U-DTW-1) with
a coupler (COUPLER2-B).
The optical interface allows the logger to be offloaded without
breaking the integrity of the seals. The USB compatibility allows
for easy setup and fast downloads.
Barometric Compensation
The HOBO Water Level Logger records absolute pressure, which
is later converted to water level readings by the software. In
this application, absolute pressure includes atmospheric
pressure and water head. Atmospheric pressure is nominally
100 kPa (14.5 psi) at sea level, but changes with weather and
altitude. Left uncompensated, barometric variations could
result in errors of 0.6 m (2 ft) or more.
To compensate for barometric pressure changes, you can use
the HOBO U20 Water Level Logger as a barometric reference.
The barometric reference is typically deployed in the same well
or at the same location as the water level of interest, but rather
than being placed in the water column, it is deployed above the
water in air.
Barometric pressure readings are consistent across a region
(except during fast-moving weather events), so you can
generally use barometric pressure readings that are taken
within 15 km (10 miles) of the logger or more, without
significantly degrading the accuracy of the compensation.
Therefore, one U20 or weather station (HOBO U30 or H21
recommended) can be used to compensate all of the water
level loggers in an area. The U20-001-01 model with its 0–9m
(0–30 ft) range or the U20-001-04 with its
0–4 m (0–13 ft)
range
are both good barometric references due to their smaller
range, temperature-compensated accuracy, and rugged
stainless steel case. HOBOware Pro includes a Barometric
Compensation Assistant for easy and accurate barometric
compensation.
LEDs
A light (LED) in the communications window of the logger
confirms logger operation.
The following table explains when the logger blinks during
logger operation:
When: The
Light:
The logger is logging
Blinks once every one to four seconds (the
shorter the logging interval, the faster the
light blinks); blinks when logging a sample
The logger is
awaiting a start
because it was
launched in Start At
Interval or Delayed
Start mode
Blinks once every eight seconds until
logging begins
Calibration
The pressure sensor in each HOBO Water Logger is individually
calibrated. During calibration, raw pressure sensor data is
collected at multiple pressures and temperatures over the
calibrated range of the logger (see the specifications table). This
data is used to generate calibration coefficients that are stored
in the logger’s non-volatile memory. The calibration coefficients
are then checked to be sure that the logger meets its stated
accuracy over the calibrated range.
The pressure sensor can be used at pressures and temperatures
that are outside of the calibrated range, but the accuracy
cannot be guaranteed.
Important:
Never exceed the burst pressure of the sensor!
Sleep Mode
The logger consumes significantly more power when it is
“awake” and connected to a base station or shuttle. To
conserve power, the logger will go into a low-power (sleep)
mode if there has been no communication with your computer
for 30 minutes. To wake up the logger, remove the logger from
the coupler, wait a moment, then re-insert the logger.
Sample and Event Logging
The logger can record two types of data: samples and events.
Samples are the sensor measurements recorded at each logging
interval (for example, the pressure every minute). Events are
independent occurrences triggered by a logger activity, such as
Bad Battery or Host Connected. Events help you determine
what was happening while the logger was logging.
The logger stores 64K of data, and can record over 21,700
samples of pressure and temperature.
Setup
Before you deploy the HOBO U20 Water Level Logger in the
field, perform the following steps in the office:
1.
Start HOBOware.
2.
Connect the logger to the computer. See the next section.
3.
Verify the status. Click Status on the toolbar and observe
that the absolute pressure is near barometric pressure for
the location and the temperature is near the actual
temperature.
