Onset U20L Operating Manual

HOBO® U20L Water Level Logger (U20L-0x) Manual

The HOBO U20L Water Level Logger is used for monitoring changing water levels in a wide
range of applications, including streams, lakes, wetlands, tidal areas, and groundwater. Using
HOBOware® Pro, you can easily configure this logger to record absolute pressure and
temperature data. This logger features a ceramic pressure sensor, durable housing, and a
protective end cap for deployment in existing wells or stilling wells. Without cumbersome vent
tubes or desiccants to maintain, this easy-to-use logger is an ideal solution for water level
studies and research.

Specifications

Pressure (Absolute) and Water Level Measurements U20L-01

HOBO Water Level Logger

Models:

• U20L-01 (30-foot depth)

• U20L-02 (100-foot depth)

• U20L-04 (13-foot depth)

Required Items:

• Coupler (COUPLER2-C) with
USB Optic Base Station (BASE­U-4) or
HOBO Waterproof Shuttle
(U-DTW-1, firmware version

3.2.0 or later)*

• HOBOware
or higher

® Pro, version 3.5

Accessories:

• Cable (CABLE-1-300 or
CABLE-1-50) and Cable Crimp
(CABLE-1-CRIMP)

• Replacement Coupler
(COUPLER2-C)

*If shuttle firmware version 3.2.0 or
later is needed, see the Onset
website or contact Onset Technical
Support.

Operation Range 0 to 207 kPa (0 to 30 psia); approximately 0 to 9 m (0 to 30 ft) of

Factory Calibrated Range 69 to 207 kPa (10 to 30 psia), 0° to 40°C (32° to 104°F)

Burst Pressure 310 kPa (45 psia) or 18 m (60 ft) depth

Water Level Accuracy* Typical error: ±0.1% FS, 1.0 cm (0.03 ft) water

Raw Pressure Accuracy** ±0.3% FS, 0.62 kPa (0.09 psi) maximum error

Resolution <0.02 kPa (0.003 psi), 0.21 cm (0.007 ft) water

Pressure Response Time (90%)*** <1 second at a stable temperature; measurement accuracy also

Pressure (Absolute) and Water Level Measurements U20L-02

Operation Range 0 to 400 kPa (0 to 58 psia); approximately 0 to 30.6 m (0 to 100 ft)

Factory Calibrated Range 69 to 400 kPa (10 to 58 psia), 0° to 40°C (32° to 104°F)

Burst Pressure 500 kPa (72.5 psia) or 40.8 m (134 ft) depth

Water Level Accuracy*

Raw Pressure Accuracy** ±0.3% FS, 1.20 kPa (0.17 psi) maximum error

Resolution <0.04 kPa (0.006 psi), 0.41 cm (0.013 ft) water

Pressure Response Time (90%)*** <1 second at a stable temperature; measurement accuracy also

Pressure (Absolute) and Water Level Measurements U20L-04

Operation Range 0 to 145 kPa (0 to 21 psia); approximately 0 to 4 m (0 to 13 ft) of

Factory Calibrated Range 69 to 145 kPa (10 to 21 psia), 0° to 40°C (32° to 104°F)

Burst Pressure 310 kPa (45 psia) or 18 m (60 ft) depth

Water Level Accuracy* Typical error: ±0.1% FS, 0.4 cm (0.013 ft) water

Raw Pressure Accuracy** ±0.3% FS, 0.43 kPa (0.063 psi) maximum error

Resolution <0.014 kPa (0.002 psi), 0.14 cm (0.005 ft) water

Pressure Response Time (90%)*** <1 second at a stable temperature; measurement accuracy also

water depth at sea level, or 0 to 12 m (0 to 40 ft) of water at
3,000 m (10,000 ft) of altitude

Maximum error: ±0.2% FS, 2.0 cm (0.06 ft) water

depends on temperature response time

of water depth at sea level, or 0 to 33.6 m (0 to 111 ft) of water at
3,000 m (10,000 ft) of altitude

Typical error: ±0.1% FS, 3.0 cm (0.1 ft) water
Maximum error: ±0.2% FS, 6.0 cm (0.2 ft) water

depends on temperature response time

water depth at sea level, or 0 to 7 m (0 to 23 ft) of water at 3,000
m (10,000 ft) of altitude

Maximum error: ±0.2% FS, 0.8 cm (0.026 ft) water

depends on temperature response time

17153-E

Specifications (continued)

HOBO U20L Water Level Logger (U20L-0x) Manual

Temperature Measurements (All Models)

Operation Range -20° to 50°C (-4° to 122°F)

Accuracy ±0.44°C from 0° to 50°C (±0.79°F from 32° to 122°F), see Plot A

Resolution 0.10°C at 25°C (0.18°F at 77°F), see Plot A

Response Time (90%) 10 minutes in water (typical)

Stability (Drift) 0.1°C (0.18°F) per year

Logger

Real-time Clock ±1 minute per month 0° to 50°C (32° to 122°F)

Battery 2/3 AA, 3.6 Volt lithium, factory-replaceable

Battery Life (Typical Use) 5 years with 1 minute or greater logging interval

Memory (Non-volatile) 64K bytes memory (approx. 21,700 pressure and temperature

Weight

Dimensions 3.18 cm (1.25 inches) diameter, 15.24 cm (6.0 inches) length;

Wetted Materials Polypropylene housing and lanyard; Viton and Buna-N O-rings;

Logging Interval Fixed-rate or multiple logging intervals, with up to 8 user-

Launch Modes Immediate start and delayed start

Offload Modes Offload while logging; stop and offload

Battery Indication Battery voltage can be viewed in status screen and optionally

samples)

Approximately 154 g (5.43 oz) in air
Approximately 53.9 g (1.9 oz) in fresh water

mounting hole 6.3 mm (0.25 inches) diameter

ceramic sensor in acetal end cap; stainless steel screws suitable
for saltwater

defined logging intervals and durations; logging intervals from 1
second to 18 hours. Refer to the HOBOware User’s Guide for
details.

logged in datafile. Low battery indication in datafile.

The CE Marking identifies this product as complying with all
relevant directives in the European Union (EU).

* Water Level Accuracy: With accurate reference water level measurement, known water density,

accurate Barometric Compensation Assistant data, and a stable temperature environment.

** Raw Pressure Accuracy: Absolute pressure sensor accuracy includes all sensor drift, temperature,

and hysteresis-induced errors.

*** Changes in Temperature: Allow 20 minutes in water to achieve full temperature compensation of

the pressure sensor. Maximum error due to rapid thermal changes is approximately 0.5%.

Plot A

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HOBO U20L Water Level Logger (U20L-0x) Manual

Logger Operation

HOBOware Pro software is required for logger operation. Using
a reference water level, HOBOware Pro automatically converts
pressure readings into water level readings (see Barometric
Compensation for more detail). The software also supports
compensation for temperature, fluid density, and barometric
pressure.

An LED in the communications window of the logger confirms
logger operation. When the logger is logging, the LED blinks
once every one to four seconds (the shorter the logging
interval, the faster the LED blinks). The LED also blinks when the
logger is recording a sample. When the logger is awaiting a start
because it was configured to start “At Interval,” “On
Date/Time,” or “Using Coupler,” the LED blinks once every eight
seconds until logging begins.

Mounting
hole

Removable end
cap covering
communications
window

Bump for
alignment
with coupler

Barometric Compensation

The logger records absolute pressure, which is later converted
to water level readings by HOBOware Pro 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 it 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
another HOBO U20L 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 HOBO U20L or U20 Water Level logger or
weather station (HOBO U30 recommended) can be used to
compensate all the water level loggers in an area. The U20L-01
model with its 0–9m (0–30 ft) range or the U20L-04 with its
0–4 m (0–13 ft) range are both good barometric references due
to their smaller range and temperature-compensated accuracy.
HOBOware Pro includes a Barometric Compensation Assistant
for easy and accurate barometric compensation. See Processing
Water Level Data using Barometric Pressure Data for more
details.

Sensor face; make
sure sensor face is in
the water when
deploying the logger
(the sensor is located
inside the logger case

16.8 mm (0.66 in.)

as represented by
dashed lines in this
diagram)

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.

Calibration

The pressure sensor in each HOBO U20L Water Level 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!

Connecting the Logger to a Computer

The HOBO Water Level Logger requires a coupler (COUPLER2-C)
and Optic Base Station (BASE-U-4) or HOBO Waterproof Shuttle
(U-DTW-1, firmware version 3.2.0 or later) to connect to the
computer. 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.

Important: USB communications may not function properly at
temperatures below 0°C (32°F) or above 50°C (122°F).

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