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HOBO Dissolved Oxygen Logger (U26-001) Manual 

                             

 2  

Logger Components and Operation 

 

Communications Cap/Lanyard.

 This removable cap protects 

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

Connecting the Logger to a Computer or 

Waterproof Shuttle

 for details on using the communications 

window. 

Mounting Hole.

 Use the hole on the communications cap to 

mount the logger. See 

Deploying the Logger

 for more 

information. 

Alignment Notch for Coupler.

 Use this notch to align the 

coupler when communicating with the logger. See 

Connecting 

the Logger to a Computer or Waterproof Shuttle

 for more 

information. 

DO Sensor.

 This optical sensor measures dissolved oxygen using 

RDO® Basic Technology. It is shipped with a red dust cap that 
must be replaced with a green sensor cap that lasts for six 
months plus a one-month grace period. See 

Installing the 

Sensor Cap

 for more details. 

Protective Guard.

 This removable guard protects the DO 

sensor. Unscrew it to install or replace the sensor cap as 
needed. See 

Installing the Sensor Cap

 for more details. 

Temperature Sensor.

 This built-in sensor (not visible in 

diagram) measures temperature. 

 WARNING: 

This logger can be damaged by mechanical 

shock. Always handle the logger with care. The logger may be 
damaged if it is dropped. Use proper packaging when 
transporting or shipping the logger. 

Do not attempt to open the logger case or sensor housing. 
Disassembling of the logger case or sensor housing will cause 
serious damage to the sensor and logger electronics. There are 
no user-serviceable parts inside the case. Contact Onset 
Technical Support at 1-800-LOGGERS (1-800-564-4377) or an 
authorized Onset dealer if your logger requires servicing.

 

Installing the Sensor Cap 

The logger ships with a replaceable sensor cap that provides six 
months of continuous use. Once the cap is initialized, an 
internal clock within the logger will count down until the sensor 
cap expiration date. When the sensor cap expires, you will need 
to replace it with a new cap (U26-RDOB-1

). 

The sensor cap is 

intended for six months of actual deployment, but the 
expiration date is seven months from the date the cap was 
initialized. This allows for any time needed between launching 
the logger and physically deploying as well as extra time in case 
you are not able to get the logger after exactly six months of 
deployment. To install the sensor cap: 

1.

 

Unscrew the protective guard covering the DO sensor (see 
diagram at left). 

2.

 

Remove the red dust cap that protects the sensor during 
shipping. 

3.

 

Take the green sensor cap out of the canister. 

4.

 

With the flat part of the DO sensor pointing down and the 
the green sensor cap oriented with the arrow up, slide the 
sensor cap over the sensor until it snaps in place. The cap 
should be snug against the logger housing without any gaps. 

 

5.

 

Screw the external protective guard back on until tight. 

IMPORTANT:

 The sensor cap expires 7 months (to the day) 

after it has been initialized. The logger will record a value of -
888 mg/L at each logging interval after the cap has expired. 
Initialization occurs automatically when the cap is installed 
while the logger is logging. You can also initialize it from the 
Status window in HOBOware or when using the Lab Calibration 
tool. To see when the sensor cap expires after being initialized, 
check the Status in HOBOware for the expiration date. The cap 
also has a shelf life; check the “Install By” date printed on the 
canister. 

Connecting the Logger to a Computer or 
Waterproof Shuttle 

To connect the logger to a computer, use either the Optic USB 
Base Station (BASE-U-4) or HOBO Waterproof Shuttle (U-DTW-
1) with a coupler (COUPLER2-C). To launch and read out the 
logger in the field, use one of these three methods: 

 

Laptop computer with Optic USB Base Station (BASE-U-4) 
and coupler (COUPLER2-C) 

 

HOBO Waterproof Shuttle (U-DTW-1, Firmware Version 
3.2.0 or later) and coupler (COUPLER2-C) 

Temperature Sensor  

(Inside Logger Housing) 

Mounting 

Hole 

Communications Cap/Lanyard 

Alignment Notch  

for Coupler 

Protective Guard 

DO Sensor 

DO sensor, flat 
side down 

Sensor cap with arrow 
positioned up; slide over DO 
sensor until it snaps in place 

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