
HOBO® U12 Temp/RH/Light/ 
External Data Logger 
(Part # U12-012) 
Inside this package: 
• HOBO U12 Temp/RH/ 
Light/External Data Logger 
• Mounting kit with magnet, hook 
and loop tape, tie-wrap mount, 
tie wrap, and two screws. 
Doc # 7661-B, MAN-U12-012 
Onset Computer Corporation 
Thank you for purchasing a HOBO data logger. With proper care, it 
will give you years of accurate and reliable measurements. 
The HOBO U12 Temperature/Relative Humidity/Light/External Data 
Logger is a four-channel logger with 12-bit resolution and can record 
up to 43,000 measurements or events. The external channel accepts a 
variety of sensors, including temperature, and split-core AC current 
sensors as well as 4-20 mA and voltage input cables (sold separately). 
The logger uses a direct USB interface for launching and data readout 
by a computer. 
An Onset software starter kit is required for logger operation. Visit 
www.onsetcomp.com for compatible software.
Specifications 
Temperature: -20° to 70°C (-4° to 158°F) 
Measurement 
range 
Accuracy 
Resolution 
Drift 
Response time 
in airflow of 1 
m/s (2.2 mph) 
Time accuracy  ± 1 minute per month at 25°C (77°F), see Plot C 
Operating 
temperature 
Battery life  1 year typical use 
Memory  64K bytes (43,000 12-bit measurements) 
Weight  46 g (1.6 oz) 
Dimensions  58 x 74 x 22 mm (2.3 x 2.9 x 0.9 inches) 
RH: 5% to 95% RH 
Light intensity: 1 to 3000 footcandles (lumens/ft
maximum value varies from 1500 to 4500 footcandles 
(lumens/ft
External input channel (see sensor manual): 0 to 2.5 DC Volts 
Temperature: ± 0.35°C from 0° to 50°C (± 0.63°F from 32° to 
122°F), see Plot A 
RH: ± 2.5% from 10 to 90%, see Plot B; conditions above 
80% RH and 60°C (140°F) may cause additional error 
Light intensity: Designed for indoor measurement of relative 
light levels, see Plot D for light wavelength response 
External input channel (see sensor manual): ± 2 mV ± 2.5% of 
absolute reading 
Temperature: 0.03°C at 25°C (0.05°F at 77°F), see Plot A 
RH: 0.03% RH 
Temperature: 0.1°C/year (0.2°F/year) 
RH: <1% per year typical; RH hysteresis 1% 
Temperature: 6 minutes, typical to 90% 
RH: 1 minute, typical to 90% 
Logging: -20° to 70°C (-4° to 158°F) 
Launch/readout: 0° to 50°C (32° to 122°F), per USB 
specification 
The CE Marking identifies this product as complying with all 
relevant directives in the European Union (EU). 
2
) 
2
) typical; 
2. Plug the small end of the USB interface cable into the bottom of the 
logger as shown in the following diagram. 
Light 
intensity 
sensor 
Important: Press this 
button for 3 seconds 
when logger is 
launched with Button 
Start or press for 1  
second to record an 
event while logging 
USB interface 
cable plugged 
into logger 
If the logger has never been connected to the computer before, it may take 
a few seconds for the new hardware to be detected. Use the logger 
software to launch and read out the logger. 
Important: If you configure the logger to start with a button start, be 
sure to press and hold down the button on the front of the logger for 
at least three seconds when you want to begin logging. 
If using an external sensor, be sure to plug it into the side of the logger 
before logging begins. Also select the correct sensor and activate the 
external channel in the logger software when configuring the launch. 
Important: If you select an external channel, but do not plug the 
probe in, false data will be recorded for that channel. 
You can read out the logger while it continues to log, stop it manually 
with the software, or let it record data until the memory is full. 
Refer to the software user’s guide for complete details on launching, 
reading out, and viewing data from the logger. 
Plot A  Plot B 
Plot C  Plot D 
Connecting the logger 
The U-Family logger requires an Onset-supplied USB interface cable to 
connect to the computer. If possible, avoid connecting at temperatures 
below 0°C (32°F) or above 50°C (122°F). 
1. Plug the large end of the USB interface cable into a USB port on the 
computer. 
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 temperature every minute). Events are independent occurrences 
triggered by a logger activity. Examples of events recorded 
asynchronously during deployment include when the logger is connected 
to the host, when the battery is low, the end of a data file once the logger 
is stopped, and button pushes. 
Press the button on the front of the logger for one second to record an 
event. Both a button up and down event will be recorded. This is useful if 
you want to mark the datafile at a particular point. For example, if the 
logger is located in an incubator, you might press the button each time the 
door is opened. 
The logger stores 64K of data, and can record up to 43,000 samples and 
events combined. 
Using external sensors 
The external input channel has a switched 2.5 V output. This signal can be 
used to power a sensor directly or it can also be used to trigger an external 
circuit. An external sensor should draw no more than 4 mA total when 
powered. The switched 2.5 V output turns on about 15 ms before the 
external channel is measured and stays powered for 48 ms after it is 
measured, as shown in the following diagram. The striped bar shows the 
16 ms period during which the logger samples the input signal. 
16 ms 
15 ms 
48 ms 
© 2003, 2004 Onset Computer Corporation 
Part #: MAN-U12-012, Doc #: 7661-B 

HOBO U12 Temp/RH/Light/External Data Logger 
Operation 
A light (LED) on the side of the logger confirms 
logger operation. 
Channel 4 
external 
input 
Light 
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 
The logger is awaiting a start 
because it was launched in Start 
At Interval, Delayed Start, or 
Button Start mode 
The button on the logger is 
being pushed for a Button Start 
launch 
shorter the logging interval, the faster the 
light blinks); blinks when logging a sample 
Blinks once every eight seconds until launch 
begins 
Blinks once every second while pressing the 
button and then flashes rapidly once you 
release the button. The light then reverts to a 
blinking pattern based on the logging interval 
Using the RH sensor 
In order to take humidity measurements, the temperature sensor must be 
used in conjunction with the RH sensor. 
Conditions outside the recommended range may offset the RH signal. 
Vapors may also affect the RH sensor. The diffusion of chemicals into the 
sensor may cause a shift in both offset and sensitivity. High levels of 
pollutants may cause permanent damage to the sensor. 
Upon returning to normal conditions, the RH sensor will slowly return 
towards calibration state by itself. However, prolonged exposure to 
extreme conditions may accelerate aging and eventually lead to a 
permanent shift. To recondition the sensor, do the following: 
1. Remove the battery 
2. Warm 24 hours 80–90°C (176–194°F) at < 5% RH 
3. Re-hydrate 48 hours 20–30°C (70–90°F) at 75–95% RH 
Protecting the logger 
The logger can be permanently damaged by corrosion if it gets wet. 
Protect it from condensation. If it gets wet, remove the battery 
immediately and dry the circuit board with a hair dryer before reinstalling 
the battery. Do not let the board get too hot. You should be able to 
comfortably hold the board in your hand while drying. 
Note! Static electricity may cause the logger to stop logging. To avoid 
electrostatic discharge, transport the logger in an anti-static bag, and 
ground yourself by touching an unpainted metal surface before handling 
the logger. For more information about electrostatic discharge, visit 
http://www.onsetcomp.com/Support/support.html. 
Mounting 
There are four ways to mount the logger using the materials in the 
mounting kit included with the logger. 
• Use the hook and loop tape to affix the 
logger to a surface. 
• Attach the magnet and then place the logger 
on a magnetic surface. 
• Use the tie wrap and tie wrap mount to tie 
the logger to an object. 
32 mm  
(1¼ inch)
• Fasten the logger to a surface with the two 
Phillips-head screws. The back of the logger 
has two inserts for the screws, 32 mm (1¼ 
inches) apart. 
Battery 
The logger requires one 3-Volt CR-2032 lithium battery. Expected battery 
life varies based on the temperature and the frequency at which the logger 
is recording data (the logging interval). A new battery will typically last 
one year with logging intervals greater than one minute. Deployments in 
extremely cold or hot temperatures or logging intervals faster than one 
minute may significantly reduce battery life. 
To replace the battery: 
1. Disconnect the logger from the computer. 
2. Unscrew the logger case. 
3. Lift the circuit board and carefully push the battery out with a small 
blunt instrument, or pull it out with your fingernail. 
4. Insert a new battery, positive side facing up. 
5. Carefully realign the logger case and re-fasten the screws. 
 WARNING: Do not cut open, incinerate, heat above 85°C 
(185°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. 
Service and Support 
HOBO products are easy to use and reliable. In the unlikely event that you have a problem with 
this instrument, contact the company where you bought the logger: Onset or an Onset 
Authorized Dealer. Before calling, you can evaluate and often solve the problem if you write 
down the events that led to the problem (are you doing anything differently?) and if you visit 
the Technical Support section of the Onset web site at www.onsetcomp.com/support.html. 
When contacting Onset, ask for technical support and be prepared to provide the product 
number and serial number for the logger and software version in question. Also completely 
describe the problem or question. The more information you provide, the faster and more 
accurately we will be able to respond. 
Onset Computer Corporation 
470 MacArthur Blvd., Bourne, MA 02532  
Mailing: PO Box 3450, Pocasset, MA 02559-3450  
Phone: 1-800-LOGGERS (1-800-564-4377) or 508-759-9500 
Fax: 508-759-9100 
E-mail: loggerhelp@onsetcomp.com 
Internet: www.onsetcomp.com 
Returning Products to Onset 
Direct all warranty claims and repair requests to place of purchase. Before returning a failed 
unit, you must obtain a Return Merchandise Authorization (RMA) number from Onset. You 
must provide proof that you purchased the Onset product(s) directly from Onset (purchase order 
number or Onset invoice number). Onset will issue an RMA number that is valid for 30 days. 
You must ship the product(s), properly packaged to protect against further damage, to Onset (at 
your expense) with the RMA number marked clearly on the outside of the package. Onset is not 
responsible for any package that is returned without a valid RMA number or for the loss of the 
package by any shipping company. Products must be clean and free of any contaminants before 
they are sent back to Onset or they may be returned to you. 
Repair Policy 
Products that are returned after the warranty period or that are damaged by the customer as 
specified in the warranty provisions can be returned to Onset with a valid RMA number for 
evaluation. 
Optional Services 
Please contact Onset for more information and prices on: 
• ASAP Repair. Onset will expedite the repair of a returned product. 
• Data-back™ Service. HOBO data loggers store data in nonvolatile EEPROM memory. 
Onset will, if possible, recover your data to a disk. 
• Tune Up Service. Onset will examine and retest any HOBO data logger. 
Warranty 
Onset Computer Corporation (“Onset”) warrants to the original end-user Purchaser for a period 
of one year from the date of original purchase that the HOBO product(s) purchased will be free 
from defect in material and workmanship. During the warranty period Onset will, at its option, 
either repair or replace products that prove to be defective in material or workmanship. 
This warranty is void if the product has been damaged by the Purchaser as a result of improper 
maintenance, abuse, misuse, mishandling, misapplication, error or negligence of Purchaser, or if 
there has been an unauthorized alteration, attachment or modification. 
THERE ARE NO WARRANTIES BEYOND THE EXPRESSED WARRANTY AS 
PROVIDED IN THIS DOCUMENT. IN NO EVENT SHALL ONSET BE LIABLE FOR 
LOSS OF PROFITS OR INDIRECT, CONSEQUENTIAL, INCIDENTAL, SPECIAL OR 
OTHER SIMILAR DAMAGES ARISING OUT OF ANY BREACH OF THIS 
CONTRACT OR OBLIGATIONS UNDER THIS CONTRACT, INCLUDING BREACH 
OF WARRANTY, NEGLIGENCE, STRICT LIABILITY, OR ANY OTHER LEGAL 
THEORY. 
LIMITATION OF LIABILITY. The Purchaser’s sole remedy and the limit of Onset’s 
liability for any loss whatsoever shall not exceed the Purchaser’s price of the product(s). The 
determination of suitability of products to the specific needs of the Purchaser is solely the 
Purchaser’s responsibility. THERE ARE NO WARRANTIES BEYOND THE 
EXPRESSED WARRANTY IN THIS DOCUMENT. EXCEPT AS SPECIFICALLY 
PROVIDED IN THIS DOCUMENT, THERE ARE NO OTHER WARRANTIES 
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO, ANY IMPLIED 
WARRANTIES OF MERCHANTIBILITY OR FITNESS FOR A PARTICULAR 
PURPOSE. NO INFORMATION OR ADVICE GIVEN BY ONSET, ITS AGENTS OR 
EMPLOYEES SHALL CREATE A WARRANTY OR IN ANY WAY INCREASE THE 
SCOPE OF THE EXPRESSED WARRANTY OFFERED WITH THE SALE OF THIS 
PRODUCT. 
INDEMNIFICATION. Products supplied by Onset are not designed, intended, or authorized 
for use as components intended for surgical implant or ingestion into the body or other 
applications involving life-support, or for any application in which the failure of the Onsetsupplied product could create or contribute to a situation where personal injury or death may 
occur. Products supplied by Onset are not designed, intended, or authorized for use in or with 
any nuclear installation or activity. Products supplied by Onset are not designed, intended, or 
authorized for use in any aeronautical or related application. Should any Onset-supplied product 
or equipment be used in any application involving surgical implant or ingestion, life-support, or 
where failure of the product could lead to personal injury or death, or should any Onsetsupplied product or equipment be used in or with any nuclear installation or activity, or in or 
with any aeronautical or related application or activity, Purchaser will indemnify Onset and 
hold Onset harmless from any liability or damage whatsoever arising out of the use of the 
product and/or equipment in such manner. 
LEGAL REMEDIES. This warranty gives you specific legal rights. You may also have other 
rights which vary by jurisdiction. The remedies provided herein are in lieu of all other remedies, 
express or implied. 
© 2003, 2004 Onset Computer Corporation. All rights reserved. 
Part #: MAN-U12-012, Doc #: 7661-B 
Onset and HOBO are registered trademarks of Onset Computer Corporation. Other products 
and brand names may be trademarks or registered trademarks of their respective owners. 
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