Section 6: Reference
HOBO Energy Logger User’s Guide
31
Section 6:
Reference
Time Accuracy
When you launch a logger, its clock is set by the host computer’s time
and time zone offset from UTC. (UTC, or Coordinated Universal Time,
is similar to Greenwich Mean Time.) The time zone offset makes it
possible to determine objective, non-local time. This prevents conflicts
and confusion when you relaunch a logger with a shuttle that may have
been launched in a different time zone, or before a Spring or Fall time
change.
Once the logger is logging, it will keep fairly accurate time. However,
the following may cause errors with time reporting.
•
Host clock error.
The most likely source of error is an incorrect
time, or time zone setting, on the computer that launched the
logger. Make sure the clock on your computer is set to the
correct date and time before launching the logger. One accurate
resource is http://www.nist.time.gov.
•
Launch time loss.
The logger may lose up to two seconds when
it is launched. This is a one-time error that occurs as part of the
start-up sequence and cannot be avoided.
•
Clock drift.
The logger’s long-term time accuracy is related to
ambient temperature. At 25°C (77°F), the worst-case error is ±8
parts per million (PPM), or about 5 seconds per week. The error
increases as the temperature deviates from 25°C, as shown in the
graph below. At a constant temperature of -20°C, the logger time
error could be as much as 35 PPM (21 seconds per week).
Section 6: Reference
32
HOBO Energy Logger User’s Guide
The following graph shows the worst-case time
error.
Figure 7: Worst-case Time Error
Resetting the Logger’s Clock
Reading out the logger and relaunching it will reset the logger’s clock.
This is useful when your logger has been running for a while and its
clock needs to be reset. Make sure the computer clock is set accurately
before relaunching.
1.
Exit HOBOware.
2.
Correct the computer’s clock.
3.
Restart HOBOware.
4.
Read out the logger, if you have not done so already.
5.
Relaunch the logger.
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