Xylem H-339 - Manual (Page 12)

H-339
Operation 1-7
Figure 5
Time Accuracy
When the data logger requests data from the H-339, the H-339 responds to the data logger with a
message indicating it can take up to 240 seconds to get valid data. The H-339 can have valid data
in as little as 30 seconds. Soon after the data is valid, the H-339 sends a service request to the data
logger so the data logger can collect the data. The service request will come sometime between 30
seconds and 240 seconds. The service request is synchronized to a timing pulse from the GPS
module to provide very accurate timing possibilities. This timing pulse is called ‘Pulse Per Second’
or ‘PPS’ and is one of the wires from the GPS module. The time returned to the data logger is the
exact time (within nanoseconds) at the rising edge of this pulse. The service request is sent to the
data logger typically at 115 microseconds after the PPS pulse, (± 10 microseconds). Figure # 4
shows these pulses.
As long as the data logger can read the data from the sensor consistently time wise, a time offset can
be established to give accuracies to less than one second. The H-339 will never return a valid time
with the value for seconds equal to X8 (08, 18, 28 etc) or higher so the data logger can increment the
seconds data without having to be concerned with roll over. This makes generating a time offset
easier as long as the offset is 2 second or less.
Words of Caution
! Keep the lead wires as short as possible.
! Use shielded cables in noisy environments.
! Always clean and inspect the GPS module when at the site.
! Never count on the time or position accuracy if the valid flag is not set.