Xylem H-3551 - Manual (Page 14)

3-4 Operation
H-3551
If the auto-purge feature of the H-350XL is enabled, the H-350XL is responsible for initiating
purges. Whenever the H-350XL makes a measurement, it compares the pressure data with the
“Purge Threshold” value. If the pressure is greater than the Purge Threshold, the H-350XL
commands the bubbler to initiate a purge sequence. During the purge, the H-350XL energizes its
internal auto-zero valve to isolate the H-350XL's precision sensor from the purge pressure. At the
completion of the purge, the H-350XL keeps its auto-zero valve energized for an additional 2.0
seconds. This is done to prevent damage to the H-350XL's precision sensor in case the orifice is
plugged and the orifice line momentarily holds the full purge pressure. A pressure relief valve in
the bubbler vents the over pressure to atmosphere during the 2 second delay.
After completion of a purge sequence, the H-350XL makes another pressure measurement. If the
pressure is still greater than the Purge Threshold value, the H-350XL knows the purge failed. It
then activates a timer which disables further purges for the next 24 hours. This is done to prevent a
plugged orifice form causing continuous purges which would quickly drain the battery.
3.3 Stand-Alone Operation
The H-3551 gas purge system can be used “stand-alone” with pressure measurement devices other
than the H-350XL. You do lose some flexibility in that you cannot edit the bubbler settings in the
field and the purge feature cannot be coordinated with a pressure measurement such that pressure
measurements are not made during a purge.
When operating stand-alone, the factory preset values are:
Bubble Rate: 60 Bub/min
Pressure to Purge With: 40 PSI
Pressure to Initiate a Purge: 20 PSI
Purge Time: 45 sec
These settings can also be monitored or edited via the SDI-12 port built into the H-3551. The H-
3551 functions as a SDI-12 “sensor”. When connected to a SDI-12 host such as a data logger or
H-4191 RS-232 side-kick interface you can issue extended SDI-12 commands to read or write
these settings. If the data logger issues and “aM!” command, the H-3551 initiates a purge
sequence. See Chapter 5 for further details.
If needed, these settings can be configured at the factory. The factory must be notified of desired
settings prior to shipment.