Xylem H-3521 - Manual (Page 21)

Chapter 3
Maintenance
H-3521
Maintenance 3-1
3.1 Maintenance
Sustained operation of the H-3521 pressure sensor is almost maintenance-free. As with any
precision instrument, the calibration should be checked on a regular basis. The instrument has no
filters or user serviceable internal parts.
3.2 Precautions
! Check for leaks, make certain the connections to the pressure port are tight.
! Make certain the atmospheric vent is not covered or blocked.
! Make certain the gauge station enclosure is open to the atmosphere. If the sensor is placed in
a cabinet or sealed enclosure, the cabinet must have a screened vent or other opening to the
atmosphere.
3.3 Offset Drift
Unlike sensors which measure dry gas pressure (H-350/H-350LITE) the H-3521 does not have an
internal valve for measuring and removing long term offset drift. The pressure sensor is very
stable however, as with any pressure sensor, long term offset drift will occur. We recommend
that the sensor be factory calibrated on a yearly basis in order to maintain the highest level of
accuracy. As an alternative, the sensor offset can be adjusted in the field using a pressure
standard or known hydrostatic pressure as a reference and using the Offset or Set Current Stage
commands.
3.4 Trouble Shooting
Experience over the years with pressure sensors has identified several common problems:
No SDI-12 response or intermittent data
1.
Check all wiring including power and ground connections. Battery connections
can become corroded. Check for corrosion in the connectors and terminal strips.
2. The H-3521 measures and reports it’s internal power supply voltage along with
Stage
and Pressure. Make a measurement and check to see if the voltage is
between 10.0 and 16.0 Volts.
3. Check the connections between your data recorder and the H-3521 sensor.
4. Verify that there is not another SDI-12 sensor on the SDI-12 bus with the same
SDI-12 address as the H-3521. Try communicating with the H-3521 without any
other SDI-12 devices on the bus. Sending a “*!” or “?!” should cause the H-3521
to return its own address as long as there are no other SDI-12 devices on the SDI-
12 bus.