15
9. IN SITU ZERO CHECK
This procedure is not recommended as a regular procedure but only one to be undertaken if there
is some serious doubt as to the zero stability of the sensor or to confirm a sudden or critical
change in the amount of settlement which is causing concern.
Disconnect the vent line from the desiccant chamber. Close the valve at the bottom of the
reservoir. Disconnect the liquid lines from the bottom of the reservoir connect one of them to a
nitrogen cylinder. Turn on the nitrogen and adjust the pressure so that the sensor reading is at its
maximum value. (Do not exceed 20% above this maximum range.) The other tube can be left
open (with long lengths of tubing (>200 m) the process can be speed up by attaching a vacuum
pump to the end of the other fluid line.) Once all the liquid has been purged from the lines, allow
the nitrogen to flow for another 30 minutes. This will tend to dry out the inside of the tubing.
Turn off the nitrogen and disconnect the ends of the tubing so that they are both open to
atmosphere along with the open vent line. Wait until the sensor reading stabilizes and then record
this zero reading. Compare this reading with the factory zero reading shown on the calibration
chart.
Refill the liquid lines following the flushing procedures described in Section 6 with the following
difference: If a vacuum pump is used, allow the vacuum pump to run for 30 minutes (or until the
sensor reading has stabilized), before opening the valve at the bottom of the filling tank to allow
liquid to enter the lines. This will greatly reduce the chances of air being trapped inside the
tubing and sensor cavities.
10. MAINTENANCE
Every 3 months:
•
Conduct a visual examination of the reservoir terminal housing. Check for leaks by observing
the water level in the reservoir sight tube. Add additional fluid as necessary by removing the
top connector of the Tygon sight tube. Alternatively, if the water level in the reservoir begins
to rise this may be due to squeezing of the tubing by ground pressures. It is important not to
let the fluid overflow from the reservoir into the vent line; this could adversely affect the
readings. Drain off any excess fluid before it reaches the top of the reservoir. If regular
maintenance is not possible then it is advisable to disconnect the tubing connecting the top of
the reservoir to the desiccant chamber and leave them both open to the atmosphere.
•
Replace the desiccant capsules in the vent line manifold or desiccant chamber. Desiccant
capsules are dark blue when active and pink when inactive.
Every 12 months:
•
Flush the liquid tubes with fresh de-aired liquid.
•
Check the in situ calibration as described in Section 8.
