Hach 910, Instrument Manual

Page: 39 / 54

Page 37
Maintenance.fm
Maintenance
Section 5 Maintenance
5.1 Sensor Calibration
5.1.1 Velocity Sensor Calibration
The velocity sensor does not require calibration. The transmit frequency is
fixed by a highly accurate quartz crystal-controlled frequency generator.
5.1.2 Submerged Sensor Calibration
Calibrating the submerged sensor synchronizes the meter electronics with the
unique characteristics of each individual probe. In addition, the calibration
compensates for a drift in the output of the sensor that may occur as the
materials in the sensor age. To ensure optimum accuracy, the manufacturer
recommends calibrating the submerged sensor when:
• The sensor is first used.
• Installing a new or different sensor on a flow meter or input receptacle.
• The difference between the level reading of the flow meter and the
independent verification (measurement with a dipstick and ruler)
is increasing.
Note: The data is constant if the difference between the level reading of the flow meter
and the independent verification is constant; recalibration is not required.
Note: Errors can occur with the level reading of the flow meter and the independent
verification. Errors are caused by variation in site conditions and measurement
abilities. These errors may cause slight variations in the difference, therefore, not
indicating a true change in the difference.
5.2 Desiccant Maintenance
5.2.1 Reusing Desiccant
The desiccant material is a silica gel indicator. When the gel is saturated with
moisture, the beads turn from blue to pink. To rejuvenate the beads for reuse,
remove them from the assembly. Heat the beads in an oven at 100 to 180 °C
(212 to 350 °F) until the beads turn blue again. Discard and replace the beads
if they do not turn blue after heating.
5.2.2 Replacing the Sensor Desiccant
Note: Before calibrating or installing
the Submerged Sensor it is
absolutely essential that the red
plastic protective cap be removed
from the atmospheric reference port
of the desiccant cartridge.
The air dryer assembly is an integral part of the velocity probe cable. A small
diameter tube is contained within the sensor cable to supply a reference port
from the logger to the transducer in the sensor. The reference air passes
through the in-line desiccant where all moisture is removed.
To remove or replace the sensor desiccant:
1. Remove the fill plug from the assembly.
2. Remove the pink desiccant beads and allow the assembly to air dry.
3. Fill the assembly with fresh (blue) desiccant.
4. Replace the fill plug.