Calibration
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10. CALIBRATION
The accuracy of dissolved oxygen measurement is directly related to the sensing surface cleanliness and calibration
technique. Oily coatings and biological contaminations are the primary cause of calibration drift.
A standard solution or a reference DO meter may be used to compare readings during calibration.
HI6421
and
HI6421P
support:
• Two‑point calibration at 100.0 % saturation (or 8.26 mg/L) and 0 .0% saturation (or 0.00 mg/L)
• Single‑point manual calibration using a standard value set by user (% saturation or mg/L)
• Single‑point calibration at 0.0 % saturation (or 0.00 mg/L)
• Single‑point calibration at 100.0 % saturation (or 8.26 mg/L)
Calibrate the probe frequently. Additionally, calibration is recommended:
• After probe replacement
• At least once a week
• Following calibration reminder
Note:
`
Perform Temperature calibration (and Pressure if required) prior to DO probe calibration.
`
If calibrating in concentration units, 8.26 mg/L (ppm) is displayed but the actual value of air
saturated water at pressure and temperature used for calibration.
`
When automatic calibration is performed it is assumed that the standard value is 100 % water
saturated air and 0% 0
2
saturated solution.
`
When a user calibration is performed it is assumed that the standard value is the DO value at the
current pressure, temperature and salinity.
General Guidelines
• Set up a routine service schedule where measurement integrity is validated.
• Do not handle the sensing surface of the sensor.
• Avoid rough handling and abrasive environments that can scratch the reactive surface of the sensor.
• Do not return the used standard to the bottle of “fresh” solution.
• For measurements across a temperature gradient (when water temperature is drastically different from
the standard), allow the sensor to reach thermal equilibrium before conducting calibrations or making
measurements. The heat capacity of the probe is much greater than the air.
• During calibration, the temperature sensor must be in the calibration solution.
• When calibrating in water‑saturated air ensure there are no droplets on the DO sensor sensing surface.
10.1. PROBE PREPARATION
Optical Probe
(
HI7641133
)
Required accessories:
Smart Cap and O‑ring, syringe and sachet with grease, purified water, lens wipe, zero
oxygen solution, calibration beakers