Hach-Lange ORBISPHERE Model 51x User Manual - Manual (Page 64)

62
Calibration menu
7.2.1 O
2
sensor calibration
7.2.2 O
3
sensor calibration
Calibration results A calibration screen will be displayed showing current measurement data
which is continually refreshed.
The value “% ideal current” is a percentage of the current against the ideal
current for the membrane type selected. If this percentage is not within the
accepted range, an error message is displayed and the calibration process
fails (see
Calibration errors (EC and TC sensors) on page 64 ). A warning
message can be displayed when this value is close to the boundaries, but
when calibration can be accepted.
The message is first displayed in the result box. The dialog box with the
error message or the warning is displayed when the finish button is pressed.
The value “% last calibration” shows the ratio between the current
measurement and the previous sensor calibration.
The value “% variation” indicates the variation during the last 3
measurements, which is the stability of the measurements. A variation as
low as possible is needed for a precise calibration.
The display shows the actual calibration parameters, and the actual
readings (temperature, barometer, current).
Note:
In case of calibration failure, consider replacing the membrane. See
the sensor maintenance manual for details.
Verification Similar to the calibration procedure, but for verification of the actual
calibration values. The result of the measurements made during the
verification is not stored and the actual calibration data is not modified.
O 2 sensor calibration
The O
2
sensor needs to be calibrated after each sensor service. Wait at least 30 minutes after mounting
a new membrane before recalibrating. The sensor is in contact with either:
•
Air at atmospheric pressure (In Air)
• O
2
at known concentration (Direct value). The gas can be dissolved or not.
In air calibration This calibration procedure places the O
2
sensor in water-saturated air, to
provide a known oxygen reference against which to calibrate.
Dry the sensor thoroughly, before placing the sensor storage cap under tap
water. Shake off any excess water, but leave a few drops inside the cap.
Verify that the screw-on protection cap is in place on the sensor head. If you
use a Dacron mesh inside the protection cap, make sure it is dry before
attempting to calibrate. Then, loosely place the storage cap back on the
sensor, holding it in place with a few turns of its collar.
Set the calibration parameters accordingly (see
Modify calibration
parameters on page 61 ), and press calibrate.
Direct calibration This procedure calibrates the oxygen sensor against a liquid sample
containing a known level of dissolved O
2
flowing through the sample line.
The instrument displays the sensitivity of the sensor as a percentage of the
sensitivity determined when calibration was last performed.
Set the calibration parameters accordingly (see
Modify calibration
parameters on page 61 ), and press calibrate.
O
3
sensor calibration
The sensor is either in contact with:
•
Air at atmospheric pressure (In Air)
• O3 at known concentration (Direct Value). The gas can be dissolved or not.
The procedure is the same as for the O
2
sensor. In the case of the In air calibration, the
sensor measures O
2
during calibration. The O
3
coefficient is deduced taking into account how
the sensor behaves in O
2
. As a different voltage is used at the anode to measure O
2
and O
3
,
the O
3
measurement takes a long time to stabilize.
To facilitate the follow up after an O
3
in air calibration, negative values can be displayed.