Hach 3650 manual Sensor Calibration, Calibration In a Span Gas

4.2 Sensor Calibration

When delivered, the sensor is pre-calibrated. However, it should be re-calibrated on site, when being used for the first time, and always after a membrane change. If you have just replaced the membrane, allow at least half an hour for the membrane to settle before attempting to calibrate.

If you want to verify the accuracy of the calibration, place the analyzer back in measurement mode and compare your displayed gas concentration against the appropriate value in the accompanying Calibration Tables booklet.

Your calibration is stored internally and is valid for the life of the sensor's membrane, thus it is not necessary to repeat the calibration procedure until after the next membrane change.

The sensor can be calibrated either by using a span gas, directly in line in a liquid sample, or in air. The mode of calibration may be selected using the Winlog97 program or directly from the instrument (see “Selecting Type of Calibration” on page 27 for more details).

4.2.1 Calibration In a Span Gas

The span gas calibration procedure may be used if you have a source of gas with a known concentration of O2 (in% units). To perform this type of calibration, the instrument must be set for calibration In a Span Gas. The Windows WinLog97 program also must be used in this procedure.

Switch on the instrument, if necessary, and wait a minute or so for the displayed measurement to settle. Then expose the sensor to a span gas sample with a known oxygen concentration. Enter the percentage of oxygen in the span gas using the WinLog97 program (see “Entering a Span Gas Value” on page 28).

1)Press the CAL button. Remember, this button may have been locked out to prevent an accidental reset (see “Locking Out the Instrument’s CAL Button” on page 28 for details).

2)A brief clearing [---]message appears.

3)Press CAL again within a 3 second period.

4)The percentage of the measured current to the ideal one is displayed.

5)When the reading is stable, press CAL again.

If the new calibration current is within 25% to 175% of the ideal current, the instrument displays CAL and returns to the measurement mode. Calibration is now complete, the sensor is calibrated, and you can proceed with your measurements.

If the new calibration current is not within this range, the instrument displays Err and returns to measurement mode. The system will not calibrate, and it is likely that a sensor service will be required.