Hanna Instruments HI 9026, Instruction Manual

Page: 12 / 14

12
Notes:
• When the meter is not calibrated or calibration is cleared (default
values loaded) there is no “expired calibration”, and the display
always shows a blinking CAL tag.
• When an abnormal condition in the RTC is detected the meter
forces the “expired calibration” status.
CONDITION
The display is provided with a 5-dot bargraph (unless the feature is
disabled) which gives an indication of the electrode status after
calibration as follows:
Bargraph indication Condition value
All 5 dots steady 81 to 100% of life
4 dots steady 61 to 80%
3 dots steady 41 to 60%
2 dots steady 21 to 40%
1 dot steady 1 to 20%
1 dot blinking 0%
Only frame is ON No info available
The “condition” bargraph remains active for 12 hours after calibration,
then only the frame is shown.
Note: When an abnormal condition in the RTC is detected, the “condition”
is cleared and only the bargraph frame is shown on the display.
CLEAN ELECTRODE
Each time pH calibration is performed, the meter internally compares
the new calibration with the one previously stored.
When this comparison indicates a significant
difference, the CLEAN message blinks on the
LCD to advise the user that the pH electrode
may need to be cleaned (see “Electrode
Conditioning & Maintenance” section).
After cleaning, perform calibration.
Note: If the calibration data are cleared, the comparison is done
with the default values.
17
Typical Electrode Life
Ambient Temperature 1 – 3 years
90 °C Less than 4 months
120 °C Less than 1 month
Alkaline Error
High concentrations of sodium ions interfere with readings in alkaline
solutions. The pH at which the interference starts to be significant
depends upon the composition of the glass. This interference is called
alkaline error and causes the pH to be underestimated. Hanna’s glass
formulations have the indicated characteristics.
1.0 Mol L
-1
Na
+
0.1 Mol L
-1
Na
+
Sodium Ion Correction for Glass at 20 - 25 ° C
Concentration pH Error
13.00
13.50
14.00
12.50
13.00
13.50
14.00
0.10
0.14
0.20
0.10
0.18
0.29
0.40
The resistance of glass electrodes partially depends on the temperature.
The lower the temperature, the higher the resistance. It takes more time
for the reading to stabilize if the resistance is higher. In addition, the
response time will suffer to a greater degree at temperatures below 25 °C.
Since the resistance of the pH electrode is in the range of 50 – 200
Mohms, the current across the membrane is in the pico Ampere range.
Large currents can disturb the calibration of the electrode for many
hours.
For these reasons high humidity environments, short circuits and static
discharges can be detrimental to a stable pH reading.
The pH electrode’s life also depends on the temperature. If constantly
used at high temperatures, the electrode life is drastically reduced.
TEMPERATURE CORRELATION
FOR pH SENSITIVE GLASS