Hanna Instruments HI 98140 - Manual (Page 13)

25 24
TEMPERATURE-RESISTANCE CORRELATION FOR
HANNA pH SENSITIVE GLASS
The resistance of glass electrodes partially depends on the temperature.
The lower the temperature, the higher the resistance. It takes longer 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 10°C.
Since the resistance of the pH electrode is in the range of 200 Mohm,
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 are 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.
Typical Electrode Life
Ambient Temperature
1- 3 years
90 °C Less than 4 months
120°C Less than 1 month
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 the
alkaline error and causes the pH to be underestimated. Hanna's
glass formulations have the indicated characteristics.
When the batteries are inserted and no power adapter is connected,
the meter can recognize different batteries charge levels.
1. Fully charged batteries - The backlight can be enabled.
2. Weakening batteries - The backlight is automatically disabled
and it is not possible to enable it until new batteries are inserted
or an external power adapter is connected. The "Batt" indication
appears when trying to enable backlight.
3. Weak batteries - "Bat" indication is displayed on lower LCD
alternating the temperature reading. Backlight is disabled and
meter can work for about 10 hours.
4. Dead batteries - LCD shuts off. The instrument stops working to
avoid erroneous readings.
Battery replacement must only take place
in a safe area and using four 1.5 V AA
alkaline batteries.
In order to replace run down batteries,
simply remove the two screws on the rear
cover of the instrument and replace the
four 1.5V AA batteries with new ones,
paying attention to the correct polarity.
A 12 Vdc power adapter can also be
used to power the unit (see accessories).
Note: The instrument uses the following configuration.
It is recommendable to purchase the Hanna voltage adapters that
use the proper polarity configuration (HI 710005 or HI 710006).
However, HI 98140 and HI 98150 can be used with other adapters.
In this case, remember to check the correct polarity of your adapter
before connecting it to the meter.
+ -
BATTERY REPLACEMENT
-20 -20
-10
0 +10+20 +30+40 +50+60+70 +80+90 ° C
-10
-10
Ω
-10
-10
-10
-10
Ω
2x10 9
1x10 9
2x10 8
1x10 8
2x10 7
1x10 7
-20 -10 0 + 10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90 °C
Alkaline Error
1.0 Mol L -1 Na +
0.1 Mol L -1 Na +
Sodium Ion Correction for the 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