40
held on to electrode cover by means of a rubber sleeve.
Electrolytic Solution
7.5 % solution of KCl is prepared by dissolving
7.5 g of AR grade KCl in 100 ml of DW. AgCl
crystals are slowly added until AgCl stops getting
dissolved.
Assembling of the Probe
At first, the bakelite nut is slid over the probe
cover. Next, a piece of the membrane (1″ × 1″) is
kept over the tip of the probe. Then, the sleeve is
pushed fitted over the electrode cover in such a
way that the membrane is flat and tight. There be
no wrinkles on the membrane. The membrane be
removed and refitted if any wrinkles are detected.
A few drops of electrolyte solution is poured and
checked for any leakage through the membrane
or near the sleeve. Now, the electrode cover
(probe cover) be filled completely with the solution. It is kept vertical and the main stem of the
probe is slowly inserted into the probe cover
which is filled with the solution. The excess
solution be let out. Next, the bakelite nut over the
main stem is tightened. It is to be ensured that
there is no air bubble. In the event of any bubble
being detected, the probe is opened and re-filled,
till no bubbles are visible.
The gold cathode should always be bright and
untarnished. It be wiped with a clean, lint-free
cloth or hard paper. Any form of abrasive or
chemical be never used.
Preparing the Instrument
1. The ‘mode’ switch be set at ‘set zero’.
2. The meter be switched ‘on’.
3. ‘Set 20’ control be adjusted until the meter
displays 20.0.
4. The mode switch be switched to ‘zero’ and the
‘zero control’ be adjusted to read 0.0. (This is
electrical zero setting of the meter.)
5. The prepared DO probe be attached to the
probe connector.
6. 15 min be allowed for the probe to be
polarised.
Air Calibration
Experience has shown that air calibration method is
quite reliable, yet, far simpler than the other methods. The air calibration method is given below:
1. The probe be placed in moist air. For this, the
probe could be wrapped loosely in a damp
cloth taking care that the cloth does not touch
the membrane. About 10 min be waited for
temperature stabilisation.
2. The mode switch be changed to ‘Temp’ and
the temperature be read and recorded. Table
showing solubility of oxygen in freshwater
against temperature be consulted, from the
manual supplied with the instrument, in order
to determine the calibration value. Also, the
altitude and the atmospheric pressure correction factor be determined from a different table
in the same manual. Calibration value from the
1st table be multiplied with correction factor
from the 2nd table. An example is given below:
Temperature be assumed as 25 °C
Altitude be assumed as 1,000 ft
From Table 1, the calibration value for 25 °C is,
say, 8.4 ppm
From Table 2, the correction factor for 1,000 ft is
about 0.96
Therefore, the correct calibration value is
8.4 ppm × 0.96 = 8.1 ppm
3. The ‘salinity’ knob be set to ‘zero’.
4. The mode switch be brought to ‘CAL’.
5. The ‘CAL control’ be adjusted until the meter
reads the correct calibration value as indicated
in step No. 2 above. Two minutes be waited to
verify the calibration stability. Readjustment
be done, if necessary.
The DO meter is now calibrated. Calibration
be checked periodically.
DO Measurement
1. With the instrument calibrated, the probe is
dipped in the water sample (whose DO is to be
measured) and the sample water be constantly
stirred. Stirring be continued with a stirrer
during the process of measurement of DO in
order to prevent depletion of DO near the
membrane of the probe.
3 Lentic Water (Physico-chemical Characteristics of Water)
held on to electrode cover by means of a rubber sleeve.
Electrolytic Solution
7.5 % solution of KCl is prepared by dissolving
7.5 g of AR grade KCl in 100 ml of DW. AgCl
crystals are slowly added until AgCl stops getting
dissolved.
Assembling of the Probe
At first, the bakelite nut is slid over the probe
cover. Next, a piece of the membrane (1″ × 1″) is
kept over the tip of the probe. Then, the sleeve is
pushed fitted over the electrode cover in such a
way that the membrane is flat and tight. There be
no wrinkles on the membrane. The membrane be
removed and refitted if any wrinkles are detected.
A few drops of electrolyte solution is poured and
checked for any leakage through the membrane
or near the sleeve. Now, the electrode cover
(probe cover) be filled completely with the solution. It is kept vertical and the main stem of the
probe is slowly inserted into the probe cover
which is filled with the solution. The excess
solution be let out. Next, the bakelite nut over the
main stem is tightened. It is to be ensured that
there is no air bubble. In the event of any bubble
being detected, the probe is opened and re-filled,
till no bubbles are visible.
The gold cathode should always be bright and
untarnished. It be wiped with a clean, lint-free
cloth or hard paper. Any form of abrasive or
chemical be never used.
Preparing the Instrument
1. The ‘mode’ switch be set at ‘set zero’.
2. The meter be switched ‘on’.
3. ‘Set 20’ control be adjusted until the meter
displays 20.0.
4. The mode switch be switched to ‘zero’ and the
‘zero control’ be adjusted to read 0.0. (This is
electrical zero setting of the meter.)
5. The prepared DO probe be attached to the
probe connector.
6. 15 min be allowed for the probe to be
polarised.
Air Calibration
Experience has shown that air calibration method is
quite reliable, yet, far simpler than the other methods. The air calibration method is given below:
1. The probe be placed in moist air. For this, the
probe could be wrapped loosely in a damp
cloth taking care that the cloth does not touch
the membrane. About 10 min be waited for
temperature stabilisation.
2. The mode switch be changed to ‘Temp’ and
the temperature be read and recorded. Table
showing solubility of oxygen in freshwater
against temperature be consulted, from the
manual supplied with the instrument, in order
to determine the calibration value. Also, the
altitude and the atmospheric pressure correction factor be determined from a different table
in the same manual. Calibration value from the
1st table be multiplied with correction factor
from the 2nd table. An example is given below:
Temperature be assumed as 25 °C
Altitude be assumed as 1,000 ft
From Table 1, the calibration value for 25 °C is,
say, 8.4 ppm
From Table 2, the correction factor for 1,000 ft is
about 0.96
Therefore, the correct calibration value is
8.4 ppm × 0.96 = 8.1 ppm
3. The ‘salinity’ knob be set to ‘zero’.
4. The mode switch be brought to ‘CAL’.
5. The ‘CAL control’ be adjusted until the meter
reads the correct calibration value as indicated
in step No. 2 above. Two minutes be waited to
verify the calibration stability. Readjustment
be done, if necessary.
The DO meter is now calibrated. Calibration
be checked periodically.
DO Measurement
1. With the instrument calibrated, the probe is
dipped in the water sample (whose DO is to be
measured) and the sample water be constantly
stirred. Stirring be continued with a stirrer
during the process of measurement of DO in
order to prevent depletion of DO near the
membrane of the probe.
3 Lentic Water (Physico-chemical Characteristics of Water)