4230 Flow Meter
Section 4 Optional Equipment
4-18
4.8.4 Storage and
Maintenance of pH
Probes
If you remove the pH probe from operation, be careful to keep the
glass sensor bulb wet. Always store the probe with the rubber
cover screwed completely over the threaded end of the sensor.
Exposure to air causes the glass membrane on the sensor bulb to
dry out. This makes it very slow to respond in solution. Pro-
longed or repeated dehydration of the bulb may ruin the probe.
The pH sensitive glass can become “conditioned” to its envi-
ronment, especially when it is continuously exposed to high pH
(10 and above) solutions. The glass does have a memory and will
respond slowly when exposed to a lower pH solution after having
been in a high pH solution for any significant period of time.
Storage of a pH probe in a 4.0 buffer solution is recommended as
this has a regenerative effect on the glass and does not put a
memory on it. Tap water will work if 4.0 buffer solution is not
available. Deionized water is good for quick rinses to clean the
element, but not for prolonged storage of an electrode. Con-
tinuous exposure of the ion-sensitive membrane to a wetted, but
deionized water solution will improperly condition the mem-
brane.
The reference electrode is also adversely affected when allowed
to dry out. Salt crystals from the electrolyte or precipitates of the
solution measured will form salt bridges either within or on the
surfaces of the liquid junction, causing the reference to be less
conductive and resulting in a higher reference impedance.
This condition will typically worsen until the unit no longer func-
tions. Soaking the reference electrode in a 4.0 pH solution, or tap
water if the buffer is not readily available, may bring the ref-
erence back to life. Boiling the electrode in 4.0 buffer solution or
tap water could revive the electrode in more severe situations. If
none of these works, it may be necessary to replace the probe.
4.9 The Dissolved Oxygen
(D.O.) Probe
This probe measures the amount of oxygen dissolved in a stream
or waterway. Oxygen dissolved in water is necessary for many
forms of life that dwell in lakes and streams. Inadequate supplies
of dissolved oxygen will cause fish and other aquatic life that
depend on them as a food source to die off or be sharply dimin-
ished in numbers.
The measurement of dissolved oxygen content is of interest to
those monitoring the condition of lakes and streams. Fish must
have a certain minimum concentration of dissolved oxygen to
thrive, typically 4 to 6 mg/l.
Various types of pollution can cause the amount of oxygen dis-
solved in water to drop sharply, placing the aquatic life forms at
serious risk. The D.O. Probe measures the amount of oxygen dis-
solved in water in a range from 0 to 20 mg/l. Note that the flow
meter can display D.O. in either mg/l (milligrams per liter) or
ppm (parts per million) depending on your choice in pro-
gramming.
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