Myron L PS6FCE - Manual (Page 47)

42 43
The problem is, on the other side of the membrane is an unknown test
solution, not potassium chloride. The outside electrode, also called
the Reference Junction, is of the same construction with a porous
plug in place of a glass barrier to allow the junction fluid to contact the
test solution without significant
migration of liquids through the
plug material. Figure 30 shows
a typical 2 component pair.
Migration does occur, and this
limits the lifetime of a pH junction,
from depletion of solution inside
the reference junction or from
contamination. The junction may
be damaged if dried out because
insoluble crystals may form in a
layer, obstructing contact with test
solutions. See pH/ORP, pg. 42.
4. The Myron L Integral pH Sensor
The sensor in the P
ool
P
ro
(see
Figure 31) is a single construction
in an easily replaceable package.
The sensor body holds an oversize
solution supply for long life.
The reference junction “wick” is
porous to provide a very stable,
low permeable interface, and is
located under the glass pH sensing
electrode. This construction
combines all the best features of
any pH sensor known.
5. Sources of Error
The basics are presented in
pH/ORP, pg. 42.
a. Reference Junction
The most common sensor problem will be a clogged junction because
a sensor was allowed to dry out. The symptom is a drift in the “zero”
setting at 7 pH. This is why the P
ool
P
ro
does not allow more than 1 pH
unit of offset during calibration. At that point the junction is unreliable.
b. Sensitivity Problems
Sensitivity is the receptiveness of the glass surface, which can be
diminished by a film on the surface. This problem also causes long
response time.
Glass surface
Figure 30
KCl solution
Electrode wire
Electrode
wire
H + ions
Junction
Plug
KCl solution
Figure 31
Junction plug
Platinum button
H + ions
Electrode wires
Glass
Glass
Surface