Myron L 9PTK - Manual (Page 67)

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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 33 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.
4. The Myron L Integral pH Sensor
The sensor in the Ultrameter III (see Figure 34) 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.