pH sensor life extended from 3 days to 75 days in fluoride mixture

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Model AP-IH/MP-41 for measuring pH in Hostile Environment


The customer was measuring pH in a chemical tank containing fluorides as well as other acids and bases.  The pH swings from concentrated acid (pH = 1) to concentrated caustic (pH = 14).  In this hostile environment, the customer would consume a non-McNab pH probe in one or two days.  Three days was the maximum probe life.


To solve this problem, the customer installed a McNab Aqua-Purometer Model AP-IH pH analyzer, along with a Model MP-41 antimony pH probe.  The antimony probe uses no glass, and is therefore immune to the problems of normal glass pH sensors.  Instead, the sensor uses a rugged piece of metal (antimony).  The first probe used got a life of two weeks, four times the previous longest life.  That first probe was then restored with a light cleaning and was used for an additional two months.

The analog 4-20 mA output from the AP-IH drives the customer's control system and the acid addition is now controlled with much higher stability. Control within + 0.2 pH unit is achieved for long periods of time.

Cost savings are achieved by the reduced consumption of pH sensors. However, the most dramatic saving is the reduction of labor because the system down time and labor required for replacement of pH probes is greatly reduced.

The stability achieved in this application gives additional savings. Consumption of caustic and base is reduced.  This stability also contributes to longer probe life because destructive pH levels (e.g. pH=1) are avoided.

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