The heart of a pH measuring loop is the electrode system. Yokogawa has designed a wide range of electrodes to ensure this heart keeps beating under the most severe conditions.
The dimensions and design meet the requirements of DIN 19263 (excluding the refillable types). A high degree of standardisation makes it possible to mount any electrode in the standard program of fittings.
The combination electrode plug and cable socket is watertight and temperature is resistant up to 125oC. It meets the requirements of IP65.
Colour coded strips on electrode and cable and clear identification of sensor specifications make incorrect installation virtually impossible.
Features
? Wide range of electrodes to suit almost all process conditions.
? The connector parts are gold plated to ensure a good electrical contact under the most severe conditions.
? A coaxial plug and socket with watertight sealing that meets the requirements of IP65.
? Colour coded strips for easy identification of electrodes and cables.
? High degree of standardisation for mounting in various flow-, insertion- and immersion fittings.
General
The principle of pH measurement is quite simple. All one needs is a glass electrode constructed of pH sensitive glass and a reference (potential) that remains stable under all conditions. This directly shows the weak point in the measuring loop. As the reference needs an ‘open’ circuit to the process, a stable reference potential is sometimes hard to keep. Most reference electrodes have a porous diaphragm to realize this open
connection to the process. Several materials are being used like ceramic, Zirconia, PTFE and wood. The porous diaphragm can
get clogged or high diffusion potentials may build up. Poisoning of the reference is another cause for unstable reference potentials.
A reference electrode can be poisoned by the penetration of
the process liquid or by diffusion of components of the process medium through the diaphragm. The stable reference potential which is formed by a Ag/AgCl reference system in a KCl solution is shifted if either gets effected. E.g. silver chloride + sulfide will form the insoluble silver sulfide. These particles give deposits, which can block the diaphragm from inside the reference electrode. Likewise the chloride concentration can be altered and therefore the reference potential is altered. The latter can be corrected by re- calibration. Poisoning of the AgCl reference system is irreversible and the reference electrode needs to be replaced.
Maintenance
For accurate pH measurement, regular (even frequent)
maintenance is required. This knowledge is decisive for the
selection of fittings and electrodes of the pH measuring loop. One needs to keep in mind that the electrode(s) must remain accessible and exchangeable. Once the right fitting has been selected, the process of selecting the right electrode(s) must take place.
Glass electrode
The SM21-AG4 will suit the great majority of the pH applications as this electrode is constructed in such a way that its sensitivity and strength are optimized. When sensitivity is required more than strength than the SM21-AG2 should be selected.
For tough industrial applications (including auto-cleaning) the dome shaped electrode is the best selection. ‘L’-glass should be selected when the electrode is used at higher temperatures (>70oC) and/or in applications that need a glass type that is resistant to chemical attack.
Reference electrode
Reference electrodes come in flowing- and non-flowing types. Flowing types such as the SR20-AC52, AC32 with a ceramic diaphragm are used in processes that have low ionic strength such as (ultra) pure water. The SR20-AS52 is a flowing type electrode with a sleeve junction. Due to the high electrolyte flow rate, the sensor is suitable for UPW application as well as in highly polluting applications. The electrolyte flow towards the process tends to keep the junction clean from clogging and poisoning.
For all other applications the SR20-AP24 is preferred. When the process cannot withstand KCl, the SR20-AP26 should be used.
