|
The SENTINEL features allows the Model LXT 330 transmitter to provide Pre-pHault diagnostic information about the accuracy of a pH, ORP or pION measurement. The SENTINEL displays a filled triangular gauge that decreases proportionally to the degradation of the reference electrode, a filled gauge indicates a properly functioning measurement while the emptying gauge indicates the remaining life of the electrode. This Pre-pHault diagnostic alerts the user to potential problems and the remaining life before the measurement fails.
Model SP3 sensors for the measurement of pH, ORP and the various pIONs use replaceable electrode cartridges specific for the measured parameter. These electrode cartridges have a measurement cell (pH gas electrode, platinum ring or ion selective membrane) and a reference cell. The reference cell is designed to produce a standard potential independent of the solution it is immersed in. While this style of electrode is typically trouble free, there are conditions that lead to failure. Diffusion through the porous liquid junction decreases the concentration of the potassium chloride inside the electrode as the electrode ages. The decreasing concentration of potassium chloride changes the potential of the cell which shows up as a dip in the measured value. Diffusion also allows chemicals in the process to infiltrate into the electrode. If these chemicals can react with silver, then the electrode will become poisoned and a large offset voltage will be generated destroying the accuracy of the measurement.
|
The SENTINEL addresses these issues by including an additional sleeved silver element into the reference cell. When the electrode cartridge is now, both silver elements are at the same potential but as the electrode ages, or becomes poisoned, the bare element changes its potential in response to the electrode depletion or poisoning. The SENTINEL monitors the potential difference between the two elements and displays the value as a gauge of the electrodes remaining life. The protected silver element is still producing the correct potential but it is in danger of failing due to the changing environment inside the reference cell. This Pre-pHault indication notifies the user of the potential electrode failure before the measurement actually fails. When a model SP3 SENTINEL sensor is connected to a SP3 transmitter, the SENTINEL functions are displayed.
The Pre-pHault diagnostic is displayed on one of the model SP3s main screens along with the process variable, % 4-20 mA output and temperature. The diagnostic value can be assigned to an optional alarm relay and / or a secondary 4-20 mA output or monitored through HART® communication. The mV limit value for the diagnostic is user configurable with a default setting of 60 mV. The Model SP3 SENTINEL sensor uses diagnostic electrodes designated by part #'s 20053XX. These electrodes use a tri-axial connector with a PV connection (pH, ORP, ION), reference connection and diagnostic connection.
|
