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It is important that the sensing end of pH and ORP electrodes remain wet,for they may be permanently damaged if allowed to dehydrate. This is true for both in-line and submersible installation configurations. However, be careful to keep the electrical interconnection between electrode and preamplifier dry and clean at all times. Moisture in this area can also cause permanent damage.
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Most pH and ORP electrodes should be installed vertically,±60 °, so the internal solutions remain in contact with the sensing ends. +GF+SIGNET electrodes that allow horizontal or inverted installation are available. Consult the factory for details.
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pH control is best when performed in a tank. This is especially true in neutralization applications since it is very important for reagents to mix thoroughly with waste fluids,and to be allowed adequate time for the reactions to occur. Limiting adjustments to fewer than 3 pH units per stage, and sizing tanks to provide at least 10 minutes retention time, will increase the probability of producing safe effluents.
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Proper electrode placement within a tank is also very important. Electrodes should be mounted in well-mixed areas, away from reagent and waste introduction.It is usually advisable to position the electrode near the discharge outlet of the tank.
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The two most common methods of controlling pH and ORP (and Conductivity)are “on/off” and “proportional” control.In on/off control, relay setpoints are defined as either high or low limits on the process variable. When the measurement value reaches a limit the relay is energized, typically for the purpose of opening a valve or starting a pump to introduce a chemical reagent to the process. This should cause the measurement value to change in the direction of the setpoint as shown in these on/off control diagrams:
Notice the relay will not de-energize until the setpoint is exceeded by the hysteresis value. This is a programmable value and is primarily used to prevent ”relay chatter”, which occurs if a relay is set to energize and de-energize at the same value. Because of hysteresis, and because reagent delivery is fairly constant while the relay is energized, a condition known as “overshoot” is inherent to the on//off control method. Overshoot refers to the introduction of more chemical reagent than is absolutely necessary for achieving a desired adjustment to the process value, and can be expensive over time.
Proportional control is a popular alternative to the on/off control method. This method typically makes use of variable-rate metering pumps to reduce overshoot and improve precision.Establishing a proportional control scenario requires the selection of setpoint(s), deviation range(s)and maximum pulse rates. The example shown here illustrates how two relays in ”pulse mode” can be used to proportionally control pH within a desired range, or to a single setpoint. This is called “Dual Proportional Control”. Of course,a single relay in proportional pulse mode can be used to establish a high or low limit and will also reduce overshoot.
Metering pumps are idle at and between setpoints. When a setpoint is exceeded, the pump begins delivering reagent at a rate proportional to the difference between the measurement value and the setpoint. The larger the difference, the faster the delivery. The programmed deviation value defines how quickly the maximum pulse rate is reached. Depending on the input requirements of the metering pump,proportional control can also be accomplished with scaleable 4 to 20 mA outputs instead of pulsing relays or open collectors.
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In-line pH control is not recommended because it is very difficult to determine the amounts of reagent necessary to achieve a desired reaction if both pH and flow are variables. However, in-line pH monitoring is very common and useful.
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For bulb-style pH and ORP electrodes, significant natural self-cleaning by turbulent eddies is achieved at velocities of 5 or more feet per second. Flat surface electrodes get adequate self-cleaning at velocities of 1 to 2 feet per second.In all cases, exposure to velocities greater than 10 feet per second can cause excessive measurement noise and electrode wear and should be avoided.
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Simply stated, the aging of pH and ORP electrodes (i.e.,reference depletion and decreased glass sensitivity) results from a series of chemical reactions. And as a general rule, the rates of chemical reactions double with every increase of 10 °C.This means shorter life expectancy for all pH and ORP electrodes as application temperatures increase.
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HF acid and strong caustics etch pH glass.High concentrations, especially at high temperatures, destroy electrodes quickly. For applications containing trace quantities of HF (<2%), use the +GF+SIGNET 3-2714-HF electrode. This electrode has a polymeric constituent in the pH glass that resists attack by HF and extends the service life considerably over “normal” electrodes..
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In applications where process temperatures will drop below 10 °C, or 50 °F, use the 3-2716 bulb-style or 32714-HF flat-style electrodes in place of the 3-2714 electrode. This is a function of the electrical impedance of the glass that increases dramatically as temperature decreases. |
