Walchem 300/400 Series pH/ORP Controllers
All W300 series controllers that include a pH or ORP input (WPH series, WDP series or WDT series) have the following requirements for the electrode signals: a preamplified electrode signal, a Pt100 or Pt1000 temperature signal (if automatic temperature compensation is necessary for your pH application) and the preamplifier must be powered by х▒ 5 VDC.
The ideal electrode is the WEL series. This offers a differential amplifier, easily replaceable electrode cartridge and optional temperature signal.
Walchem can provide a preamplifier that can accept the necessary power, and can connect with any standard electrode with a BNC connector on the cable.
Once the electrode signal is amplified, the cable may be extended up to 1000 feet (300 meters). If using a standard, nonamplified electrode connected to a preamplifier, the electrode should be located as close to the preamplifier as possible. A maximum of 20 feet is recommended. With the WEL, the amplifier is right in the electrode, for maximum reliability. An electrode cable must be routed away from any AC wiring, fans, motors, or other sources of electrical noise.
Keep in mind that the pH/ORP electrode will require calibration on a regular basis. The longer the distance between the electrode and the controller, the more painful the calibration procedure will be. At 1000 feet of separation, you will either need two people with 2-way radios or some good track shoes.
It is difficult to predict pH/ORP electrode life in certain applications. These electrodes are similar to batteries, and eventually they will run down. There are some factors that will influence the life:
Temperature: The higher the temperature, the lower the life of the electrode. If the electrode is exposed to temperatures over 140 degrees F (60 degrees C) on a regular basis, the expected life may only be several months.
Shelf Life: The span of a pH electrode (the mV that the electrode puts out per pH unit) will decrease with time, even if the electrode is stored in the original soaker bottle. Eventually, two pH buffers will create almost the same mV output, and the electrode will fail calibration. Walchem electrodes are warranted to have a 12 month shelf life.
Abrasion: Scratches on the pH glass will reduce the span of the electrode. Our flat surface design minimizes this effect.
Chemical Attack: Acid Fluorides will dissolve the pH sensitive glass. Our HF-resistant glass option will minimize this effect. An electrode that might dissolve in a few days with normal pH glass, might last several months with HF-resistant pH glass. Take careful notice of all the wetted materials of construction of an electrode, and make sure that they are compatible with whatever may be found in your solution.
Contamination of Reference: The pH/ORP electrode utilizes a silver/silver chloride reference to measure the process signal against. Certain chemicals (heavy metals, sulfides, proteins, etc.) will interact with the silver, and either clog the junction or dissolve the reference.
Coatings: An application where the electrode frequently coats will require frequent cleaning, and this increased handling will result in increased abrasion, and shorter life. The flat design will reduce the frequency of cleaning. Our probe wash feature will decrease the handling required when cleaning
Proportional control will always provide more accurate control of your process, but the controller and metering pumps will be a little more expensive. Here are some guidelines to determine if you can save the money or not.
On/off control can generally be used for applications where the response time of the system to chemical additions is fast, and/or where the acceptable range of the pH/ORP is wide. In a small, well mixed tank, on/off control can maintain the pH within 0.1 or so of the set point. In a well designed industrial wastewater pretreatment system, where there is sufficient residence time of the water flowing through the tank, and the acceptable level of pH may be х▒ 0.5 or more, on/off control will also be fine.
Use proportional control whenever there is an extremely tight control range (х▒ 0.1 or less), where there are long delays between adding the chemical and detecting the addition, or when the set point is at an inflection point in the titration curve such that when a small addition is made the pH changes dramatically.
It is possible, but not simple, depending upon the application. If at all possible, include a tank with a mixer in the system. The tank should be sized to provide at least 20 minutes of retention time (for example, if the water flow rate is 20 gallons per minute, use a 400 gallon tank).
If you control the pH on the fly, without a tank, there are a number of considerations. First, if the flow rate and the incoming pH are both variables, then you need to have a pH transmitter and a flow transmitter sending signals to a PLC, which can integrate these two signals to determine the flow rate of the metering pump required. The injection point of the pH adjusting chemicals must be downstream from the pH electrode. A second pH transmitter further downstream can provide verification that the pH is correct.
If flow is constant, then a proportional pH controller can handle the application, with set points adjusted such that the pump operates at the correct speed for the incoming pH. Again, the injection point must be downstream from the pH electrode. If the pH needs to be adjusted by more than 2 pH units, then incorporating a tank is highly recommended.
You can, however the sensor must be powered by +/- (not just +) 5 VDC and use Pt100 or Pt1000 for ATC.