There are hundreds of manufacturers of pH (or ORP) probes. Some cost as little as $100. The probes that SCT distributes cost more than six times that much. You would have to burn through six or more traditional probes to justify buying just one of ours. Our favorite probes are the ones from Aquametrix, and they go on to explain why.
Ever buy a $99 printer only to find that you spend that much every month just for the ink? Then you understand that the initial cost can be very different from the total cost of ownership. So, let’s talk total cost of ownership.
Can SCT and Aquametrix really claim that one of their pH probes will outlast six of the other guys? Well…yes. Most pH/ORP probes are combination probes. (They get that confusing name because the reference electrode and process electrode are “combined” into one glass or plastic body. A typical combination probe may last for two years in clean water, but it will be blessed if it lasts 6 months in a moderately challenging environment, like wastewater. That’s a cost of about $200/year. Typically, the things that kills a typical combination pH probe include contamination of the reference solution, poisoning of the reference electrode or plugging of the liquid junction.
A differential probe, however, replaces the bare silver wire that serves as a reference electrode with a completely enclosed glass electrode that is identical to the process (measuring) electrode. This means it can’t be poisoned. The liquid junction (the tiny porous structure that allows current to flow from the process to the measuring electrode) is replaced by a big, beefy, replaceable salt bridge. This means that, when the junction gets plugged up, you can just throw it away and screw in a new one. It also means that, when the reference solution becomes contaminated with the process, you can just unscrew the salt bridge, pour out the old solution and add new solution.
Remember, that in a combination probe, the reference electrode and the process electrode are “combined” into one complete circuit. So, when a stray voltage from, say, a ground loop, finds its way into the process it messes up the exposed reference electrode (a bare silver wire coated with silver chloride). But a differential probe uses two identical electrodes—both sealed in glass and immersed in a salt solution—and each is connected to a ground rod to make two half-circuits. When a stray voltage creeps into the process it goes straight to the ground rod and is subtracted from both electrodes. See the schematic below. So, no faulty readings. No ground loops.
The differential probe has an embedded pre-amplifier. That means instead of being restricted to a 15-foot length cable typical of a combination probe, the cable of one our our partner probes can be 1000 feet or longer.
So the decision is yours. Remember one failure can be costly in compliance or system upsets with chemical overuse, and other bad things. One related thing to keep in mind is - how often will a person run out to your site to train your operators, help with problems, fix minor issues? SCT will. You can count on that.