Maintaining and Troubleshooting pH Control Systems

The chemical makeup of a nutrient solution is a critical factor in the success of any hydroponic installation. The nutrient element concentrations and the acid-base chemistry of the solution should be well controlled. This is needed for several reasons: optimum conditions for plants, lowest cost, and minimized waste.

The acidity of any nutrient solution is a critical parameter. Too much acid (low pH) and the roots will get damaged and may even leak cytoplasm into the nutrient solution. If the low pH persists, the plants will wilt and die. Not enough acid (high pH) and nutrients may not be as available for uptake by plants. Of course, a very high pH will also severely damage plant roots. The bottom line is controlling the pH of a nutrient solution is of prime importance if one wants the best results.

It is possible to manually test and adjust the pH of a nutrient solution. Often, this is how small or prototype systems are initially set up. If the pH is tested several times per day (at least once is recommended) and an adjusting solution added accordingly, this is generally an adequate approach. However, medium-sized and larger commercial operations invariably use (or should use) automated pH control systems. Although automated systems reduce labor and when operating well, give the grower one less thing to worry about day to day, they can, and do, present problems. This article is concerned with those systems that include a measuring component and a dosing component to automatically control the solution pH in a nutrient reservoir.

Loss of pH control can be caused by many defects, but usually is either caused by the pH sensing part of the system, or the pH adjusting part of the system. Incorrect instrument readings can result in improper operation of the adjusting system, even though there is nothing wrong with the adjusting hardware or software. On the other hand, if the adjusting solution is not well controlled, the system can be unstable with wider-than-desired fluctuations in the pH. Not all malfunctions fit neatly into either of these two possible failure modes, but many do, and they provide a good starting place for troubleshooting procedures.

Troubleshooting pH Control Systems

When troubleshooting a pH control system, no matter what the problem is, it usually pays to go through a few simple checks before getting too far involved, just to make sure there is not a simple solution. Do the following first:

1. Confirm that any circuit protection devices are not tripped and make sure there is electrical power to the system.

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2. Check all electrical connectors for tightness. Look for damaged wiring.

3. Check all plumbing fittings for tightness, breaks, or leakage.

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4. Check the location of the pH electrode. Make sure it hasn’t come loose.

5. Check the supply of adjusting chemicals (acid and/or base) to make sure they haven’t run out and that they are the correct concentration.

These checks should take only a minute or two. How many times in the world will something not work only because it’s not plugged in or the power switch is turned off? Many, many times, to be sure.

Calibration

To calibrate simply means to check a measuring device against a standard to determine its accuracy and precision. Contrary to popular belief, it does not mean “adjust the system” to improve its performance, although that is often done. If there is a discrepancy between the measured value and the true value, if the amount of error is known, it can be compensated for.

Also, pH controllers are either of the single-point (tested and adjusted at one pH value) or two-point (tested at two pH values and adjusting one and the slope between them) calibration type. Either method works with two-point preferred for better accuracy and precision. Calibration solutions should be fresh (used before their expiration dates) and proper techniques must be used to avoid cross-contamination of solutions which will slowly degrade the accuracy of the calibrations.

Pumps

Pumps used to inject acid or base into a nutrient tank do not run continuously so they should last for many years before failure. Occasionally, a pump may need to be partially disassembled and cleaned. Most pumps are lubricated for life but check the manufacturer’s recommendations on that.

Peristaltic pumps are sometimes used for liquid injection. The internal parts of a peristaltic pump never contact the fluid itself and use a roller squeezing action on flexible tubing to move the fluid. For these pumps, the tubing should be moved periodically (if possible, depending on design) to avoid wearing out the same section of tubing as the pump does its work. If the tubing cannot be moved, inspect and replace it before it wears out.

Electrodes

Electrodes, commonly called probes, submerged in a nutrient solution are exposed to a relatively harsh chemical environment of dissolved ions. Solution constituents can precipitate out of solution onto the probe, which will degrade probe performance after a while. Probes should be inspected regularly and cleaned as needed in accordance with the manufacturer’s recommendations. Electrodes don’t like to be operated in air, so make sure the solution level does not drop below the level of the installed probe. Automatic fluid level control and a low-level warning system are good ideas. It is not unusual to have to replace an electrode every few years or so.

Electronics

The good news is semiconductor components (transistors and integrated circuits) have a low failure rate and are probably the least likely parts of a system to fail. This is especially true of microprocessors, which are designed to operate reliably for many years (decades even). If there is to be a failure in a controller it is most likely to be in the power supply or electrical connections. Connectors can come loose or be fouled with contaminants if they are in an exposed location. Occasionally, an internal relay used to turn the pump on and off may fail.

Power supplies in controllers do most of the hard work as far as electronics go. The power supply provides all the current used by every part of the circuit and generates the most heat. So, if a controller fails, the power supply is a good starting point for isolating the problem. Unless you have good documentation (schematics and circuit descriptions), it is difficult to make much progress on troubleshooting an electronic controller and, in most cases, you are better off replacing a defective controller with a new one. High-end controllers can often be returned to the factory for repair, but unless you have a spare unit, your system will be down for the repair period.

Update the Software

Most pH controllers are not easily modifiable after purchase and installation. What you have is what you’ve got. However, some systems may have firmware that can be periodically updated. If the system interfaces with a PC-based software package for data display and system control, it is likely this software will be updated periodically. If software updates become available, install them.

Planning for Maintenance

A preventive maintenance routine should be scheduled and diligently adhered to — not only the nutrient monitoring and control system, but the entire facility. Periodic calibration, inspection and cleaning do much for system reliability.

No one in charge of pH control wants a failure of their system to be the cause of a major loss. For operations where crop failure would have great economic consequences, spare parts should be kept on hand (unless it can be assured they are available locally) so a serious defect can be quickly corrected. Systems that interface with the internet or cellular networks to send out emergency notices are a good idea.

Litmus-type pH paper should be on hand in case all else fails. Test paper can also be used to informally check against the electronic reading to see if they agree. If they don’t, further investigation is warranted.

Loss of pH control and crop destruction is one of the most dreaded scenarios imaginable to any horticulturist. Get to know and love your pH control system and maintain it well.