Meters and their maintenance

Managing pH and electrical conductivity (EC) in your hydroponic cropping systems is essential to maintaining crop health and productivity. There are a number of different ways you can measure pH and EC, and the goal of this article is to cover the most common types of pH and EC meters, as well as calibrating and maintaining them.

There are several different types of pH and EC meters used in controlled-environment agriculture. Broadly speaking, they may be classified as hand-held, benchtop or in-line (or in-tank). Hand-held pH and EC meters, as the name implies, are units that can be held. There are a few different types of hand-held meters for measuring pH and EC. In one type of meter, the probe that is placed into solutions and measures pH and/or EC is integrated directly into the meter itself; these are commonly referred to as “pen” meters. Another popular style of hand-held meter is the kind where the probe and the meter that displays the readings are separate and connected by a cable. These are easy and convenient for carrying around a facility.

Benchtop meters are an efficient choice for measuring pH and EC of numerous samples. These units are larger than hand-held meters and sit stationary on a bench, with pH and/or EC probes connected by a cable and sitting in an adjustable arm. Samples are collected in the production area and brought to a single, central location for measuring pH and EC.

Finally, those meters with pH and EC probes permanently placed in irrigation lines (in-line) or in nutrient solution reservoirs (in-tank) are most frequently used in conjunction with some type of controller, such as dosers injecting an acid to lower pH or concentrated fertilizer stock to increase EC. One thing to consider with these in-line or in-tank probes is that they are generally tied to something that is making an adjustment to something (like pH or EC). Consider relying on a pH or EC measurement coming from probes independent of those used for controlling a device. This adds a layer of independence and insurance, as problems with the control units may be caught with the separate sensor.

One of the most important aspects of any pH and EC meter is regular and correct calibration of the probes. As pH and EC meters are used, the values of measurements displayed on the meter can drift from the actual value of the parameter being measured. To maintain the accuracy of meters — as well as the value of the information collected by using them — perform regular calibrations.

For hand-held and benchtop meters, calibrate them prior to initial use. (This does not mean in between each measurement.) For those in-line or in-tank probes that are running continually, develop a regular weekly (or more frequent) routine for calibration. pH may either be a one- or two-point calibration. As the name suggests, only one reference value is used for calibrating the pH range for one-point meters, and two values (one high, one low) are used for two-point meters. For EC, a single-point calibration is the norm.

No pH or EC meter is going to last forever. However, their functional life can be extended by regular maintenance. More specifically, attention needs to be given to the pH/EC probes themselves. First, probes should be regularly cleaned, regardless of whether hand-held, benchtop, or in-line. For those pH and EC meters which are in-line, the probes are continually submersed in water, whether clear or amended with fertilizers, and the tips are always submersed in liquid. For hand-held and benchtop meters where probes are not continually in solution, storage solution should be used. This solution helps keep probes from desiccating over time in between uses. In addition to using appropriate storage practices, periodically clean the probes for hand-held meters. This will avoid the buildup of any solutes on probes, such as fertilizer salts or the calcium or magnesium from hard water.

Maintenance isn’t just for equipment. Do not forget about reference and calibration solutions, too. Mark the containers with the date they were received. Since growers often purchase supplies in bulk for reduced unit prices or to save on shipping, avoid using old reference and calibration solutions. Expiration dates are often printed on containers and should be checked prior to use.

The lifeblood of any hydroponic system is the nutrient solution, and pH and EC are important chemical properties to maintain throughout production. Appropriate selection, use and maintenance of pH and EC meters are essential for soilless success.

Christopher (ccurrey@iastate.edu) is an associate professor of horticulture in the Department of Horticulture at Iowa State University.