pH Probe Calibration and How to Care for One

Electronic pH measurement of aquarium water used to be something only a laboratory could perform. Thanks to advancements in technology, the cost of high-quality pH meters has come down to an affordable level. Hand-held pH meters, like the Hanna Pocket pH Tester (HI98108) are affordable and easy to use.  It’s not uncommon for reef aquarists and aquatic plant enthusiasts to use a continuous pH monitoring system to keep track of and make adjustments to their tank’s water chemistry. But all this great technology is useless if not properly cared for. If your pH meter is not periodically calibrated, the readings will not be accurate. Here’s what you need to know about pH probe calibration and care.

How pH Meters Work

There are three main parts to a pH meter:

• a pH measuring electrode
• a reference electrode
• a meter that calculates and displays the pH

pH probes generate a weak electrical signal based on the pH of the water. Typically inside a plastic probe encasement is a specialized glass with two electrodes within it: a reference electrode and a pH-measuring electrode. The reference electrode and pH-sensing electrode are inside the probe and work together to create the electrical signal that is sent to the meter. The reference electrode sits within a solution of potassium chloride. The pH sensing probe looks like a small glass bulb at the tip of the probe. The tip of the probe is coated with a special type of gel membrane that allows hydrogen ions to diffuse inside. This electrode is filled with a pH 7 solution. When the probe is in pH 7.0 water, it sends no voltage to the meter. The meter is programmed to display pH 7.0 when the probe sends no voltage, so as the pH drifts from 7, the voltage sent to the pH meter changes. The meter is designed to match the probe voltage to a specific pre-calibrated pH level. The meter displays the pH on an LCD or LED display.

Water Temperature and pH Meters

pH electrodes and measurements are temperature sensitive. Meters with a temperature sensor automatically compensate for water temperature. This is called Automatic Temperature Compensation (ATC). Many of the inexpensive hand-held pH meters claim to have ATC but in fact, do not. This is why aquarists find the readings inconsistent and unreliable. Some high-quality pH meters don’t have ATC but have a manual temperature adjustment knob that allows you to set the temperature of the water you’re testing.

Why Calibration of pH Meters is Important

pH is one of the most important parameters to test, freshwater or marine. One way to ensure your always getting an accurate pH measurement is to regular calibration and care for the probe. pH probes are highly sensitive instruments that send a tiny electrical signal to the meter. Over time, pH probes age, which changes their voltage output. The meter matches the voltage sent from the probe to a certain pH level. If the voltage is wrong, the meter will display an inaccurate pH. Calibration makes sure the probe and meter are speaking the same language and providing the correct pH level.

How to Calibrate a pH Meter

Your pH meter is programmed to use specific reference pH levels in the calibration process. The instruction manual will list the pH buffers to use for calibration. Buffer solutions have pre-set pH values. Like 7.0, 10.0 and 4.0. They’re used to calibrate the electrode and meter, using the meters calibration program. Buffers are available as ready to use liquids or powders you dissolve in RO/DI water. Better-grade pH meters allow you to calibrate at two different pH levels. This is called two-point calibration. The idea is to dial-in the meter near the pH level in your tank. One calibration is made at pH 7 and a second is typically performed at either pH 4 or pH 10. It is best to select buffers that “bracket” the actual pH value of the water type to be measured. For a planted tank dosing carbon dioxide, use pH 4 and pH 7 buffers. For marine aquariums use pH 7 and pH 10 buffers. Some pH meters come with calibration solutions.

How Often Should I Calibrate?

Laboratories calibrate their pH meters every day. For aquarium testing once every three weeks is a reasonable calibration schedule. You may see advertisements promoting how fast and easy pH meters are for testing your tank or multiple aquariums. This is true, once you’ve properly calibrated the meter. The point of using an electronic pH meter is to get precise and accurate pH measurements or continuously monitor your planted or reef aquarium. Skimping of calibration defeats the purpose of a pH meter.

pH Probe Calibration and Care Tips

• When calibrating, never place the pH probe into the container of buffer solution. Pour out what you need into a separate container. Discard the used buffer solution after calibrating the meter. Save the unused buffer for next time.
• Never wipe the probe’s glass bulb with a cloth or brush. It can damage the probe.
• If you need to store your probe, the tip should always be moist. But don’t use RO/DI. Use a special probe storage solution. Potassium chloride (KCl) is formulated to safely preserve the probe and make sure it functions properly when you need to use it again. If your probe cap came with a sponge, soak it in storage solution to keep the probe tip wet.

Final Thoughts

A pH meter is a great tool for monitoring the pH in your aquarium and tap water. With proper care it will provide years of reliable service. We suggest avoiding no-name meters and go with a well-known brand with a good reputation. They’ll have helpful instructions, a warranty and product support after the sale.