Calibration: Getting your pH and EC on point
By Heidi Lindberg
pH and EC values are important, but they’re only useful if your meters are accurate.
By Heidi Lindberg
With all of the activity in the greenhouse, it is easy to forget the basics. Make sure your pH and electrical conductivity (EC) meters are calibrated correctly.
In my experience, growers are carrying out their daily activities until they see a nutritional problem in the crop – and then it is time to dust off the pH and EC meter. The measurements you take with your meters are only as good as the calibration. Remember that pH is based on a logarithmic scale, so a pH of 6.0 is 10 times more basic than a pH of 5.0. What appears to be small shifts in pH are actually larger than you might think and may quickly affect the nutrient uptake of your plants. For example, I had not calibrated my meter in over 6 months and the readings were 0.5 units off. If your pH and EC meter also provides you with an inaccurate reading, you might falsely make a management decision about your spring crop – costing you time and money – and you still will not achieve your desired results.
There are a variety of EC and pH meters on the market, but the meter that I have is a Hanna Instruments HI 9313-6. Michigan State University Extension recommends that you calibrate your meter before every measurement session, but at a minimum calibrate your meter a couple of times throughout the season. Calibrating your meter is quick and easy. Simply follow this step-by-step process:
- Gather all materials needed to calibrate your pH and EC meter: pH and EC meter, storage solution, standard solutions, distilled water, and beakers or cups (if applicable).
- If you have not used your pH and EC meter in a while, follow the manufacturer’s instructions on how to get it ready to go. First, check to see if the probe needs a new battery. After a long period without use, the electrode in the probe of the meter will have likely dried out. If the probe dried out but still works, soak the probe in a storage solution for a minimum of an hour (for example, HI70300 Storage solution).
- Make sure your standard solutions are not expired! Frequently, we are tempted to draw out of the same bottle for years on end. Be careful, as the integrity of the solution may have changed over time. The manufacturers often sell these calibration solutions in variously sized bottles or as convenient individually packaged disposable sachets.
- If your standard solution came in a larger bottle, pour the solution into a clean, dry container such as a small cup. If your standard solution came in an individual sachet, place the electrode directly into the packet. Never dip the probe directly into the larger bottle as the known pH or EC will become inaccurate over time. Also, growers should take note that the pH of the solution changes with temperature, so be sure to check the temperature of the solution before calibrations.
- Once you have “re-wet” the probe, you will now need to calibrate the pH of the meter. For my meter, I needed the pH standards of 4.01 and 7.01.
- Rinse the tip of the probe with distilled water or reverse osmosis (RO) water prior to dipping it in any standard solution.
- After inserting the electrode in the standard solution, adjust the dial to make sure your meter is reading the pH of the known 7.01 standard (see photo).
- Rinse between calibrations with distilled water or RO water.
- Now calibrate your meter using the 4.01 standard solution. Similar to step 6, adjust the calibration dial so that your pH meter reads 4.01.
- Finally, calibrate your meter to a known EC. My meter displays the number in mS/cm, but the calibration packet is labelled in µS/cm (1413 µS/cm, in my case). Remember, 1413 µS/cm is equivalent to approximately 1.41 mS/cm. Place the tip of the probe into the EC standard solution and adjust the EC calibration dial to the known standard (1.41 mS/cm).
You are now ready to use your pH and EC meter! For guides and videos on meter calibration, crop-specific factsheets (annuals, vegetables/leafy greens, and potted crops), and pH and EC management recommendations, check out fertdirtandsquirt.com.
Heidi Lindberg is a greenhouse and nursery educator at Michigan State University Extension. She can be reached at email@example.com.