How to Calibrate a pH Pen or Meter, Check EC and Store the Probe

A pH pen drifts as residue builds up on its probe and the probe wears, and a pen that has drifted gives a wrong reading for every feed you mix. Calibrating it in buffer, a solution of known pH, brings it back into line, once the probe is clean. This guide covers the routine step by step, how often to do it, how to use a pH meter and check an EC meter, and how to store and clean a probe so it lasts longer. The pens, calibration fluids and probe solutions are all in our nutrient management section.

Buffers, standards and probe solutions

pH meters are calibrated in buffer. EC meters, which read feed strength, are checked or calibrated in conductivity standard, a solution of known EC.

Calibration, storage and cleaning solutions we sell, as their product pages describe them.
Solution	What it is for	Sizes on the page
pH Pen Calibration Fluid	pH 4 or pH 7 buffer, one per bottle	300 ml and 1 litre
EC Pen Calibration Fluid	Conductivity standard, 1.41 or 2.76 mS/cm	300 ml
Hanna Calibration/Buffer Solutions	pH 4.01 and 7.01 buffers; 1.413 and 5 mS/cm standards; Quick Cal Solution (ask us which meters take it)	120 and 230 ml; Quick Cal also 500 ml
pH Probe Storage Solution 300ml	Keeps the probe hydrated between uses	300 ml
Groline Electrode Storage Solutions	Keeps electrodes hydrated	120, 230 and 500 ml
pH Probe Cleaning Solution 300ml	Removes salts, minerals and organic residue	300 ml
Groline Electrode Cleaning Solutions	Removes dirt, residues and contaminants	120, 230 and 500 ml

Their pages say the pH Pen Calibration Fluid suits all pH pens, the EC Pen Calibration Fluid all types of nutrient meter, the two Growth Technology probe solutions all pH probes, and the two Groline solutions all types of pH and EC electrode.

How often to calibrate a pH pen

Weekly, to keep nutrient readings accurate, say the pages for our pH pens and calibration fluids. Calibrate again after cleaning the probe, and whenever a reading looks wrong.

One point or two? Some pens calibrate at one point and some at two, the pH Pen Calibration Fluid page says, so check your pen's instructions and buy the buffers it asks for: a two-point pen needs a bottle of each. If your pen can do both, use two points, as general practice: pH 7 and pH 4 sit either side of the pH most feeds are adjusted to (the makers' targets are in our pH and EC meters guide).

Our pH and EC meters, and the fluid each product page names.
Meter	Reads	Calibrate or check in	How often
Bluelab pH Pen	pH	pH Pen Calibration Fluid	Weekly
Bluelab Truncheon	EC, CF and TDS	EC Pen Calibration Fluid, as a check only	Regularly
Aqua Master pH Temp Meter P50 Pro	pH and temperature	pH Pen Calibration Fluid	Weekly
Aqua Master EC Temp Meter E50 Pro	EC and temperature	EC Pen Calibration Fluid	Weekly
Aqua Master Combo Pen P110 Pro	pH, EC and temperature	pH Pen Calibration Fluid; EC Pen Calibration Fluid for the EC side	Weekly
GroLine pH/EC Monitor	pH, EC and temperature, continuously	Hanna Calibration/Buffer Solutions	Regularly

How to calibrate a pH pen or meter, step by step

This is the general two-point routine, not any one maker's. The buttons vary from pen to pen, so where your pen's leaflet says otherwise, follow the leaflet.

Pour a little pH 7 and a little pH 4 buffer into two clean containers. Never dip the pen into the bottle, and throw the used buffer away afterwards rather than pouring it back.
Take the cap off, rinse the probe in clean water and shake off the drops.
Stand the probe in the pH 7 buffer, stir gently and let the reading settle. Then calibrate the pen at that point the way its leaflet describes.
Rinse again, then do the same in the pH 4 buffer.
Rinse once more, put a little storage solution in the cap and put the cap back on.

With a one-point pen, pour only the buffer its leaflet names and skip step 4.

How to check an EC meter

Check an EC meter the same way, in conductivity standard: rinse the probe, shake off the drops so no water dilutes the standard, dip it in a little standard poured into a clean container, and read. Use the strength your meter's instructions name. If the reading is off and the meter can be calibrated, set it to the standard's value as its leaflet describes.

The Bluelab Truncheon cannot be calibrated as such, its page says: the standard shows whether it is still reading correctly, and if it reads low it most likely needs cleaning. Its page also says to clean the sensor at least once a month, or it will under-read.

How to use a pH meter and store the probe

Mix the feed completely first. The pH is checked last, once everything is in.
Rinse the probe, dip it in the feed, stir gently and wait for the reading to settle.
Adjust with a little Growth Technology pH Down or Growth Technology pH Up (both can be corrosive neat: follow the label), stir and read again.
Rinse the probe, then put a little storage solution in the cap, or stand the probe in it, until next time.

Never let the probe dry out: one that dries out loses accuracy and wears out sooner, the storage solution page says. As general practice, never store it in RO or distilled water either. Our meter pages ask for storage solution only on pH probes.

Cleaning the probe, and when to replace it

Salts, minerals and organic residue build up on the sensor and make the readings drift, the pH Probe Cleaning Solution page says. Clean first, then calibrate:

Stand the probe in cleaning solution for the time on the label.
Rinse it in clean water.
Recalibrate in buffer before you use it again.

The Groline Electrode Cleaning Solutions page gives the same order, and says it suits all types of pH and EC meter, so it is the one for an EC meter such as the Truncheon. Clean a pH probe now and then, as the Bluelab pH Pen page says, and whenever readings drift or the pen will not calibrate.

If a pen still will not calibrate after a clean and a soak in storage solution, the probe is probably worn out. The Aqua Master pH Temp Meter P50 Pro takes a replacement probe, sold on its own, so you need not buy a new meter.

Quick answers

How often should I calibrate a pH pen?

Weekly, as the pages for our pH pens and calibration fluids say, and again after cleaning the probe or whenever a reading looks wrong.

What is pH probe storage solution for?

It keeps the sensor hydrated between uses and helps prevent salt build-up, its page says. Rinse the probe and put a little in the cap.

Why won't my pH pen calibrate?

Often a dirty or dried-out probe. Clean it, rinse it, stand it in storage solution for a while, then try again with fresh buffer. If it still will not calibrate, the probe is probably worn out.

Can you calibrate a Bluelab Truncheon?

Not as such, its page says: dip it in conductivity standard, and if it reads wrong, clean it. A dirty one reads low.

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