BRAND · THE METER CASE AND THE pH SHELF
A Bluelab pen is only as good as the bottle beside it.
Bluelab pens, the soil pH meter and the Guardian monitor live in our meter case, and the calibration and storage solutions that keep them honest sit on the pH shelf a few steps away. Here is what each one measures, and the short routine that keeps a pen reading true for years.
How we stock it at Green Zone
The Bluelab pens, the soil pH meter and the Guardian Monitor Wi-Fi are in the glass meter case. The pH 4.0 and pH 7.0 calibration solutions, the KCl storage solution and the 2.77 EC standard are on the pH shelf.
We'd rather you leave with a bottle of KCl beside the pen. Most of the "broken" pH pens that come back to the counter aren't broken. They were stored dry, or calibrated months ago.
The care routine, in one paragraph
A pH pen needs two things. Calibrate it about once a month with two solutions, pH 7.0 first and then pH 4.0, and keep the tip wet between uses with a few drops of KCl storage solution in the cap, once a week. Never store it in RO, distilled or plain water. If it does dry out, Bluelab says to soak it in KCl for 24 hours and recalibrate. An EC pen asks for less: Bluelab ships it calibrated, so just clean the probe every couple of weeks and check it now and then in the 2.77 EC solution.
The lineup
Bluelab pH Pen
Reads pH and temperature in water and nutrient solution, to 0.1 pH, and compensates for temperature on its own. It floats, it's waterproof, and it reminds you to calibrate every 30 days. The probe isn't replaceable, so care is what makes it last.
Where it fits: Your feed water and runoff
Bluelab Conductivity Pen
Reads EC, ppm 500 or ppm 700, plus temperature. It comes factory calibrated, and a dirty probe is the usual reason for a low reading, so clean it every two weeks.
Where it fits: How strong the feed is
Bluelab Soil pH Meter
A spear-tip probe that reads pH straight in the root zone, in soil, coco, rockwool or potting mix, as well as in solution. The probe is replaceable and sits in a KCl-filled cap. The meter body is splash resistant, not waterproof.
Where it fits: Soil, coco and potting mix
Bluelab Guardian Monitor Wi-Fi
Sits on the wall and watches pH, EC and temperature in the reservoir all day and night. High and low alarms go to your phone through Bluelab's Edenic app, and it keeps its settings through a power cut.
Where it fits: Reservoirs and hydro systems
Bluelab calibration and storage solutions
pH 4.0 and pH 7.0 for the two-point calibration, the 2.77 EC standard to check an EC pen, and KCl to keep a pH probe hydrated. Use fresh solution each time and never pour it back in the bottle.
Where it fits: Every pen, every month
Which pen for which grow, and how Bluelab compares with the Milwaukee and Hanna meters in the same case, is on our pH and EC meters page: pH and EC meters.
What the numbers should read
We aim for an input pH of 6.2 to 6.8 in soil and peat mixes, and 5.8 to 6.2 in coco and hydro. When you read EC as ppm, always say which scale: ppm 500 is EC times 500 and ppm 700 is EC times 700. The Bluelab pens read both scales, so set the one your feed chart uses.
How it fits your grow, in 3 steps
Step 1. Start with the pH pen and a bottle of KCl.
pH is the number that locks food out of reach fastest. Hydrate a new pen in KCl before first use, then calibrate it.
Step 2. Add EC when you feed from a bottle.
If you mix a bottled nutrient, the EC pen tells you how strong the mix is before it goes in the pot.
Step 3. Move to the Guardian when you run a reservoir.
A reservoir drifts while you're away. The Guardian watches it and tells your phone when pH, EC or temperature leaves the band you set.
Who it's for, and who it's not for
A good fit if
You feed from a bottle or run hydro and want numbers you can trust.
You want a pen that floats, shrugs off a spill and reminds you to calibrate.
You run a reservoir and want an alarm before the plants show a problem.
Not the right fit if
You grow in an amended living soil and only water: a pen is a nice check, not a must.
You won't calibrate or store it wet: any pH pen will drift, Bluelab included.
Common mistakes we help people avoid
Storing the pH pen dry or in plain water. It reads wrong long before it looks broken. KCl in the cap, once a week.
Calibrating with pH 7.0 only. The pen won't hold a one-point calibration. 7.0 first, then 4.0.
Pouring used solution back in the bottle. It contaminates the rest. Use a little, then toss it.
Blaming the EC pen for a low reading. Clean the probe first.
Frequently asked questions
How often should I calibrate a Bluelab pH pen?
About once a month. The pen shows a reminder 30 days after the last calibration. Use pH 7.0 first, then pH 4.0.
Does the Conductivity Pen need calibrating?
Bluelab ships it calibrated, so it's optional. If readings look off, clean the probe and check it in the 2.77 EC solution.
My pH pen dried out. Is it done?
Often not. Bluelab says to soak it in KCl storage solution for 24 hours, then do a two-point calibration. Probes do age out eventually, so if it still won't calibrate, bring it in and ask at the counter.
Can I check soil pH with the pen?
The pen reads liquid. For soil, use the Soil pH Meter with its spear probe, or test your runoff with the pen.
Keep going
Departments: pH, water & meters · Hydro systems & watering
Grower answers: My runoff number is high. Should I flush? · How can I tell when my plant wants more food?
More deep dives: pH and EC meters: which one fits your grow · General Hydroponics · Cutting Edge Solutions
Want the short version of how we keep a grow steady? Get the free Stability Playbook
Before you drive
In the meter case: Bluelab pH Pen, Conductivity Pen, Soil pH Meter and Guardian Monitor Wi-Fi.
On the pH shelf: Bluelab pH 4.0, pH 7.0, KCl storage solution and the 2.77 EC standard.
If an old pen won't calibrate, bring it with you and ask at the counter.
Need one exact meter? Call (508) 459-9609 first.
Photographed in the store on October 9, 2026.