ON OUR SHELF · THE METER CASE
Read the water before you blame the plant.
Our meter case holds pH pens, EC pens, a soil pH meter and a reservoir monitor, and the fluids that keep them honest sit on the pH shelf beside it. Here is which meter fits your grow, what the numbers mean, and how to keep a pen reading true.
How we stock it at Green Zone
Meters live in a glass case in our store: Bluelab and Milwaukee pens, a Bluelab soil pH meter and the Guardian reservoir monitor. The calibration solutions, probe storage and pH up and down are on the pH shelf.
Most people buy a pen the day a leaf looks wrong. We'd rather sell it before that day, and sell the bottle of calibration solution with it. A meter is only as good as its last calibration, and a pen that's been stored dry will tell you something that isn't true.
pH, EC and ppm, in one paragraph
pH is how acidic the water at the roots is, and it decides which nutrients the roots can take up. EC, electrical conductivity, is how much dissolved food is in that water, read in mS/cm. ppm is EC converted to a number on one of two scales: ppm500 is EC × 500 and ppm700 is EC × 700. EC is the master unit, so always name the scale. A meter measures the water. It doesn't measure the plant, so read it next to what the new growth is telling you.
The lineup
Bluelab pH Pen
Reads pH and temperature, fully temperature compensated, with two-point calibration and a 30-day reminder. Waterproof, and it floats. Bluelab says it needs a solution of 0.2 EC or above, so it isn't for plain RO water.
Where it fits: pH in your feed water
Bluelab Conductivity Pen
Reads EC, ppm500 or ppm700, plus temperature, with automatic temperature compensation. Factory calibrated; Bluelab says to clean the probe every two weeks, and that low readings usually mean a dirty probe. Waterproof, and it floats.
Where it fits: Feed strength and runoff
Milwaukee pH600 Digital pH Pen
Reads pH 0.0 to 14.0 to a tenth. Calibrated at one point, in pH 7.01 buffer, with a small screwdriver. Milwaukee says to calibrate after cleaning or storage and to keep storage solution in the cap.
Where it fits: A first pH pen, kept simple
Milwaukee pH55 PRO pH and temperature tester
Reads pH and temperature with automatic temperature compensation and one- or two-point calibration; Milwaukee recommends two. Waterproof to IP65, with a replaceable probe.
Where it fits: pH, waterproof, with a replaceable probe
Milwaukee EC59 PRO EC, TDS and temperature tester
Reads EC, TDS and temperature, waterproof to IP65, with a replaceable probe. Factory calibrated, with one-point recalibration in 1413 µS/cm solution. Its TDS factor is selectable and starts at 0.5, the 500 scale. Milwaukee says to rinse the probe in tap water after every use.
Where it fits: Feed strength, waterproof, replaceable probe
Bluelab Soil pH Meter
Reads pH straight in soil, coco, stone wool and potting mix as well as in solution, through a spear-tip probe on a cable. Two-point calibration with a 30-day reminder, and a replaceable probe. The meter body is splash-resistant, not waterproof. Sold until recently as the Multimedia pH Meter.
Where it fits: pH in the root zone itself
Bluelab Guardian Monitor Wi-Fi
Mounts on the wall and reads pH, EC and temperature in your reservoir around the clock, with high and low alarms and phone alerts through Bluelab's Edenic app. The pH probe is replaceable.
Where it fits: Reservoirs that run all the time
Calibration solutions
Bluelab pH 7.0 and pH 4.0, Bluelab 2.77 EC standard, and Hanna GroLine pH 4.01 and 1413 µS/cm. Bluelab says to calibrate pH 7.0 first, then 4.0, with fresh solution you never pour back.
Where it fits: Every pen, every month
pH probe storage solution
Keeps a pH probe's tip wet. Bluelab's is KCl, potassium chloride, and Bluelab says to refresh the cap weekly. Bluelab says never to store a probe in RO, distilled or deionized water, and Hanna says the same of distilled and deionized.
Where it fits: Between readings
General Hydroponics pH Control Kit
pH Up, pH Down and a drop test: a few drops in the vial, then match the color, from pH 4.0 to 8.5, with no batteries or calibration.
Where it fits: A first check, or a backup
General Hydroponics pH Up and pH Down
pH Down is food-grade phosphoric acid; pH Up is potassium hydroxide and potassium carbonate. GH says to mix your nutrients first, then adjust: a small amount, stir, test, repeat.
Where it fits: Moving the number
Replacement Bluelab pH probes are in the case too. Bluelab says every pH probe ages and eventually fails, and how long it lasts depends on use and care.
Runoff: what the pen tells you after the pot
Water until about 10 to 20% of what you put in runs out, and catch it. Runoff EC above your input is normal, and up to about 20 to 30% above is just information. In coco or other inert mixes, watch at about 0.5 EC above input, and get concerned at 0.7 to 1.0 or more above only when the gap is climbing and the plant shows stress. A trend takes 2 to 3 waterings in a row. One reading never calls for a flush. Runoff pH normally sits a couple of tenths from input. More on this in My runoff number is high. Should I flush?.
How it fits your grow, in 3 steps
Step 1. Pick the meter by what you're reading.
Feed water in a bucket: a pH pen and an EC pen, or the drop kit to start. The root zone itself: the Bluelab Soil pH Meter. A reservoir that runs day and night: the Guardian. If you grow in an amended soil and only ever water, you need these less. The soil buffers its own pH, and input EC isn't the lever there.
Step 2. Calibrate, store it wet, then trust it.
Calibrate pH at two points, with pH 4.0 and 7.0 buffers. Check EC in a 1.41 mS/cm standard (1,413 µS/cm at 77°F), or the higher standard your meter's maker sells; Bluelab's is 2.77. Bluelab says to calibrate monthly, and more often with heavy use or organic nutrients. Milwaukee says to calibrate after cleaning or storage. Between readings, keep the pH tip wet with storage solution.
Step 3. Read the lane, then move one knob.
In soil and peat mixes fed from bottles, keep input pH around 6.2 to 6.8; in coco and hydro, about 5.8 to 6.2. For bottled mineral feed in coco, hydro or soilless mixes, start around 0.4 to 0.8 EC for seedlings, 0.8 to 1.4 in veg and 1.2 to 1.8 in flower. Those are starting lanes, not caps, and the plant and the bottle's own chart move them. Change feed about 0.2 EC at a time. First signs show after one full watering cycle, and new growth gives the verdict in about 7 to 10 days.
Who it's for, and who it's not for
A good fit if
You feed from bottles, in coco, hydro or a peat mix, and want to know what's going in.
You read runoff and want a number you can trust over a few waterings.
You run a reservoir and want to know when it drifts before the plant does.
Not the right fit if
You grow in an amended, water-only soil and never mix a feed. Start with the bag instead: Which soil bag?.
You want one number to diagnose a plant. A meter reads the water, not the leaf. Start at Yellow leaves: is it a deficiency, and is this too much?.
Common mistakes we help people avoid
Storing a pH pen dry, or in RO water. Bluelab and Milwaukee both say never to store a probe in RO or distilled water, and Hanna says never in distilled or deionized. Bluelab says a dried-out probe needs 24 hours in KCl, then a two-point calibration.
Reusing calibration solution. Bluelab says to use fresh solution every time, and never to pour it back into the bottle.
Reading a 500-scale number off a 700-scale chart. The same 2.77 EC solution reads 1,385 on ppm500 and 1,940 on ppm700. Name the scale, or just use EC.
pH-ing before you add the nutrients. GH says to mix the nutrients into the water first, then adjust.
Testing plain RO water with a pH pen. Bluelab's pH Pen needs a solution of 0.2 EC or above.
Chasing one runoff reading. It takes 2 to 3 waterings in a row to make a trend.
Frequently asked questions
Do I need pH and EC, or just pH?
If you feed from bottles in coco, hydro or a peat mix, both. pH tells you whether the roots can take the food up, and EC tells you how much food is there. In an amended, water-only soil, neither input number is the main lever.
What's the difference between EC and ppm?
ppm is EC converted on a scale: ppm500 is EC × 500 and ppm700 is EC × 700. Bluelab's pens let you pick either scale, and Milwaukee's EC59 starts on the 500 scale. Charts printed in one scale don't match a pen set to the other.
How often should I calibrate?
Bluelab says monthly, and its pens remind you at 30 days. More often with heavy use, or with organic soils and nutrients. Milwaukee says to calibrate after cleaning or storage.
Can I check the pH of my soil?
Yes, with the Bluelab Soil pH Meter, which reads right in soil, coco and potting mix. Bluelab says to moisten dry media with distilled water, not tap water, before you read it.
My pen reads way off. Is it broken?
Not always. Milwaukee says a big error can mean the pen wasn't calibrated, the electrode dried out, or the batteries are low. Bluelab says a low EC reading usually means a dirty probe. Every pH probe ages and eventually fails. Bring it in and we'll look at it with you.
Can I replace the probe instead of the meter?
On some. The Milwaukee pH55 and EC59, the Bluelab Soil pH Meter and the Guardian take a replacement probe. The Bluelab pH Pen doesn't.
Do you carry every model?
Not every model is in the case at once. Call (508) 459-9609 to check before you drive.
Keep going
Departments: pH, water & meters · Nutrients · Hydro
Grower answers: My runoff number is high. Should I flush? · How can I tell when my plant wants more food? · Yellow leaves: is it a deficiency, and is this too much?
More deep dives: General Hydroponics family · Athena family · Which soil bag? The soil room compared
Want the short version of how we keep a grow steady? Get the free Stability Playbook
Before you drive
In the meter case: Bluelab and Milwaukee pH and EC pens, the Bluelab Soil pH Meter and the Guardian Monitor. On the pH shelf: calibration and storage solutions, and pH up and down from General Hydroponics, TNB Naturals and Advanced Nutrients.
Not every meter model is out at once.
Need one exact meter or probe? Call (508) 459-9609 first.
Photographed in the store on October 9, 2026.