Milwaukee MW302 EC meter

Portable EC meter with a separate SE520 probe on a cable, built for the rough conditions of horticulture and hydroponics.

The Milwaukee MW302 reads 0.0 to 10.0 mS/cm at a resolution of 0.1 and an accuracy of ± 2% of full scale, with automatic temperature compensation between 5 and 50 °C. It ships complete with the SE520 probe, 1413 µS/cm buffer, a calibration screwdriver, a 9V battery and instructions. Around 300 hours per battery.

A dirty probe lies, and you will not notice

A conductivity probe barely needs calibrating and lasts years. Which is exactly why it gets neglected, and exactly why this is the most common mistake.

An EC probe measures how well water conducts between two electrodes. The distance between them and their surface area determine what comes out of the measurement. As long as both stay constant, the meter stays right, and that is precisely what stops happening once a film builds up.

What accumulates there:

• Nutrient salts that dry on when the probe is put away wet.
• Limescale from hard tap water, which sets stubbornly.
• Biofilm from a reservoir carrying algae or bacteria.

The effect is treacherous: the meter reports no fault, it reports a tidy value that is wrong. A fouled probe usually reads low, because the film obstructs contact between water and electrode. And a low reading provokes exactly the wrong response. You conclude you are underfeeding and add nutrient to a solution that was already at strength.

The routine that prevents it is short:

1. Rinse after every use with clean water. This alone prevents most of it.
2. Clean periodically with a solution intended for conductivity probes. No abrasives and no brushes on the electrodes.
3. Calibrate after cleaning, every time. A clean probe reads differently from a dirty one, which is the whole point.
4. Store it dry and protected. Unlike a pH probe, an EC probe does not need storing wet.

Technical Specifications:

• Range: 0.0 to 10.0 mS/cm
• Resolution: 0.1 mS/cm
• Accuracy: ± 2% of full scale
• Temperature compensation: automatic, 5 to 50 °C
• Probe: SE520 conductivity probe with 1 metre cable (included)
• Operating temperature: 0 to 50 °C, max. 95% relative humidity
• Battery: 1 x 9V (included), approx. 300 hours continuous use
• Dimensions: 145 x 80 x 40 mm
• weight: 220 g with battery
• Supplied with: SE520 probe, 20 ml 1413 µS/cm buffer, screwdriver, battery, instructions

Frequently asked questions

How often should I clean the probe?
With daily use in nutrient solution, monthly, and sooner if readings start to differ from what you expect.

How often should I calibrate?
Less often than a pH meter, but always after cleaning.

Why a separate probe on a cable?
So you can measure in a deep reservoir without holding the meter in the water, and so the probe can be replaced on its own.

Can I use it in soil?
Not directly. For soil you measure the conductivity of a sample or of runoff, or use a meter built for the job.

Does it measure pH?
No. For both, look at a combi meter or a set with a separate pH meter.