EC, TDS and PPM: Check the Scale Before You Change Your Reservoir

A nutrient reading is only useful when you know what the number means. Two meters can test the same solution and display different PPM values without either meter being broken. The difference may be the conversion scale.

That matters because an unexplained number can tempt you to add nutrients, dilute a reservoir or chase a target that was written in another unit. Before changing the solution, identify the measurement and the scale.

EC is the common starting point

Electrical conductivity, usually shown as EC in mS/cm or µS/cm, measures how well the solution conducts electricity. Dissolved mineral salts affect that conductivity. EC does not identify individual nutrients, but it gives you a repeatable way to track the overall strength of a nutrient solution.

Penn State Extension explains that monitoring EC can help detect nutrient-strength and soluble-salt problems in hydroponic systems. It also makes clear that the reading is part of diagnosis, not the diagnosis by itself: Hydroponics Systems and Principles of Plant Nutrition.

If EC moves away from your established baseline, investigate what changed. Check your starting water, top-offs, mixing order, reservoir volume and plant response before assuming that one more dose is the answer.

A meter's PPM number is often a conversion

Many handheld nutrient meters also display TDS or PPM. On these meters, the PPM value is commonly calculated from conductivity rather than obtained by counting each dissolved substance.

The official Hanna Instruments HI98330 manual documents both a 500 PPM scale and a 700 PPM scale for the same EC range. That is the practical source of a lot of confusion. A solution with one EC value can produce different displayed PPM numbers when the meter uses a different conversion factor.

This is also why a PPM target copied from a chart, forum or another grower's notes is incomplete unless you know the scale behind it.

Use one comparison system

General Hydroponics' current guide, How to Read a General Hydroponics Feed Chart, recommends knowing whether a meter uses PPM 500, PPM 700 or EC before comparing it with a feed chart. The guide points to EC as a useful common reference because it avoids that conversion-scale mismatch.

A practical record can be simple:

1. Write down the nutrient program and exact chart. 2. Record the target unit exactly as printed. 3. Note the meter model and selected display mode. 4. Save the EC reading even if you normally watch PPM. 5. Record reservoir temperature, pH and the time of the reading. 6. Note what you added or removed before the next measurement.

The goal is consistency. If you change meters or charts, overlap them for a few readings and write down both values. Do not silently replace a 500-scale PPM baseline with a 700-scale number.

Measure after the solution is mixed

The General Hydroponics guide also recommends adding products according to the program's mixing instructions, allowing each product to disperse, measuring EC or PPM after mixing and checking pH on the finished solution.

This sequence gives the number context. A reading taken before the nutrients are fully mixed should not be treated as the final reservoir strength.

Use the target supplied for the nutrient program and crop you are actually running. This article does not set a universal EC or pH target, and HydroPip has not tested the meters discussed in the source manuals.

When the number looks wrong

Before correcting the reservoir, ask four questions:

- Is the meter showing EC, PPM 500 or PPM 700? - Does the target use the same unit and scale? - Was the meter maintained and calibrated according to its own manual? - Did the starting water, reservoir volume or mixing process change?

If the units match and the result is still unexpected, repeat the measurement with clean equipment and review the crop and nutrient instructions. A second measurement can prevent a real solution from being changed to fix a display-setting problem.

Keep the decision tied to your setup

HydroPip's role is to help you organize the setup and the decisions around it. Keep the meter model, scale and recent readings with your grow notes so that a number remains traceable.

Visit HydroPip when you are planning or troubleshooting your system. The better your records, the easier it is to compare today with your own baseline instead of somebody else's unexplained PPM number.