hydroponics.howFIELD GUIDES 02
FIELD GUIDE / Nutrient film technique

Choosing EC and pH meters: range, resolution, calibration, probes

An EC or pH meter is bought against the solution band you must hold and the calibration routine you will keep, not against the digits on its display.

8 min + guided practiceWorked quantities & explicit assumptionsReview status ↗
In this chapter
3D FIELD MODEL / NFT–01
Nutrient film technique: Start with the solution → Lift and distribute → Pass the root zone → Recover the outflow. Two shallow channels with branched supply and gravity recovery.1 / RESERVOIRStart with the solution2 / PUMPLift and distribute3 / CHANNELSPass the root zone4 / RETURNRecover the outflowConceptual sequence. Follow the guide for full operation and return.

Loading the interactive model. The complete lesson is available below.

All components and instructions are available without 3D. Illustrative geometry and flow. Not a simulation.
AFTER THIS CHAPTER
  • Separate what an EC or pH meter can tell you from what only a water test can.
  • Set range, resolution, accuracy, temperature compensation and probe requirements from the published band you must hold.
  • Write a spec sheet, with calibration and storage consumables, to compare across suppliers.
WORK THE NUMBERS

EC and meter-ppm converter

Meter ppm = EC (mS/cm) × meter factor.

800500-scale meter ppm
1,024640-scale (TDS) meter ppm
1,120700-scale meter ppm
1,600EC (µS/cm)

These are meter conversions, not measured elemental nutrient concentrations. 640 is the factor some general-purpose TDS meters and Penn State’s water-test guidance use; check which scale your meter applies.

01 / What each instrument measures, and what it cannot

An EC meter reads how well the solution conducts electricity. Oklahoma State’s EC and pH guide explains that dissolved fertilizer salts break into ions, conductivity rises with added ions, and EC is therefore a good measure of the amount of salts in solution. Penn State’s irrigation-water guidance calls the same quantity “soluble salts” and treats millimhos and millisiemens per centimetre as equivalent units.

University of Missouri G6984 states the limit: EC is a proxy for total dissolved ions and does not confirm that each nutrient is at its target. A pH meter reads acidity only; G6984 adds that alkalinity cannot be measured with a pH meter, and that pH test strips should be avoided because fertilizer dyes affect their color. So the list is a conductivity meter, a pH meter or a combination, plus a separate alkalinity test for source water.

Reading: Electrical Conductivity and pH Guide for Hydroponics ↗ · Interpreting Irrigation Water Tests ↗ · Hydroponic Nutrient Solutions (G6984) ↗

02 / Buy for mS/cm; treat ppm as a display option

The EC, pH and meter scales chapter explains the 500, 640 and 700 ppm factors, and the calculator on this page converts between them. The buying consequence: specify a meter that displays mS/cm or µS/cm, with any ppm factor selectable and shown. OSU prints its crop table in mS/cm; UF/IFAS HS1422 gives lettuce as 1.2–1.8 mS/cm with 560–840 ppm beside it; that pair matches neither a 500 nor a 700 factor (1.2 × 500 = 600), which is the case for logging mS/cm and treating ppm as a labelled display; Penn State estimates TDS with a factor of about 640.

Manufacturer pages accessed 17 September 2026 show the spread: Bluelab’s Combo Meter lists EC, CF, ppm 500 and ppm 700; Hanna’s GroLine HI9814 a selectable 0.5 or 0.7 factor; Apera’s PC60 an adjustable 0.40–1.00. A fixed-factor pen is not wrong, but every reading must be relabelled before comparison with a published range.

Reading: Electrical Conductivity and pH Guide for Hydroponics ↗ · Growing Lettuce in Small Hydroponic Systems (HS1422) ↗ · Interpreting Irrigation Water Tests ↗ · Bluelab Combo Meter (product page, technical specifications) ↗ · GroLine Waterproof Portable Hydroponics pH/EC/TDS Meter, model HI9814 (product page) ↗ · PC60 Multiparameter pH/EC/TDS/Salinity/Temperature Pocket Tester Kit (product page) ↗

03 / Range, resolution and accuracy that match the solution

OSU’s Table 2 spans 0.8–4.0 mS/cm across its listed crops and lists lettuce at 1.2–1.8 mS/cm; HS1422 prints lettuce at 1.4–1.8 mS/cm in one passage and 1.2–1.8 mS/cm in another; G6984 wants source water below 1 dS/m. A range of 0–5 mS/cm covers all of that; the example pages list 0.0–9.9 (Bluelab), 0.00–6.00 (Hanna) and 0–20 mS/cm (Apera). Crop bands are quoted to 0.1 mS/cm, so 0.1 resolution reads them; 0.01 earns its place when you also log low source-water EC, for example a supply reading 0.4 mS/cm, as in the exercise below.

Accuracy is stated two ways. Bluelab lists ±0.1 mS/cm under its 25 °C accuracy column; Hanna and Apera list ±2% and ±1% of full scale. Percent of full scale is an absolute figure in disguise: 2% of a 6.00 mS/cm scale is 0.12 mS/cm at any reading, so a displayed 1.5 may sit anywhere in 1.38–1.62, still inside a 1.2–1.8 band. For pH, the pages list 0.1 resolution with ±0.1 (Bluelab) and 0.01 with ±0.02 (Hanna) or ±0.01 (Apera); against OSU’s crop bands of 5.0–7.0 and G6984’s 5.5–6.5, a tenth is the working unit.

Reading: Electrical Conductivity and pH Guide for Hydroponics ↗ · Growing Lettuce in Small Hydroponic Systems (HS1422) ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · Bluelab Combo Meter (product page, technical specifications) ↗ · GroLine Waterproof Portable Hydroponics pH/EC/TDS Meter, model HI9814 (product page) ↗ · PC60 Multiparameter pH/EC/TDS/Salinity/Temperature Pocket Tester Kit (product page) ↗

04 / Temperature compensation, waterproofing and replaceable probes

Conductivity rises as a solution warms, so an uncompensated meter reads a warm reservoir higher than the same solution cold. Automatic temperature compensation (ATC) corrects to a reference temperature, which is why accuracy is stated at 25 °C; Hanna’s page gives its correction as 1.9% per °C and Apera’s compensates over 0–50 °C. Specify ATC on both EC and pH, and still log the temperature.

G6984’s buying advice is short: a waterproof and shockproof pH meter with replaceable probes. The reason is probe life: G6984 gives 1–2 years for a pH probe, replaced when it can no longer be calibrated, and 2–5 years for the EC probe in a combination meter. A sealed pen is discarded with its electrode. The example pages each name a part: Bluelab’s pH probe on a BNC connector, Hanna’s HI1285-7, Apera’s PC60-E. Hanna and Apera list IP67; where a page lists no rating, ask in writing.

Reading: Hydroponic Nutrient Solutions (G6984) ↗ · GroLine Waterproof Portable Hydroponics pH/EC/TDS Meter, model HI9814 (product page) ↗ · PC60 Multiparameter pH/EC/TDS/Salinity/Temperature Pocket Tester Kit (product page) ↗ · Bluelab Combo Meter (product page, technical specifications) ↗

05 / Calibration solutions, storage and drift

A pH glass electrode drifts and must be recalibrated against known buffers: G6984 says weekly at pH 4 and 7; OSU lists buffers of pH 4, 7 or 10, puts a calibration step inside each of its EC and pH procedures, and checks EC and pH daily. Conductivity standards are single points: OSU notes every EC buffer has a specific value, usually 1.41 mS/cm; Hanna calibrates at 1.41 or 5.00 mS/cm; Apera’s kit ships 1413 µS/cm and 12.88 mS/cm; Bluelab describes its conductivity probe as factory-calibrated. Buy a standard near your working band.

Storage decides whether calibration holds. G6984 stores the pH probe in storage solution or pH 4 buffer, never submerged in water; rinses with distilled or deionized water before use, after calibration, between samples and before storing; and uses the manufacturer’s cleaning solution when readings slow. Apera’s kit lists 3 M KCl storage solution; Bluelab ships a KCl-filled cap. So the purchase is meter, two pH buffers, one conductivity standard, storage and cleaning solution. Log each pre-calibration reading; a growing offset is drift.

Reading: Hydroponic Nutrient Solutions (G6984) ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗ · PC60 Multiparameter pH/EC/TDS/Salinity/Temperature Pocket Tester Kit (product page) ↗ · GroLine Waterproof Portable Hydroponics pH/EC/TDS Meter, model HI9814 (product page) ↗ · Bluelab Combo Meter (product page, technical specifications) ↗

06 / Pens, combination meters and the spec sheet

HS1422 notes that some meters read conductivity, pH and dissolved oxygen together; G6984 says a combination pH, temperature and EC meter, though often dearer, gives more monitoring options. One unit means one calibration routine; two pens mean a working EC meter while the pH probe is replaced.

  1. Write the band you must hold, its crop source and stage, and your source-water EC.
  2. Choose mS/cm as the logged unit; require a selectable, displayed ppm factor if ppm is offered.
  3. Set range, resolution and accuracy from the band, converting percent-of-full-scale figures to mS/cm.
  4. Require ATC, a written IP rating and a named replacement probe with its expected life.
  5. Order two pH buffers, a conductivity standard near your band, storage and cleaning solution.
  6. On arrival, calibrate, then read a second standard and record the offset.
  7. Fix the calibration schedule and the replacement trigger: a probe that will no longer calibrate is replaced.
CriterionWhat to write downWhere the figure comes from
Working bandCrop EC and pH band with stage; source-water ECOSU Table 2 (lettuce 1.2–1.8 mS/cm); HS1422 1.2–1.8 and 1.4–1.8 mS/cm; G6984 below 1 dS/m
RangeLowest and highest EC you will readBand plus headroom; cited examples 0–6, 0–9.9, 0–20 mS/cm
Resolution and accuracyBoth figures at reference temperatureManufacturer sheet; percent of full scale converted to mS/cm
Units and factormS/cm displayed; ppm factor selectable and shownScales chapter; Penn State 640; manufacturer factor lists
Temperature compensationATC on EC and pH; rangeManufacturer sheet (Hanna 1.9%/°C; Apera 0–50 °C)
WaterproofingIP rating in writingManufacturer sheet; G6984
ProbeReplacement part number; expected lifeG6984 pH 1–2 years, EC 2–5 years; part list
CalibrationBuffer and standard values; scheduleG6984 weekly pH 4 and 7; OSU 1.41 mS/cm
ConsumablesStorage solution, cleaning solution, sparesG6984; kit contents on cited pages
FIELD QUESTIONA sealed EC pen reads 0–3999 µS/cm at ±2% full scale; a combination meter reads 0.0–9.9 mS/cm at ±0.1 mS/cm with a replaceable pH probe. Lettuce on tap water at 0.4 mS/cm. Which sheet items decide it?

Range: both cover the 1.2–1.8 mS/cm lettuce band (OSU Table 2, HS1422) and the 0.4 source water. Accuracy: 2% of 3.999 mS/cm is 0.08 mS/cm, close to the meter’s ±0.1, so neither wins on the crop band. Probe: the pen is discarded when its electrode fails; G6984’s 2–5 year EC and 1–2 year pH probe lives apply to the replaceable unit. The decision turns on probe replacement and whether you already own a pH meter; if you do, the pen serves the crop band.

Reading: Growing Lettuce in Small Hydroponic Systems (HS1422) ↗ · Hydroponic Nutrient Solutions (G6984) ↗ · Electrical Conductivity and pH Guide for Hydroponics ↗ · Interpreting Irrigation Water Tests ↗ · Bluelab Combo Meter (product page, technical specifications) ↗ · GroLine Waterproof Portable Hydroponics pH/EC/TDS Meter, model HI9814 (product page) ↗ · PC60 Multiparameter pH/EC/TDS/Salinity/Temperature Pocket Tester Kit (product page) ↗

ILLUSTRATED SYSTEM INVENTORY

Parts & buying criteria

Build your own parts & cost worksheet →

Showing nutrient film technique. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.

01Reservoir1 · illustrated quantity+

Holds the recirculating nutrient solution. This is the common sampling and refill point for both channels.

Inspect: In cutaway view, compare the water surface with the pump intake. The drawing leaves room above the water; it does not specify a working volume.

Maintain: Record the level before adding water. Inspect the cover, clean accessible deposits and keep the intake submerged during use.

BUYING CRITERIA

Opaque, cleanable container rated by its manufacturer for the intended use; accessible lid and drain.

Check size, materials and operating conditions with your chosen supplier before ordering.
02Pump & supply1 · illustrated quantity+

Lifts solution from the reservoir to the channel inlets. Cyan packets indicate the direction of delivery.

Inspect: Follow the rising supply pipe and its branch to each channel. Both inlets need an observable supply.

Maintain: Disconnect power before opening the pump. Inspect its inlet screen and confirm both branches flow after reassembly.

BUYING CRITERIA

Compare the pump curve at the installed lift and pipe resistance; include service access and replacement availability. Supply the pump, timer and any air pump through ground-fault protection (GFCI/RCD), keep plugs and cord connections above the water line with a drip loop, and switch off and unplug before reaching into the solution.

Check size, materials and operating conditions with your chosen supplier before ordering.
03Growing channels1 · illustrated quantity+

Contain the shallow stream beneath the plants and guide it toward the return.

Inspect: Open the cutaway to find the channel floor beneath the roots. The slope is exaggerated for reading.

Maintain: Look along each channel for ponding, debris or an obstructing root mat; inspect the outlet before changing the pump.

BUYING CRITERIA

Removable covers, supported slope, smooth cleanable surfaces and outlets accessible with plants in place.

Check size, materials and operating conditions with your chosen supplier before ordering.
04Plant baskets6 illustrated · illustrated quantity+

Bridge the opening in the channel cover while allowing roots to pass through.

Inspect: In exploded view, the baskets lift above the channels. The open slats reveal the relationship between support and roots.

Maintain: Check that each rim sits securely and that roots are not pinched at transplanting or inspection.

BUYING CRITERIA

Match the basket rim and hole diameter; allow removal without damaging adjacent plants.

Check size, materials and operating conditions with your chosen supplier before ordering.
05Support mediumFor 6 baskets · illustrated quantity+

The illustrated expanded clay supports the seedling in its basket. The channels are not filled with aggregate.

Inspect: Select the brown pebbles and separate the basket in exploded view. Media texture is simplified.

Maintain: Remove loose particles before installation. Inspect whether the starter plug stays appropriately moist.

BUYING CRITERIA

Select a stable grade that stays in the basket; compare water retention, dust and cleaning requirements.

Check size, materials and operating conditions with your chosen supplier before ordering.
06Gravity return1 · illustrated quantity+

Collects channel outflow and brings it back to the reservoir without a second pump.

Inspect: Trace the low ends of both channels into the common return. Inspect the full path before assuming the circuit is clear.

Maintain: Remove obstructions and check for leaks. Test drainage with the pump off during commissioning.

BUYING CRITERIA

Removable unions, supported pipe and sufficient drain capacity under the actual installation conditions.

Check size, materials and operating conditions with your chosen supplier before ordering.
SOURCES & EDITORIAL STATUS

Evidence beside the lesson.

Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.

Independent specialist review is pending.

Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.

  1. Oklahoma State University ExtensionElectrical Conductivity and pH Guide for Hydroponics ↗

    Water analysis, salinity, pH and monitoring.

  2. Penn State ExtensionInterpreting Irrigation Water Tests ↗

    Distinguishing alkalinity, pH, salts and source-water problems.

  3. University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗

    Solution preparation, elemental nutrition and measurement.

  4. University of Florida IFAS ExtensionGrowing Lettuce in Small Hydroponic Systems (HS1422) ↗

    Lettuce system choice and management.

  5. Bluelab · manufacturerBluelab Combo Meter (product page, technical specifications) ↗

    Manufacturer specification accessed 17 September 2026: pH range 0.0–14.0 with 0.1 resolution and ±0.1 accuracy at 25 °C; EC range 0.0–9.9 mS/cm with 0.1 resolution and ±0.1 accuracy; conductivity displayed as EC, CF, ppm 500 (TDS) or ppm 700; two-point pH calibration with a 30-day reminder; a conductivity probe described as factory-calibrated; a replaceable pH probe on a BNC connector; a KCl-filled pH probe cap; no IP rating stated on the page; a specification example, not an endorsement.

  6. Hanna Instruments · manufacturerGroLine Waterproof Portable Hydroponics pH/EC/TDS Meter, model HI9814 (product page) ↗

    Manufacturer specification accessed 17 September 2026: pH range −2.00 to 16.00 with 0.01 resolution and ±0.02 accuracy at 25 °C, one- or two-point calibration with pH 4.01, 7.01 or 10.01; EC range 0.00–6.00 mS/cm with 0.01 resolution and ±2% full-scale accuracy, one-point calibration at 1.41 or 5.00 mS/cm; TDS factor selectable 0.5 (500 ppm) or 0.7 (700 ppm); automatic EC temperature compensation with β = 1.9% per °C; IP67 casing; a replaceable three-in-one probe HI1285-7 on a DIN connector; a specification example, not an endorsement.

  7. Apera Instruments · manufacturerPC60 Multiparameter pH/EC/TDS/Salinity/Temperature Pocket Tester Kit (product page) ↗

    Manufacturer specification accessed 17 September 2026: pH range −2.00 to 16.00 with 0.01/0.1 resolution and ±0.01 pH ±1 digit accuracy, automatic one- to three-point calibration; conductivity range 0–20 mS/cm in three auto-ranging scales with 0.1/1 µS and 0.01 mS resolution and ±1% full-scale accuracy; TDS factor adjustable 0.40–1.00; automatic temperature compensation 0–50 °C; IP67 waterproof rating; replaceable combination probe PC60-E; kit contents including 3 M KCl storage solution, pH 4.00 and 7.00 buffers and 1413 µS/cm and 12.88 mS/cm conductivity standards; a specification example, not an endorsement.

CONTINUE LEARNING

The next useful connections.

Nutrients & mixing

Nutrient mixing: from grams to elemental ppm

Scale two documented fertilizer examples, read the bag analysis and calculate what actually enters the final solution.

Interactive tool · 6 min + guided practice →Nutrients & mixing

EC, pH and the meter “ppm” scales

Calibrate your instruments, record unambiguous units and interpret concentration without pretending EC is a nutrient analysis.

Interactive tool · 6 min + guided practice →Nutrients & mixing

Reservoir management and solution replacement

Use a water-and-nutrient ledger to distinguish top-ups, rebalancing and a complete batch change.

Interactive tool · 6 min + guided practice →Cycling & water balance

Water quality: pH, ammonia and alkalinity

Track the interacting measurements that govern fish, roots and nitrification.

Interactive tool · 6 min + guided practice →Source water

Source water and alkalinity

Read a laboratory report, distinguish equivalent units from elemental nutrients and prepare water deliberately.

6 min + guided practice →

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