Concept-clarification buyer’s guide · Procurement perspective · Published: October 12, 2026
Table of Contents
Key Takeaways
- The three meters answer three different questions: an EC meter measures conductivity directly, a TDS meter multiplies that conductivity by an assumed factor, and a salinity meter interprets conductivity (or refraction) against the fixed ionic recipe of seawater.
- Common TDS conversion factors are 0.5 (NaCl / US 500 scale), 0.64 (EU 640 scale), and 0.7 (442 / hydroponics scale). The same water reads 960 ppm on one scale and 750 ppm on another from the identical 1,500 µS/cm sample [1].
- The factor is not a constant: across natural waters it typically spans roughly 0.5 to 0.9 depending on ionic composition [2], and reference tables put it between about 0.50 and 0.82 over a 100–10,000 µS/cm range [3].
- Seawater averages about 35 ppt salinity (typically 31–38 ppt), with sodium and chloride making up roughly 91% of its ions — a composition TDS pens were never scaled for [4].
- Many pocket testers top out at 10.0 ppt, which is why taking a TDS pen to the sea returns a blinking error or a nonsense number; seawater calls for a salinity meter or a calibrated refractometer [5].
- The buying rule: pick the instrument for the water and the decision — EC for process control, TDS for ionic trend screening, salinity for brines and marine systems — and know which conversion scale any displayed ppm figure is using.
Three Meters, Three Different Questions
Sales floors blur them together, but the three instruments exist because they answer different questions.
How well does this water conduct electricity? That is the EC meter’s question, answered directly in microsiemens per centimeter (µS/cm). Roughly how much ionic material is dissolved in it? That one belongs to the TDS meter, which takes the EC reading, multiplies it by a conversion factor, and prints an estimate in ppm or mg/L. And how salty is this water compared with the ocean? — the salinity meter answers it either by conductivity interpreted through the ionic proportions of seawater or by optical refraction.
None of the three is “more accurate” in the abstract. Each is exact about its own quantity and approximate about everything else. Trouble starts when a buyer asks one instrument a question it was never designed to answer — most famously, dipping a TDS pen into the sea.
What an EC Meter Actually Measures
Electrical conductivity is a physical property: dissolved ions carry current, so the meter’s electrodes report how readily the sample does so. It is the raw, assumption-free measurement — the number the other two instruments are built on.
Because different ions carry different charge-to-mobility ratios, conductivity responds to what is dissolved as well as how much. That is exactly why process engineers prefer it as a control signal: it moves when the water changes, even before anyone knows why.
EC earns its keep in boilers, cooling loops, RO plants, and ultrapure water systems, where the operating question is exactly “has the ionic content moved?” For pure-water work the instrument needs the right cell constant and temperature compensation, but the principle is unchanged: EC measures what is there.
What a TDS Meter Actually Measures (and Assumes)
A TDS meter is an EC meter wearing a costume. It measures conductivity, then converts it with a predefined factor to an estimated concentration of dissolved ionic solids [2]. That factor is a bet about the water’s ionic makeup, and the betting odds are standardized: 0.5 corresponds to a sodium-chloride calibration (the “500 scale” common on US meters), 0.64 is the European “640 scale,” and 0.7 is the “442 scale” built on a potassium-chloride–dominated natural-water blend [1].
The size of the bet is easy to demonstrate. The same sample reading 1,500 µS/cm becomes 960 ppm on the 0.64 scale but 750 ppm on the 0.5 scale — a 28% difference produced by identical water and identical electronics [1]. The true factor is not a constant at all: reference material puts the typical range across natural waters at roughly 0.5 to 0.9 [2], and conversion tables for natural waters show the factor sliding from about 0.50 at 100 µS/cm to about 0.82 at 10,000 µS/cm as composition and concentration change [3]. Real TDS, meanwhile, is defined gravimetrically — evaporate a filtered sample and weigh the residue — which is what laboratories do and pocket meters only approximate [2].
Read any TDS display the way an accountant reads a currency conversion: useful, consistent, and only as truthful as the exchange rate it assumes.
What a Salinity Meter Actually Measures
Salinity is defined against seawater, whose ionic proportions are remarkably stable across the oceans. Sodium and chloride account for roughly 91% of the ions, and average ocean salinity sits near 35 ppt, varying between about 31 and 38 ppt by location [4]. Because the recipe is fixed, salinity instruments can exploit it. A conductivity-based salinity meter measures EC and interprets it through the seawater model, reporting in practical salinity units (PSU) or ppt; a refractometer takes a drop of sample and reads salinity from the refractive index of the light bending through it [5]. Both are legitimate. Both need the scale matched to the sample.
The defining property is the reference water. Salinity meters are calibrated for the ocean’s ionic blend — not for river water, not for boiler water, not for the heterogeneous ionic soup a TDS pen assumes. That makes them accurate for marine aquaculture, seawater intakes, and brine streams, and it makes them the wrong tool for freshwater chemistry.
Matching the Meter to the Water
| Instrument | What it really does | Typical output | Right water / right job |
|---|---|---|---|
| EC / conductivity meter | Direct measurement of ionic conductance, corrected for temperature | µS/cm or mS/cm | Process control in boilers, cooling loops, RO permeate and ultrapure water; any application where change is the question |
| TDS meter | EC multiplied by an assumed factor (0.5 / 0.64 / 0.7 scales) [1] | ppm or mg/L (estimated) | Ionic trend screening, RO salt-rejection checks, drinking-water consistency checks — trend tool, not verdict tool [2] |
| Salinity meter | EC interpreted through the seawater ionic model | ppt, PSU | Seawater, brackish and marine aquaculture, brines, desalination intake and product water [4] |
| Refractometer | Optical refraction converted to salinity | ppt or specific gravity | Field checks on marine systems and salt production; needs periodic calibration [5] |
A few pairings deserve spelling out. Drinking water: a TDS reading is a legitimate quick screen for ionic content and RO performance, but a TDS figure alone never establishes safety, because the parameter it estimates covers only a slice of what matters [2]. Irrigation: advisory practice grades irrigation water in dS/m against crop-specific tolerance tables, which is why farms standardize on EC — the raw signal — rather than on a factor-derived ppm estimate. Aquaculture and marine systems: species live within narrow salinity windows in the 30–38 ppt band typical of the sea [4], so conductivity-derived salinity meters or calibrated refractometers are the standard, not freshwater-scaled TDS pens. Boilers and ultrapure loops: conductivity with temperature compensation is the control parameter of record, and many plants drive blowdown and polishing directly from it.
The Classic Mistake: Taking a TDS Pen into the Sea
The most common cross-application error in this space is pointing a freshwater TDS pen at seawater. Two things break at once.
First, range. Seawater’s salinity of about 35 ppt corresponds to a conductivity far beyond instruments built for tap and well water — many pocket testers simply top out at 10.0 ppt, a ceiling that already excludes saltwater aquariums and full-strength seawater [4][5]. Second, the conversion: the pen’s 0.5- or 0.64-scale factor assumes an ionic blend that seawater does not have, so even an in-range reading would be converted against the wrong chemistry [1][4]. A refractometer or a salinity meter scaled for the marine range returns an answer the pen structurally cannot.
The same logic runs in reverse for buyers of process instruments: an instrument advertised as “three meters in one” is still three conversions sharing one sensor. Ask which scale each conversion uses before trusting any displayed number.
The Procurement Checklist
Three questions separate a good purchase from a drawer filler.
What parameter does the decision actually need — raw conductance, estimated ionic mass, or seawater-referenced salinity? Buy the instrument whose native output is that quantity, not one that prints a converted cousin of it. What scale is behind the number — 0.5, 0.64, 0.7 [1], or a verified water-specific factor [3]? Get it in writing, because two meters on the same bench can legitimately disagree by a quarter of their reading [1]. How does the data leave the instrument — a pocket display, or a digital output (Modbus RTU/TCP, 4–20 mA) that a historian, controller, or AI water model can consume?
For fixed plant monitoring, integration-first instruments pay for themselves. Shanghai ChiMay’s in-line conductivity and salinity instrumentation publishes ranges, accuracy, temperature compensation, and communication options on its product pages, and its multi-parameter probes bundle conductivity with pH, ORP, and temperature in a single body so correlated readings arrive synchronized.
TDS, EC, and salinity are not competing products. They are three honest answers to three different questions — and plants that buy the answer first, the instrument second, end up with meters that agree with the water instead of merely displaying it.
References
- CalculatorLib — “EC to TDS (ppm) Calculator”: conversion factors 0.5 (NaCl/US 500 scale), 0.64 (EU 640 scale), and 0.7 (442/hydroponics), with the 1,500 µS/cm → 960 vs 750 ppm worked example. URL: https://calculatorlib.com/ec-to-tds-ppm-calculator
- Water Data Group — “Total Dissolved Solids”: TDS determined gravimetrically or estimated from conductivity with conversion factors typically between 0.5 and 0.9. URL: https://waterdatagroup.com/wiki/total-dissolved-solids
- Commercial Water Lab — “Measuring TDS in Water”: NDSU Extension WQ1923 conversion table for natural waters, factor spanning ~0.50 to ~0.82 across 100–10,000 µS/cm. URL: https://commercialwaterlab.com/guides/measuring-tds-in-water
- Biology Insights — “How to Measure Salinity of Seawater”: average ocean salinity ~35 ppt (31–38 ppt range), PSU definition, and sodium/chloride at ~91% of seawater ions. URL: https://biologyinsights.com/how-to-measure-salinity-of-seawater/
- Apera Instruments — “Measuring Salinity in a Saltwater Aquarium”: pocket testers capped at 10.0 ppt, refractometer accuracy and calibration needs, and electronic salinity meters for saltwater. URL: https://aperainst.de/?lang=en&p=28472
- Shanghai ChiMay — product pages for in-line conductivity and salinity instrumentation used in industrial cooling, aquaculture, and irrigation monitoring. URL: https://www.chimaycorp.com/durable-digital-conductivity-meter.html
About the author: Written by the Shanghai ChiMay Technical Editorial Team — Shanghai ChiMay’s water-analysis instrument group, which builds in-line conductivity, pH, salinity, and multi-parameter analyzers for process control, desalination, and aquaculture applications.
