Measurements · Water Quality
TDS
Also known as total dissolved solids
Total dissolved solids.
What it is
What is TDS?
Total dissolved solids (TDS) is the combined concentration of all dissolved substances in water (mineral salts, metals, and small amounts of dissolved organic matter), reported in milligrams per liter. It captures everything that remains dissolved after suspended particles are excluded, and it is a long-standing general indicator of water quality, taste, and suitability for drinking, irrigation, and industry. In continuous monitoring TDS is not weighed directly but estimated from conductivity: because most dissolved solids are ionic and carry current, a sonde multiplies its specific conductance by a conversion factor to give an approximate TDS. It sits in the same family as conductivity, specific conductance, and salinity, all of which describe the dissolved load of the water from the same underlying measurement.
Because it is easy to derive and easy to communicate, TDS is a familiar headline figure for drinking-water and irrigation supply.
Health and operational effects
Moderate TDS is normal and even desirable, since it gives water its taste and supplies minerals, but high TDS makes water taste salty, bitter, or metallic and can make it unsuitable for drinking or sensitive industrial use. In irrigation, high TDS reduces crop yields and, over time, degrades soil structure, so it is a key measure of water for agriculture.
Limits
TDS Limit Values
The published values, by averaging period; each links to the standard that sets it.
Secondary
TDS is reported in milligrams per liter. Drinking-water frameworks commonly set a secondary (aesthetic) guideline for TDS, based on taste and usability rather than acute health, and irrigation guidance sets its own limits. The acceptable figure depends on use; see the limit table on this page. Interpret TDS as an approximate, easily communicated proxy for dissolved load: read it alongside the conductivity and specific conductance it derives from, treat trends and sudden changes as the informative signals, and remember that the value is an estimate that shifts with the mix of salts present.
How it is measured
How TDS Is Measured
The reference laboratory method for TDS is to evaporate a filtered sample and weigh the residue. For continuous monitoring, TDS is estimated from conductivity: the multiparameter sonde multiplies its temperature-corrected specific conductance by a conversion factor to report an approximate TDS in milligrams per liter. It is therefore a derived channel of the sonde connected to a Thiamis gateway, sharing the conductivity cell and temperature sensor. Because the conversion factor depends on which salts dominate, the estimate is approximate, and its reliability rests on the same maintenance as conductivity: periodic cleaning against biofouling and calibration against a standard solution to keep the underlying reading true.
An instrument on Thiamis5
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing TDS sensors
TDS comes from the conductivity cell of the sonde, so it is measured in situ at the same representative, well-mixed point: near an intake for drinking-water protection, in a supply channel for irrigation, or at an outfall. Continuous logging turns TDS into an early-warning and trend tool: a steady climb flags growing salinity or a pollution source, while a sudden step mirrors the conductivity spike of a discharge or intrusion. As with all conductivity-derived values, keeping the sonde clean and the cell calibrated is what keeps the estimate meaningful over a long deployment.
Every device and option that measures TDS 5
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 YSI · EXO1, EXO1S, EXO2… | Water Quality Sondes | Thiamis | |
 In-Situ · Level TROLL 300, Level TROLL 400, Level TROLL 500… | Water Quality Sondes | Thiamis | |
 Eureka Water Probes · Manta+ 20, Manta+ 25, Manta+ 30… | Water Quality Sondes | Thiamis | |
 Spectra Scientific · Spectra Litmus 6, Spectra Litmus 4 | Water Quality Sondes | Thiamis | |
 YSI · 6600 V2, 6600EDS V2, 6920 V2… | Water Quality Sondes | Thiamis |
Where it is monitored
Solutions That Monitor TDS
Each one links to the solution page showing what that industry measures it for.
Water Quality Monitoring
Often addedRivers, lakes, drinking water, and stormwater
Related
More in Water Quality
Questions
TDS FAQ
What is the difference between TDS and conductivity?
They measure the same thing from different angles. Conductivity is the direct electrical measurement of dissolved ions, and TDS is an estimate of the total mass of dissolved solids obtained by multiplying conductivity by a conversion factor. TDS is more intuitive to communicate, but it is derived from, and only as good as, the conductivity reading.
Is high TDS dangerous to drink?
High TDS is usually an aesthetic issue rather than an acute health hazard. It makes water taste salty, bitter, or metallic and can cause scaling, which is why drinking-water limits for it are typically secondary or guideline values. Very high TDS can indicate contamination worth investigating, so it is best read alongside the specific ions involved.
Why is TDS only an estimate in continuous monitoring?
TDS is calculated from conductivity using a conversion factor, and that factor depends on which dissolved salts are present. Different waters need different factors, so the sonde's TDS is an approximation of the true gravimetric value, adequate for trends and screening but not a precise weight.
How do I monitor TDS?
Deploy a multiparameter sonde whose conductivity cell derives TDS, connected to a Thiamis gateway and logging continuously, with regular cleaning and calibration. Use the configurator to pair a logger with a sonde covering TDS alongside conductivity, specific conductance, salinity, and temperature.
Monitor TDS on the Record
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