Measurements · Water Quality
Resistivity
Also known as specific resistance
Resistivity is how strongly water resists an electric current (the inverse of conductivity), so the purer the water, the higher its resistivity.
What it is
What is resistivity?
Resistivity is how strongly water resists an electric current (the inverse of conductivity), so the purer the water, the higher its resistivity. On this platform it is part of the conductivity set a multiparameter water-quality instrument reports, alongside conductivity, specific conductance, salinity, and TDS.
Three instrument families in the sensor catalog report it:
- the In-Situ Aqua TROLL CTD loggers and multiparameter sondes (the Aqua TROLL 100, 200, 400, 500, and 600), in Ω·cm;
- the YSI 6-Series multiparameter sondes (the 6600, 6920, 6820, and 600-series), in MΩ·cm, kΩ·cm, or Ω·cm;
- the Spectra Scientific Spectra Litmus configurable sondes (Litmus 6 and Litmus 4), in MΩ·cm.
How it is measured
How Resistivity Is Measured
Each is a sonde reporting its conductivity cell's readings, read by a Thiamis gateway. The In-Situ and Spectra Litmus instruments are polled over Modbus RS-485. The YSI 6-Series is read over SDI-12. Its response is an ordered list of values with no labels, so the sensor order configured in YSI's software when the sonde is prepared determines which value is resistivity. A deployment reports only the parameters it was set up for. The YSI 6-Series sondes and the Spectra Litmus are discontinued by their makers, so on the platform they mostly serve existing deployments.
An instrument on Thiamis3
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Every device and option that measures Resistivity 3
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 In-Situ · Level TROLL 300, Level TROLL 400, Level TROLL 500… | 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 |
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Questions
Resistivity FAQ
Which instruments report resistivity?
The In-Situ Aqua TROLL 100, 200, 400, 500, and 600, the YSI 6-Series sondes, and the Spectra Scientific Spectra Litmus 6 and Litmus 4.
How does a YSI 6-Series sonde report resistivity over SDI-12?
As one value in an ordered, unlabeled list. Its meaning is set by the sensor order configured on the sonde, and a deployment reports only the parameters it was prepared for.
In what units is resistivity reported?
Ω·cm on the In-Situ instruments, MΩ·cm on the Spectra Litmus, and MΩ·cm, kΩ·cm, or Ω·cm on the YSI 6-Series.
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