The HYDROS 21 is a low-cost, durable, and easy-to-use tool for monitoring EC, temperature, and depth in both groundwater and surface water.


  • Continuously monitor groundwater and surface water level changes
  • Integrated electrical conductivity and temperature measurement
  • Digital SDI-12 communication
Your Price $511.00
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METER HYDROS 21 CTD Sensor40683-P HYDROS 21 conductivity, temperature & depth sensor, 10m cable
Usually ships in 3-5 days
HYDROS 21 conductivity, temperature & depth sensor, 10m cable
Usually ships in 3-5 days

The HYDROS 21 (CTD) is designed for groundwater monitoring wells or piezometers, surface water monitoring, and wetland monitoring. The sensor cable is vented to remove the effects of barometric pressure changes. The sensor also features a precision thermistor to measure temperature. The integrated 4-probe electrical conductivity transducer accurately senses EC up to 120 mS/cm.

HYDROS 21 uses EC to measure the concentration of salts in the water and for information about dissolved solids. The HYDROS 21 measures EC by applying an alternating electrical current to two electrodes, measuring the current flow through those electrodes, and then measuring voltage drop with a separate set of electrodes. The conductance is the ratio of current to voltage. A thermistor near the EC sensor senses the temperature of the water. The HYDROS 21 uses the temperature to adjust the EC measurements to the 25 °C default value and provides the temperature output for the data stream.

For water depth/level, the sensor uses a ceramic peizoresistive differential pressure transducer to measure the pressure applied by the water column above the sensor. The HYDROS 21 uses a direct relationship between pressure and water depth to output water depth. The reference port of the pressure transducer is vented through the cable to atmospheric pressure, so the HYDROS 21 does not require a reference barometric pressure. A porous Teflon vent near the data logger end of the cable provides the reference. The Teflon keeps liquid water out of the cable but allows air to enter and leave.

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