12
SYSTEM
Keep the vent open to the same atmospheric pressure that applies to the water and out of
the water. Since the cable conducts reference air between the sensor and the atmosphere, it
is extremely important to protect the cable from any damage that allows water to enter.
3.3.2 TEMPERATURE
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.
3.3.3 ELECTRICAL CONDUCTIVITY
The 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. The HYDROS 21 EC measurements are corrected to EC at 25 °C:
Equation 1
EC
25
=
EC
T
1
+
0.019(
T
−
25)
⎡⎣
⎤⎦
where
EC
25
is the normalized EC at 25 °C,
EC
T
is the EC measured by the probe at
temperature
T
, and
T
is the temperature at the time of measurement.
Conductivity is conductance multiplied by a cell constant (obtained by using conductivity
standards).
NOTE: A four-electrode sensor gives unpredictable readings in air because there is no connection between the voltage
and current electrodes.