11
HYDROS 21
3.2 COMPONENTS
The HYDROS 21 sensor monitors water level, temperature, and EC in both ground and surface
water. The HYDROS 21 uses a vented ceramic peizoresistive differential pressure transducer
to measure the pressure from the water column to determine water depth.
A thermistor in thermal contact with the probe provides water temperature, while the
stainless steel screws on the surface of the sensor form a four-electrode array to measure
EC. With a range of 0 to 120 dS/m, the HYDROS 21 has the ability to make accurate EC
measurements in a broad range of applications. The HYDROS 21 has a compact 3.4-cm
diameter body made of rugged Delrin
®
resin. The electronic circuitry is encapsulated in a
marine-grade epoxy to protect the sensor in corrosive environments.
Figure 4 HYDROS 21 component diagram
3.3 THEORY
This section explains the theory of water depth, temeprature, and electrical conductivity.
3.3.1 WATER DEPTH
The HYDROS 21 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.