Theory Of Operation; Figure 1 - Model 3400 P - Geokon 3400 Series Instruction Manual

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1. THEORY OF OPERATION

Geokon Model 3400 Piezometers are intended for dynamic measurements of fluid and/or pore
water pressures in standpipes, boreholes, embankments, pipelines, pressure vessels, reservoirs,
etc. They are also used for static pressure movement where the readout system is incompatible
with vibrating wire type transducers. The piezometer assembly is shown in Figure 1.
Figure 1 - Model 3400 Piezometer Assembly
The basic pressure transducer is semiconductor based. The output from the transducer may be
100 mV/volt, 0 to 5 volts, or 4 to 20 mA at the option of the user. The transducer is packaged
inside a 1 1/4" (32 mm) diameter stainless steel tube (standard 304 SS or optional 316 SS for
aggressive environments). At one end of this tube is a filter housing to allow the passage of water
while preventing the entry of soil particles. At the other end is located a bulkhead seal and cable
entry seal to prevent water from reaching the backside of the transducer. A thermistor included
inside the main housing allows the measurement of temperature.
The output cable is multi conductor with from two to four shielded pairs depending on the
transducer output. Voltage types are generally read using remote sensing techniques. Low-
pressure models may also be vented to the atmosphere through a vent tube inside the cable.
Venting of the transducer is necessary if the effects of barometric pressure changes on the
transducer are to be eliminated.
Where venting is used, the outer end of the vent tube is connected to a desiccant chamber to
prevent moisture from migrating to the transducer interior.