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Piezometer Comparison

 
 Standpipe  Vibrating Wire  Pneumatic  Titanium
Range Depth of standpipe 10,25, 50,100,250,500 psi
.07,1.8, 3.5, 7, 17, 35 bar
180 psi
12 bar
20, 50 100, 250, 500 psi 1.35, 1.8, 3.5, 7,17,35 bar
Response Time* Slow Fast Fast Fast
Reading Time * Minutes Seconds 5 minutes with 200 ft (tubing Very Fast
Readout* Water Level Indicator. Size and weight depend on cable length. Small, battery-powered readout. Large, heavy readout. Weight due to internal gas bottle Data Logger
Remote Access * No. Reading is obtained at top of standpipe Yes. Signal cable can be run to remote readout station. Yes. Signal cable can be run to remote readout station. Yes
Data Logging* No Yes No Yes
Accurate readings Requires technique Easy Requires patience and technique Easy
Vulnerability
to Lighting
No Yes (electric sensors) No Yes (electric sensor)
Main Advantage Simple. Easy to install and easy to read Not affected by lightning Corrosion proof
Titanium body
Main Limitation No remote access Some sites may require lightning protection Slow reading time. Some sites may require lightning protection
Main Expense Drilling Borehole Drilling Borehole Drilling Borehole Drilling Borehole

* Starred items are explained below.

Response Time

Response time is relevant mainly in cohesive soils, such as clays, that do not easily release their water. When pore-water pressure changes, some volume of water flows into or out of the piezometer until an equilibrium is established between the pore-water pressure in the soil and the water in the piezometer. The time required for this equilibrium to occur is called the piezometer’s response time.

Factors affecting response time are the hydraulic conductivity of the soil and the volume of water that must flow into or out of the piezometer. Diaphragm piezometers require less water and therefore have faster response time. Standpipe piezometers are intrinsically slower than diaphragm piezometers because the standpipe requires a larger volume of water. For example, a small-diameter 3/4-inch standpipe requires an inflow of 240 ml of water to show a 1 psi change in pore-water pressure. A VW piezometer, which has a very sensitive diaphragm, requires an inflow of only 0.00002 ml (12 million times smaller) to show the same 1 psi change. In highly permeable soils where water is readily available, the difference in response times may be insignificant. However, in clays and other soils where permeability is very low, the response times will be very different, making the VW piezometer or some other electric piezometer the only useful choice.

In certain conditions, the response time of a diaphragm piezometer can be slowed if there is an air bubble between the filter and the diaphragm. The air bubble must deform (change volume) before it can transmit the pressure of the water, and this requires a greater inflow or outflow of water than if there were no air bubble. In saturated soils with high permeability, the presence of a bubble has little effect, since water is in good supply, and the bubble is easily compressed. In tight soils, such as clays, response time may be slowed by the presence of a bubble, because the free water needed to compress the bubble, is scarce.

Remote Access

Measuring the water level in standpipe piezometers requires direct access to the top of the pipe. If regular and continued access is not possible, then a diaphragm piezometer (VW, Pneumatic, or 4-20mA), which can be operated from a remote readout station, is the only choice.

Reading Time

Standpipes can occasionally present problems if the pipe is not straight or if there are false signals due to organic growth on standpipe walls. VW sensors can normally be read in seconds. Pneumatic sensors take longer to read, since gas must be sent from the readout to the piezometer and then returned to the readout.

Readout

Readout size and weight become an issue because field technicians often take several readouts with them. The size and weight of the water level indicator depends on the quantity of cable on its reel.The VW readout is the smallest and lightest. The pneumatic indicator is the largest and heaviest (11 kg). Its internal tank requires regular refills. All the readouts use use batteries, which must be changed from time to time.

Data Logging

Data loggers allow readings to be taken automatically without a technician present. Also, data loggers allow many more readings to be taken, sometimes revealing trends that otherwise would go unnoticed. Standpipe piezometers are not usually automated. VW piezometers are easy to automate, but require loggers that are capable of exciting and reading them. Pneumatic piezometers are not suitable for automation. Titanium piezometers can be connnected to 4-20mA loggers.

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