* Starred items are explained below.
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.
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.
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 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 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.