10.5.2 Pneumatic Piezometer
A pneumatic piezometer operates by gas pressure. The piezometer is sealed in a borehole, embedded in fill, or suspended in a standpipe. Twin pneumatic tubes run from the piezometer to a terminal at the surface. Readings are obtained with a pneumatic indicator.
The piezometer contains a flexible diaphragm. Water pressure acts on one side of the diaphragm and gas pressure acts on the other. When a reading is required, a pneumatic indicator is connected to the terminal or directly to the tubing. Compressed nitrogen gas from the indicator flows down the input tube to increase gas pressure on the diaphragm. When gas pressure exceeds water pressure, the diaphragm is forced away from the vent tube, allowing excess gas to escape via the vent tube. When the return flow of gas is detected at the surface, the gas supply is shut off. Gas pressure in the piezometer decreases until water pressure forces the diaphragm to its original position, preventing further escape of gas through the vent tube. At this point, gas pressure equals water pressure, and the pneumatic indicator shows the reading on its pressure gauge. Figure 10 shows the working principle of pneumatic piezometer.
Various advantages of this piezometer include reliable operation, possibility of remote reading, no electrical component and indicator can be calibrated at any time. However, it suffers from a number of limitations such as accuracy depends on skill of operator, difficult and expensive to automate, requiring personal attention on site, reading time increases with length of tubing. There is also a chance that the pneumatic tubing can get blocked by condensation of dry nitrogen gas.
Figure 10: Working Principle of Pneumatic Piezometer