Measurement of Hydrostatic Pressure Using Piezoresistive Level Transducers

Water defines life on Earth in many aspects. Given the obvious importance of this element, reliable monitoring becomes necessary. What cannot be measured cannot be effectively managed. From the supply of fresh water, drinking water purification, storage and consumption, to wastewater treatment and hydrometry, effective operation and planning are not possible without accurate baseline parameters. Currently, a wide range of devices and processes are available to cover comprehensive hydrometric infrastructure. The classic water level measurement device, without a doubt, is a level gauge with an accuracy of +/- 1 cm, operating as an analog device – without electronic data transmission and verified visually. Today, much more advanced and accurate instruments provide remote transmission of measured data, including piezoresistive pressure sensors for measuring levels in both groundwater and surface waters.

Level measurement with pressure sensors

Pressure sensors for level measurement are installed at the bottom of the water body to be monitored. Unlike the above-mentioned "analog" level gauges, they can be read without the need to contact the measured medium. Piezoresistive level sensors were developed to meet today's requirements for automation and control processes. The water level can be measured without human intervention, enabling continuous monitoring in hard-to-reach places.

Hydrostatic level sensors measure the hydrostatic pressure at the bottom of the water body, where the pressure is proportional to the height of the water column. Additionally, it depends on the density of the liquid and gravitational force. According to Pascal's law, this leads to the following formula:

p(h) = ρ * g * h + p0

p(h) - hydrostatic pressure

ρ - liquid density

g - acceleration due to gravity

h - height of the liquid column

Piezoresistive level transducers are protected from surface effects as they are located at the bottom of the measured water body. Neither sea foam nor floating debris can affect the measurements in this case. Of course, sensors must be adapted to the expected underwater conditions. For example, in saltwater, it is preferable to use a level sensor in a titanium housing. If galvanic effects are likely, the best choice is a polyvinylidene fluoride housing (DCX-25 PVDF). In freshwater, high-quality stainless steel is mostly sufficient. Finally, adequate grounding of level sensors is essential to prevent damage from lightning strikes.

Modern level sensors: all data from a single device

Piezoresistive level transducers can be used in open waters such as lakes, in groundwater, as well as in closed tanks. In open water, appropriate pressure sensors with a built-in capillary in the cable for atmospheric pressure compensation will be used. In tanks, differential pressure sensors are used due to the need to account for gas pressure on the liquid. Based on the significant self-sufficiency and simplicity of piezoresistive pressure sensors, as well as their optimization for very high pressures, it has become possible to perform measurements at great depths. Theoretically, the only limitation to depth is the length of the sensor cable.

Based on this fact, it can be stated that these modern measuring instruments are also extremely versatile. Among other things, we are interested not only in the water level. Its quality is equally important for groundwater monitoring. For example, the purity of an underground reservoir can be determined by conductivity: the lower the conductivity, the cleaner the water (DCX-22 CTD). In addition to conductivity sensors, level sensors with integrated temperature measurement are also available today (36XW). Piezoresistive level transducers cover a wide range of monitoring tasks and are undoubtedly preferable to level gauges of other principles of operation in many applications.

If you have any theoretical or practical questions about piezoresistive level transducers, please contact our specialists for a free consultation. Also, we recommend reviewing the informational resource on water applications of Keller instruments: keller-h2o.com