Water pressure sensors are often used to measure the level of water in a tank, or the rate of change in that level (as shown below). The sensor is fitted to the top of an open-ended tube submerged within the container. As the water level rises, the air above the water in the tube is compressed, increasing the pressure on the sensor. An analog-to-digital converter (ADC) is used to convert the signal from the sensor into a digital value.
They can also be used to gauge the pressure in pipes where water is flowing-for example, in water distribution systems, to automatically determine whether pumps need to be activated to increase the flow rate. And they can be used to gauge the depth of a submerged object-for example, in deep-sea diving.
Water pressure sensors can measure pressure in several different ways:
- Absolute water pressure is measured against zero. This is similar to the way gas pressure sensors measure gas pressure as compared with a vacuum.
- Gauge pressure measures water pressure against the atmospheric pressure around the sensor. If the water pressure sensor is completely submerged in water, a vent line is used to allow air from above the surface to enter the sensor, to provide the reading for atmospheric pressure. The vent line can often be run through the power cable supplying the sensor.
- Differential pressure reflects the difference between two bodies of water-for example, in two separate tanks or containers, or two water pipes. This can be used to measure pressure drops across filters, or measure flow rates by measuring the difference in pressure across a restriction.
Water pressure sensors usually contain a physical diaphragm, often made of silicon, which bends as pressure is applied. The diaphragm is a strain gauge, which varies its electrical resistance when force is applied. This resistance is used to modify the output voltage of the sensor.
Some water pressure sensors provide zero-based outputs, where zero pressure results in no output signal at all. For example, their output might be in the range 0–5V. Others offer voltage at zero pressure, with a range such as 1–5V.