Pressure sensors for different media types
Applications
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 in the diagram on the right). 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 analogue-to-digital convertor (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.
Measurement options
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.
Technology
Water pressure sensors are transducers, generating an electrical signal in proportion to the pressure they measure.
A typical water pressure switch |
Water pressure sensors typically 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.
One drawback of zero-based output is the difficulty of identifying problems with the sensor itself. For example, in a water-pumping system, a pump might be configured to activate when the water pressure rises above a certain point, perhaps indicating that a certain depth of water has accumulated. If the water pressure sensor has a 0V signal, that might indicate zero pressure – or the sensor may have failed completely, which would mean that the pump did not activate as water levels rose, perhaps leading to a flood. In contrast, a zero reading from a ‘voltage at zero pressure’ sensor would clearly indicate a fault.
Options and specifications
Specifications to consider when choosing a water pressure sensor include:
- Type of measurement (absolute vs. gauge)
- Pressure measurement range
- Accuracy (usually expressed as a percentage)
- Media compatibility
- Moisture resistance
- Operating temperature range
- Venting (see below)
- Vibration resistance
For wireless water pressure sensors, additional relevant options are:
- Transmitter wiring (cable or flying lead)
- Transmitter accuracy
- Radio frequency
- Electromagnetic interference
Sensors that measure gauge pressure must be vented, so you may need to consider sealing methods for outdoor applications, high-pressure water jets (in a car wash or industrial process, for example), or where the sensor will be exposed to a lot of water vapour. An alternative is to use a sealed gauge, or a sensor that measures absolute pressure.
Pumping systems may sometimes be subject to ‘water-hammer’, where the opening or closing of valves sends a shock-wave through the water, causing a pressure transient or pressure spike that may exceed the measurement range of the sensor. To protect against this, some sensors can be fitted with restrictor plugs to slow down water flow and mitigate the effects of the spike.
Some water pressure sensors can indicate that they have developed a fault by sending their signal output out of range (either below the lowest point or above the highest point). In pumping applications, this can help to prevent flooding, or protect the pump from running dry or incurring extra damage.
Limitations
Since water pressure sensors may come into contact with different kinds of water, you may want to consider their suitability for different pH levels (acidic or alkaline), salt water, chemicals or other contaminants.
Not all pressure sensors may be suitable for use with potable (drinking) water. The regulations concerning what materials can come into contact with drinking water vary from country to country.
If you want to learn more about the different types of media that pressure sensors can measure, the applications of each type, and the different sensor options for your design, click the links below to jump to the section you're interested in.
Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.
