Air pressure sensors
Pressurised air is used for multiple purposes in engineering. And air pressure sensors offer measurement and control of these important pressure levels.
Measuring atmospheric air pressure, or barometric pressure, is a different topic that’s covered in a separate section. This section focuses solely on applications in which air pressure is deliberately raised above normal atmospheric levels.
A very simple example of air under pressure is the air compressed in a football, without which the ball wouldn’t bounce. In a tyre, compressed air gives a combination of rigidity and cushioning. In inflatable boats, it creates a rigid but light and buoyant structure.
The ideal functioning of any of the above depends on reaching and keeping the right level of air pressure – hence the need for air pressure sensors.
While a manual pump will – with a lot of effort – inflate smaller items, you’ll naturally turn to the power of a compressed air system for larger inflatables.
Compressed air, processed from atmospheric air using an electrical or engine-powered compressor, has been used in industry since the mid-1800s and does much more than pump up footballs.
Tanks of compressed air can be used by divers for breathing underwater, by firefighters to survive in smoke-filled buildings, and by medics to ensure ventilation of patients’ lungs.
However, it’s the ability of compressed air to store and transfer energy that accounts for most of its industrial applications. Think of powerful air-driven tools, air brakes, air-assisted paint spraying, and heating, ventilation and air conditioning (HVAC) systems. These are just a few examples of compressed air’s power and versatility.
Applications
In each of the examples above there’s an ideal air pressure for operation, which air pressure sensors will monitor. Knowing the air pressure also enables you to measure other variables, like air flow rate, indirectly. Crucially, there’s a maximum safe air pressure level for any piece of equipment containing compressed air, above which it becomes dangerous.
Consider the energy stored by compressed air in an industrial tyre, for instance. If the tyre is filled beyond bursting point, you can imagine the explosive results when all that energy is suddenly released. That’s why air pressure sensors are so vital as part of a control and switching system.
The capacity of compressed air for transmitting high levels of energy is very obvious in tools like pneumatic drills, and it can even be used to drive vehicles. Safety is obviously a key concern when handling that kind of power.
For trains and heavy goods vehicles, air brakes are often specified for their known efficiency and reliability. To maintain their failsafe performance, pressure levels must be monitored.
Concentrated, controlled, accurate levels of power are needed for devices such as the air tools now used by many dentists. Air pressure is one of the measurable criteria which affect overall precision and performance.
Systems supplying breathing assistance for patients require similarly reliable air flows. In cleanrooms, maintaining a small but very specific positive pressure is critical to keeping out dirty air.
From sandblasting rusty sheet metal (left) and grimy stone buildings to gently removing dust from small spaces, compressed air is moving all around us. In every case, air pressure sensors are helping to ensure the optimum air pressure, flow rate and effect.
To give an idea of the more detailed functions of air pressure sensors, let’s take a closer look at their use in an air conditioning system.
Recommended air flow rates exist for public buildings and industrial plants. If the pressure, and therefore the flow, drops below the ideal, air pressure sensors will register the change and adjustments can be made or causes investigated.
Differential air pressure sensors are used specifically to monitor pressure before and after filters. In doing so, they can indicate when a dirty filter needs replaced.
Sensors also help avoid energy wastage through unnecessarily high pressure in a system. A small reduction, while staying within the optimum pressure range, can lead to significant savings on electricity.
Measurement options
Depending on your system’s needs, you’ll normally choose between the following types of sensor (which measure pressure in different ways):
- Absolute air pressure sensor – measuring pressure in relation to that of a perfect vacuum (this is your least likely choice, except perhaps for experimental work).
- Gauge air pressure sensor – measuring pressure in relation to atmospheric pressure (in tyre pressure measurement, for example, a reading of zero means the internal air pressure is equal to that of the atmosphere).
- Differential air pressure sensor – measuring the difference in pressure between two points (such as before and after a filter in an air conditioning system).
Technology
In the simplest air pressure gauges, there’s a direct mechanical connection between the pressurised air and a pointer on the gauge. Common mechanisms for this include bourdon tubes and pistons. The higher the pressure, the further the pointer moves.
For greater accuracy and control, however, you should look at air pressure sensors which convert pressure’s effect into a proportional electrical signal. This type of pressure sensor tends to be referred to as a pressure transducer.
Many different technologies and operating principles have been used to achieve precise pressure measurements, each with their own advantages and disadvantages. Summaries of the three most common approaches used in the air pressure sensor field are given below.
Resistive air pressure transducer or strain gauge
A representative diagram of a resistive air pressure sensor
A diaphragm, in contact with the air whose pressure is being measured, deforms as pressure increases (see diagram above). Strain gauges attached to the non-contacting surface of the diaphragm are similarly deformed. The piezoresistive effect, in which the strain gauge material’s resistance alters when deformed, is converted into an electrical signal.
A cross-section of a capacitive air pressure sensor |
Capacitive air pressure transducer
In this approach, two capacitive plates are separated from each other by a small gap. One is fixed, while the other, which is in contact with the air, acts as a flexible diaphragm (see diagram to the right). Increasing air pressure deforms the diaphragm, which narrows the gap and decreases capacitance. The change in capacitance is converted into an electrical signal.
Inductive air pressure transducer
Here the deformation of a diaphragm is converted into linear movement of a ferromagnetic core using the principle of inductance. The core’s movement causes variation in the induced current which is generated by an AC-powered coil on another secondary pick-up coil. This change is, in turn, converted into an electrical signal.
Options and specifications
No two applications are exactly alike. You’ll have to assess how well the air pressure sensors on your shortlist meet your needs and make the best – if not perfect – choice. Here are some criteria to consider:
- Pressure range. At the very least, this should cover the maximum pressure permissible in your equipment. Excess pressure is a common cause of air pressure sensor failure.
- Precision. Most air pressure sensors are reasonably accurate at room temperature but less so at higher temperatures. If accuracy is a priority, choose a sensor with a high-precision specification and use digital electronics. Sensors offering calibrated and temperature-compensated signals are available.
- Location flexibility. Vibration, shocks and high temperatures may affect sensors placed close to the equipment they monitor. If installation at a distance isn’t possible, look for a robust design. If sensors may be exposed to hot or wet weather, ensure their housing offers suitable protection.
- Compatibility. Will the sensor’s fittings allow it to thread easily onto your existing set-up? Is its analogue output compatible with your signal conditioning instrumentation?
- Response time. Some pressure transmitters allow easy adjustment of response time, which can be useful in eliminating false triggering.
- Price. Don’t pay more than you need to, but bear in mind that with low-cost sensors you’ll have to compromise on things like durability and accuracy.
- Lifetime cost. Factors like ease of installation, low maintenance needs, robustness and longevity should be assessed alongside the purchase price.
Limitations
The chances are, your broad choice will be between resistive and capacitive air pressure transducers.
Of these, resistive – or strain gauge – is the category most used. It offers advantages in terms of overpressure protection, effective resolution, and resilience in the face of vibrations, shocks and dynamically varying pressures.
However, when selecting a resistive air pressure transducer you should be aware of the different material choices. These vary in stability with respect to temperature, humidity and sensor output.
Capacitive air pressure transducers also cope well with overpressure and are better than strain gauges for use at low pressures. They score well on hysteresis, linearity, stability, repeatability and measurement of static pressure.
On the other hand, they are larger and more expensive to make. They can also be adversely affected by particulates and humidity in the gap between their capacitive plates.
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.
