Everybody is familiar with pressure. Not the figurative type, of course: looming deadlines and impossible-to-hit metrics. Rather, the literal, scientific force; that is the pressure exerted by fluids all around us. Pressure is simply force applied to a unit of area, but the way to measure that pressure can vary greatly.
Are all pressure sensors created equal? It might be easy to say yes, considering they all measure pressure, but pressure sensors can become highly specialized to their desired application. Pressure sensors all attempt to convert a pressure measurement into a signal that can be used by another device. Let’s take a look at differential and absolute pressure sensors to see what makes them special.
What is pressure?
The molecules in fluids, both gases and liquids, push apart from each other in all directions. The closer these molecules get, the harder they push away as their kinetic energy increases. Measuring pressure is similar to measuring voltage in many respects. Both pressure and voltage measurements need a reference measurement. While it may seem like a simple decision, choosing a good reference is key to an effective sensor.
What’s the difference between absolute and differential pressure?
When measuring pressure, it is important to know which reference point to choose. When a pressure is measured in reference to an absolute vacuum (0 psi), the measured pressure is referred to as absolute pressure (psia). Think of this pressure as measuring a voltage relative to ground. Differential pressure uses a different pressure as the reference point. A common differential pressure is gauge pressure which uses atmospheric pressure as the reference point (psig). This can often be thought of as measuring a voltage relative to another voltage, for example measuring a 1.7 V differential between 5 V and 3.3 V voltage sources.
When adding a pressure sensor to a design, designers can typically tell what type of pressure sensor it is by its form. Note the differential pressure sensor with two ports, the gauge sensor with one port and the absolute pressure sensor without a port.
So how do they work?
All electronic pressure sensors rely on a physical reaction to an applied pressure in order to generate a change in measurement electronically. This change in measurement could be a change in voltage, resistance, capacitance or other value. Let's look at four of the most popular types of pressure sensors.
Capacitive pressure sensors
Capacitive pressure sensors have two key parts: a rigid plate and a flexible membrane. The flexible membrane divides the two regions of pressure. Sometimes a dielectric medium is used to fill the space between the plates. As the pressure differential across the flexible membrane changes, the flexible membrane deflects toward or away from the rigid plate, changing the capacitance.
Strain gauge pressure sensors
A strain gauge-type pressure sensor also uses a flexible diaphragm to capture a change in pressure. Foil or silicon gauges are arranged in a formation known as a Wheatstone bridge. Strain gauges are designed to change their ohmic resistance in response to a change in deflection. By adhering the strain gauge directly to the surface of the diaphragm separating the two pressures, the change in resistance can be used to calculate the change in resistance.
Metal and ceramic diaphragms are often used with this type of sensor. The metal diaphragm typically results in higher over-pressure capabilities and higher burst pressures than the ceramic-type diaphragms.
Piezoresistive pressure sensors
Pressure sensors that use piezoresistive components for the sensing elements are arranged in a formation similar to the Wheatstone bridge. These piezoresistive components generate a change in resistance as they are deformed also. By attaching these components to a flexible membrane, the deflection of the membrane or diaphragm can be calculated in a similar manner to the strain gauge-type pressure sensors. The key difference between piezoresistive and strain gauge-type sensors is the magnitude of the effect. Piezoresistive sensors can have a 100 times greater change in resistance compared to strain gauge-type sensors, making piezoresistive sensors a better choice when sensing small changes in pressure.
Optical pressure sensors
Pressure sensors that use optics for their measurement are newer and tend to be more expensive but have key characteristics that set them apart. By using interferometry to measure changes due to pressure in an optical fiber, these types of pressure sensors are unaffected by electromagnetically noisy environments. Because of this unique characteristic, optical pressure sensors can often be found near radiography equipment and can also be medically safe for implantation. Another unique characteristic is that pressure can be measured at multiple points on a single fiber.
Are MEMS sensors even better?
MEMS stands for Micro Electro-MEchanical System. MEMS sensors use the same piezo or capacitive principles as other sensors, but in a much smaller form factor. MEMS sensors are fabricated on silicon at a micron-level resolution and packaged with amplifying and noise reduction circuitry to provide a conditioned signal. These sensors can be tiny, measuring just 2 mm or 3 mm on each side. These types of sensors find their way into many consumer goods including cell phones and vacuum cleaners.
Which sensor is the right one for my application?
Before selecting the type of sensor needed for an application, consider why the measurement is needed in the first place. If changes in atmospheric pressure should not affect pressure reading, consider using an absolute pressure sensor. Absolute sensors work great for many applications including reading barometric pressure for weather applications and for use in an altimeter for products like drones and phones.
In some applications, the absolute pressure is not as important as the gauge pressure. Consider applications like filling tires with air or many medical applications. For a tire to be inflated properly, the pressure has to be measured with respect to the atmospheric pressure. For medical applications, atmospheric pressure is pushing on the body. To pump fluids in and out of the body, the pressure needed must be higher or lower than atmospheric pressure. The absolute pressure just isn’t as important here.
Finally, many applications require differential pressure sensing with a reference other than atmospheric pressure. Some applications include HVAC systems and many production processes. In an HVAC system, differential pressure is often used to determine the flow rate of a fan using a piezo ring and also to measure the life left in a filter. By measuring the pressure differential from the inlet of a fan to the outlet of the fan, the volumetric flow rate of the fan can be calculated by using factors related to the design and size of the fan. In filtration applications, pressure drop across a filter is an easy indicator for when it is time to change the filter.
Before selecting a pressure sensor be sure to clearly define the requirements of the application. By knowing the range of pressures the sensor will be exposed to, the required resolution, the desired reference pressure and the space available for the sensor, a better selection can be made.