Even though this is a lesson about pressure sensors, let's talk about pressure gauges for just a moment. Pressure gauges are devices that react to pressure in a measurable way. Unlike sensors, which generally convert their results to a signal, gauges are usually read manually. Don't expect a gauge to interface with your PLC or ECM, but they can still be reliable, extremely useful, and cost-effective.
The manometer is a gauge that uses a liquid column to measure pressure. The first recorded manometer was developed in 1643.
Don't let those dates fool you; manometers have not been banished to museums. These simple pressure sensors are still in use today in classrooms, laboratories, and even on high efficiency furnaces. Their appeal is in their simplicity. Basic manometers have no moving mechanical parts, never need calibration, and don't require power to provide a reading. And because manometers rely on physical properties (density of the measurement liquid, and Earth's gravity), U-tube manometers are considered to be a NIST standard of accuracy.
NIST stands for National Institute of Standards and Technology. It has the distinction of being one of the oldest physical science laboratories in the United States, and is an authority on measurement accuracy.
The U-tube style manometer uses the displacement of a liquid to measure the pressure of a trapped fluid relative to the atmosphere, or another reference pressure. Try out this virtual manometer to see how it operates.
Manometers can be great for simple applications. As mentioned, basic manometers
Sorry, mobile machinery. This one's not for you.
The diaphragm in this diaphragm pressure gauge pushes against a ball joint. As the ball joint moves, its motion is translated into the gauge needle rotation. Try it out!
Diaphragm pressure gauges are common, for good reason:
Of course, these gauges have limitations:
A Bourdon-tube gauge is similar to a diaphragm gauge; both use a mechanical linkage and gears to turn the gauge needle. But the heart of the Bourdon-tube gauge is an oval-shaped tube instead of a thin membrane. When the pressure inside of the tube (the measured pressure) is greater than the pressure outside (the reference pressure), the tube deforms slightly. Try it out!
Even though they were invented in the 1850s, Bourdon-tube pressure gauges are still one of the most commonly used pressure gauges around. Unlike a diaphragm pressure gauge, Bourdon-tube gauges are able to read very high pressures. The tube can read liquids and gasses up to 100 000 psi/689 500kPa!
But don't throw out your diaphragm pressure gauge just yet; Bourdon-tube gauges are not a good choice when you need to measure very low pressures.. They are unable to read pressures below 8.7 psi/60 kPa.
No surprises here — pressure sensors are a family of sensors that are used to measure pressure in the physical world, and send out a corresponding electrical signal. You'll find them everywhere, including
There are endless uses for pressure sensors, so it's not surprising that there are multiple different technologies used to measure pressure. Let's look at a few of the most common sensor technologies.
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