Objectives

• Define "pressure sensor"
• Summarize common pressure gauge technologies
• Differentiate between 3 common pressure sensing technologies
  • Variable Capacitance Sensors
  • Strain Gauges
  • Piezoelectric Sensors
• Recall common pressure units and vocabulary

What Is a Pressure Gauge?

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.

Manometer

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.

Try It!

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.

Low Density Indicator Fluids

Pro
• Low density indicator fluids, like water, can provide very accurate meter resolution.

Con
• Low-density indicator fluids cannot measure a high pressure differential.

High Density Indicator Fluids

Pro
• When the indicator fluid has a higher density, like mercury, the manometer is capable of measuring greater pressure differentials.

Con
• Indicator fluid with high density provides a lower meter resolution.

Pick me!

Manometers can be great for simple applications. As mentioned, basic manometers

• do not require calibration.
• have no mechanical moving parts.
• do not require power.

• Manometers must be level (relative to their initial calibration).
• The glass tube is fragile.
• Indicating fluids do not function well outside of ideal temperature ranges.

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!

Pick Me!

Diaphragm pressure gauges are common, for good reason:

• They are easy to read and do not require power.
• They can read low pressures accurately, even pressures as low as 0.2 psi/1.6 kPa

Of course, these gauges have limitations:

• Excessive motion or vibration will make the needle jump.
• If the gauge is overpressured, the diaphragm will rupture.
• Corrosive fluids can destroy the diaphragm.

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!

Pick Me!

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.

• large industrial vessels, measuring the pressure of trapped gases for process control applications,
• mobile equipment such as loaders, reporting hydraulic system pressure.
• consumer vehicles, tracking the tire pressure
• inside buildings and bridges in seismically active areas, tracking foundation strain.
• at the base of liquid filled tanks, where fluid level is inferred from the pressure measurement.
• combustion engines, reporting the oil pressure.