All about Pressure Gauges
A pressure gauge is a common component in industrial and commercial operations across the world. But not every gauge is created equally or made for every situation.
After temperature, pressure is the second most common process measurement in industrial and commercial applications – and pressure gauges are the simplest way of measuring pressure.
While such other pressure measurement devices as sensors, transducers, and transmitters convert pressure into an electrical signal that is then sent to a data-acquisition device, pressure gauges are simple and local – displaying the pressure inside a bike tire or home fire extinguisher just as they serve an important role in industry.
Pressure, simply defined, is force applied over a unit area. Most commonly associated with gases and liquids, pressure is a critical component of a diverse array of applications, from those relying on accurate pressure control to those deriving other values based upon pressure (depth, level, or flow, for example).
The most common pressure measurement is PSI (Pounds per Square Inch). Other measuring units Pascal, Torr, Standard/Technical atmosphere, and many others.
Many other different types of pressure exist, all referring to the zero reference point of a measurement. For example:
- Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. Sealed gauge sensors may use a fixed pressure different than ambient atmospheric temperature.
- Absolute pressure is zero-referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure.
- Differential pressure is the difference in pressure between two points.
Pressure Gauge Technology
While pressure gauges are fairly simple devices, many considerations go into choosing the best instrument for your application. For starters, some pressure gauges are digital, while others are analog. While they carry out the same function, digital and analog gauges use different technology and are optimal under different conditions.
These days more common than analog devices, digital pressure gauges use microprocessors and advanced sensors to display highly accurate pressure data on a digital screen. Though generally more expensive than their analog counterparts, digital gauges offer features that make them more attractive for many applications.
First, digital gauges are easy to read. Rather than having to count hashes in order to determine the pressure, digital gauges provide resolutions of up to 0.001, making them ideal for low pressures or small incremental pressure changes, such as those found with leak testing.
Digital pressure gauges also have fewer moving parts than analog gauges, making them more reliable and simple to operate. Nonetheless, they can be programmed for multiple pressure units, and include outputs for transmitting results to a computer or data logger for analysis and storage.
Most digital pressure gauges rely on one of two measurement technologies:
Strain gauge: Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure and changes in the electrical resistivity of a semiconductor or metal—silicon, polysilicon thin film, bonded metal foil, thick film, or sputtered thin film—when mechanical strain (pressure) is applied. Most commonly this device comprises a diaphragm with patterned metallic strain gauge embedded into it. Increasing pressure causes the diaphragm (and subsequently, the gauge) to deform, which affects its resistivity. The change is measured and converted into an electrical signal proportional to the pressure.
Piezoelectric: Uses the piezoelectric effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure.
Analog pressure gauges, sometimes referred to as mechanical gauges, use a needle that points to a number on a scale corresponding to the pressure sensed by the measuring element. From bike pumps to pressure cookers, analog pressure gauges are basic and common: they are accurate, inexpensive, require no power source and little maintenance.
Analog gauges are accurate enough to use as test gauges, reliable enough to use in complex process environments, inexpensive enough for commercial use, and rugged enough for industrial use.
Most analog gauges rely on one of two measurement principles:
Bourdon Tube: The Bourdon pressure gauge uses the principle that a flattened tube tends to straighten or regain its circular form in cross-section when pressurized. Although this change in cross-section may be hardly noticeable, and thus involving moderate stresses within the elastic range of easily workable materials, the strain of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is pressurized.
Bourdon tube pressure gauges work very well for most applications, particularly those involving medium to very high pressures; however, they can respond slowly, and lack the sensitivity for highly accurate readings at low pressures and can be also be sensitive to shocks and vibration as well as subject to hysteresis.
Bellows: Bellows gauges are a great solution when measuring pressure ranges below what is ideal for Bourdon pressure gauges, because they contain an elastic element that radially expands and contracts to respond to pressure changes. The internal bellows is connected to a pointing device so that subtle pressure fluctuations are indicated on a pressure calibrated scale on the dial.
Bellows gauges excel in low pressure applications, have the accuracy and sensitivity for precise measurement, and are rugged and reliable with low hysteresis and creep. And like Bourdon tubes, bellows gauges are sensitive to vibration and shock.
Though they cannot match the features of digital gauges, analog gauges are also often available with temperature compensation for greater accuracy, liquid fill to dampen pointer movement, and multiple dial sizes.
Considerations for Selecting a Pressure Gauge:
- What is the type and range of the pressure?
- What accuracy is required?
- Is an output required? If so, which type?
- What units of measurement are preferred?
- Which process connection is required?
- What is the temperature range? Is compensation needed?
- What burst pressure is required?
- What is the preferred dial size?