Pressure is the second most common process measurement, after temperature, in industrial and commercial applications. Even around the house, when inflating a tire or checking the status of a boiler system, accurately and immediately knowing the pressure is of importance. Pressure gauges are the simplest, most direct way of measuring and displaying pressure. While other pressure measurement devices such as sensors, transmitters and transducers convert pressure into an electrical signal to be sent to a controller, recorder, or another type of data-acquisition device, pressure gauges are meant to be a local display showing, at a glance, the pressure inside your fire extinguisher, tires, boiler, pressure cooker, or an important process in an industrial setting.
Understanding Pressure
Pressure is defined as the amount of force applied over a unit area. Usually involving liquids and gases, pressure is a critical component of a diverse array of applications, both those that rely on accurate pressure control as well as those that derive other values (such as depth/level or flow) based upon pressure.
Pressure measurements can be made in a number of units. Most commonly, we see PSI ( pounds per square inch) or bar. Other units of measure include kg/cm2, inH2O, mmHg, Pa, and many others.
There are also different types of pressure to consider. The type of pressure refers to the zero reference point of a measurement. For example:
Gauge pressure: The pressure gauge is referenced against atmospheric pressure so it does not include the effects of that pressure making it equal to absolute pressure minus ambient air pressure. Sealed gauge sensors may use a fixed pressure different than ambient atmospheric temperature.
Absolute pressure: The pressure gauge is referenced against a perfect vacuum so it, therefore, includes the effects of atmospheric pressure. It is equal to gauge pressure plus atmospheric pressure.
Differential pressure: The pressure gauge contains two process connections to measure the difference between two pressures, such as each side of a filter to measure pressure drop.
Pressure Gauge Technology
Pressure gauges are fairly simple devices though there are many considerations that go into selecting the best instrument for your particular application. The most obvious difference, when looking at pressure gauges, is that some are digital while others are analog. While they fulfil the same basic role, analog and digital gauges use different technology and excel under different conditions.
Analog Gauges
Analog pressure gauges, often 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. Analog pressure gauges are found everywhere as they provide an accurate, inexpensive option that requires no power and little, if any, maintenance.
Analog gauges can be tailored to fit nearly any application. They can be accurate enough to use as test gauges, reliable enough for use in complex process environments, rugged enough for industrial use, and inexpensive enough for commercial use.
Most analog gauges rely on either of two measurement principles:
Bourdon Tube: Gauges with bourdon tubes are the most common type of analog gauges in use. Bourdon tubes rely on the principle that a curved tube tends to straighten out when subjected to pressure. The tube is connected to a pointing device so that subtle movements due to pressure fluctuations are indicated on pressure calibrated scale on the dial.
Bourdon tube pressure gauges work very well for most applications, particularly those involving medium to very high pressures. They are simple in construction, which keeps them inexpensive and easy to use. Bourdon tubes also offer superior linearity and can be accurate up to ±0.1% making them suitable for precision measurements.
Bourdon tube pressure gauges also have limitations, though. They 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. Bourdon tubes can also respond slowly, so applications involving rapid pressure fluctuations are not ideal. Also, like all analog gauges, Bourdon tubes cannot make absolute pressure measurements nor are they particularly adept at precision measurements.
Bellows: Bellows gauges are a great solution when measuring pressure ranges below what is ideal for Bourdon tube gauges. Bellows gauges 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 movements due to pressure fluctuations are indicated on pressure calibrated scale on the dial.
Bellows gauges excel in low pressure applications and have the accuracy and sensitivity for precise measurement. Additionally, bellows gauges are rugged and reliable with low hysteresis and creep. Like Bourdon tubes, bellows gauges are sensitive to vibration and shock.
Analog gauges are widespread for a reason, they offer accuracy across wide range at a good price. Though they cannot match the features of digital gauges, analog gauges are often available with temperature compensation for greater accuracy, liquid fill to dampen movement of the pointer, multiple dial sizes to improve visibility and space requirements.
Digital Gauges
Digital pressure gauges use advanced sensors and microprocessors to display highly accurate pressure readings on a digital indicator. Though generally more expensive than analog gauges, digital gauges offer a number of features that make them attractive alternatives for a number of applications.
Digital gauges provide quick and easy to read results. Rather than having to count hashes to read the pressure, digital gauges provide resolutions of up to 0.01 or 0.001 making them ideal for very low pressures or small incremental pressure changes, such as those found when leak testing, that would be impossible to identify with an analog gauge.
Digital pressure gauges have fewer moving parts than analog gauges making them more reliable. Simple to operate, they nonetheless can be programmed for multiple pressure units and include outputs for sending results to a computer, data logger, or other instrument for storage or analysis.
Most digital pressure gauges rely on one of two measurement technologies:
Strain gauge: Strain gauge sensors rely on the piezoresistive effect which describes changes in the electrical resistivity of a semiconductor or metal—commonly silicon, polysilicon thin film, bonded metal foil, thick film, or sputtered thin film—when mechanical strain (pressure) is applied. Most commonly this technology consist of a diaphragm with patterned metallic strain gauge embedded into it. Increasing pressure causes the diaphragm, and subsequently, the gauge to deform which effects it’s resistivity. That change is measured and converted into an electrical signal proportional to the pressure. Generally, strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and to reduce sensitivity to errors.
Piezoelectric: Piezoelectric sensors rely on the piezoelectric effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure. As pressure is applied a charge develops across the sensor in proportion to the force.
Things to Consider When Selecting a Pressure Gauge:
What is the type and range of the pressure?
Is an output required? If so, which type?
What accuracy is required?
What units of measurement are preferred?
Which process connection is required?
Are there any issues with material compatibility or chemical resistance?
What is the temperature range? Is compensation needed?
What burst pressure is required?
Are any agency approvals needed?
What is the preferred dial size?
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