Fluid pressure dynamics are part and parcel of modern engineered systems, which in turn calls for advanced instrumentation to monitor, control, and manage these systems against very precise pressure requirements. Pressure gauges – both analog and digital – are particularly important examples of such instrumentation, which we’ll be taking a close look at in this article.
Let’s begin with a few quick definitions to outline what pressure gauges are and what they measure:
- Pressure Gauge – a mechanical instrument that attaches to piping, tanks, channels, vessels, or any other type of fixture that holds a fluid needing its pressure measured. Fluid pressure is an engineering variable that communicates the force or potential energy exerted by the fluid onto a surface contacting that fluid. Higher pressure means that the fluid has more potential energy stored up which can be used to perform mechanical work (or present hazards if unexpectedly released). Pressure gauges have a visible display on them from which operators can read a legible pressure reading.
- Pressure Units – pressure can be described in multiple units depending on their accuracy, location, and function. Imperial pressure units are read in PSI (pounds per square inch), whereas metric units are read in BAR. Other variations include mmHg (millimeters of mercury), Pa (pascals), ATM (atmospheres), and WC (inches of water column).
- Pressure References – pressure gauges are available in different reference ranges, known as absolute, gage, and differential pressure. Absolute pressure starts measuring from complete vacuum and includes atmospheric pressure in its reading (where measuring the air outside at sea level would read 14.7 PSI). Gage pressure starts measuring from atmospheric pressure (where measuring the air outside at sea level would read 0 PSI). Differential pressure measures the value between two points.
Selection Considerations for Pressure Gauges Common applications for pressure gauges include:
- Municipal Utilities – such as water and natural gas distribution to residential and commercial buildings.
- Industrial Utilities – plant water, steam, wastewater, compressed air, and fuel piping systems.
- General Manufacturing – compressed air, nitrogen, cleaning water, and hydraulic systems.
- Heavy Industrial – ore and mineral mining, construction material preparation, and building materials production.
- Food & Beverage – dairy, snack, alcohol, confectionary, and ingredient process systems.
- Petroleum – crude oil, natural gas, and heavy hydraulic systems.
- Civil Infrastructure – major waterways, reservoirs, river and basin management.
- Semiconductor – sterile water, cooling water, coating deposition, chemical treatment, and inert gas flushing systems.
- And many more!
Comparing Analog and Digital Pressure Gauges
Now that we understand what purposes pressure gauges serve, let’s have a closer look at the differences between these two designs.
Sensing Technology:
- Analog Gauge – an analog pressure gauge uses a mechanical mechanism to translate fluid pressure into a visual readout. Two mechanisms are prevalent (a mechanical diaphragm or a Bourdon tube), both of which directly transmit fluid pressure exerted on their sensing element to an accurately calibrated indicating needle, moving the needle to align with a written pressure figure around the gauge’s dial.
- Digital Gauge – a digital pressure gauge uses an electrical transducer to convert physical fluid pressure into an electrical signal, and this electrical signal is then transmitted to an electronic display where a pressure value is then displayed. No mechanical translation of pressure is involved as with an analog gauge. Many electronic variations exist for digital gauges including piezoelectric, inductive, capacitive, and resistive transducers.
- Readability – analog gauges are often smaller and more difficult to read, which make them great for incidental, general use. Digital gauges often have larger, clear LCD screens that can also be backlit, making them easier to read for critical, frequent uses, though at a higher cost.
- Accuracy – any gauge is subject to calibration drift over time, where the displayed value is not exactly the real variable being measured. Analog gauges tend to be more susceptible to drift stemming from process upsets, temperature and pressure extremes, mechanical wear, and general aging. Digital gauges tend to be less vulnerable to such drift as there are fewer (or no) mechanical moving parts involved, and some models can even alert operators to self-diagnosed calibration drift on their displays.
- Precision – in general, digital gauges can be more precise than mechanical gauges, but with a caveat. Some digital gauges are misleading in their precision, as their readout may display a value that their transducer cannot actually measure down to. Similarly, mechanical gauges need to be read looking square at their dial, otherwise the viewing angle may lead to an incorrect read.
- Calibration – readers can infer from the above points that calibration is critical no matter what style gauge is utilized. Generally, mechanical gauges must be calibrated more often than digital gauges.
- Compound Gauges – for decades, analog gauges have been offered with compound readouts, combining multiple variables into one gauge. Combination vacuum and pressure gauges are very common, as are combo temperature-pressure, multi-scale pressure, and absolute-gage pressure gauges. Digital gauges can also offer these features, though at increasingly high costs and complexity (given all the additional electronics required).
- Application Nuances – some applications require specific gauge features as a function of their operational nuances. For example, some regulated processes such as dairy pasteurization require gauges to be certified against particular standards for accuracy and reliability, making analog gauges the de facto standard. Another example would be hazardous environments where electrical power is prohibited since it can serve as a source of ignition, virtually ruling out digital gauges.
- Application Environments – further to the above point, the environments in which pressure gauges will be installed may also influence gauge selection. For example in extremely hot applications, analog gauges may immediately drift due to thermal expansion of their metallic parts. Conversely, extremely humid environments may prefer analog gauges, as condensation can damage the electronics within digital gauges.
- Power Source – digital gauges need power, either from internal batteries or external electrical sources. Often this makes analog gauges preferred so as to avoid the additional cost, complexity, and risk potential of forgetting to replace batteries or relying on external power.
- Cost – digital gauges come at a price premium over analog gauges, though often with many more features and options as well. Analog gauges are still the primary choice for routine, incidental pressure monitoring applications if for no other reason than their great dependability at affordable costs.
Conclusion
Modern industrial and commercial fluid process systems are designed to operate within specific pressure ranges, and any deviations above or below these ranges must be immediately detected and addressed. Pressure gauges give operators, engineers, and maintenance personnel direct ways to monitor and control these systems, helping to keep systems operating smoothly while avoiding potentially catastrophic issues. Across all of the many nuances of these technical systems, there are places for both analog and digital pressure gauges, each serving a specific balance of cost, features, maintainability, and reliability. While analog gauges are a mature technology with an affordable price, digital gauges are a newer contender rich with customizable options and advanced features. Selecting between gauge designs comes down to each individual application’s requirements, and above all else, the safety and protection of personnel working around these applications.