Gas pressure is measured using gauges that detect the force a gas exerts on a surface, then translate that force into a readable number. The most common tools are mechanical gauges (like the Bourdon tube), manometers, and digital pressure sensors. Which one you need depends on how precise your reading must be and what you’re measuring.
Before picking a tool, though, it helps to understand what “pressure” actually means in this context and which type of pressure reading you’re after, because there are several.
Types of Pressure Readings
Pressure is defined as force acting uniformly over a defined area. That sounds simple enough, but there are three distinct ways to express a pressure measurement, and they differ based on what you’re comparing against.
Gauge pressure is the most common type in everyday and industrial use. It measures the difference between the gas you’re testing and the surrounding atmospheric pressure. When you check the pressure in a car tire or a propane tank, you’re reading gauge pressure. Because it’s measured relative to the atmosphere around you, the reading can shift slightly with weather changes or altitude.
Absolute pressure measures against a perfect vacuum, meaning zero pressure. This eliminates the influence of weather and elevation entirely. The formula is straightforward: absolute pressure equals gauge pressure plus atmospheric pressure. At sea level, atmospheric pressure averages 1013.25 millibars (about 14.7 psi), so a tire reading 32 psi on a gauge actually has an absolute pressure of roughly 46.7 psi.
Differential pressure compares two pressure sources against each other rather than against the atmosphere or a vacuum. This is useful in systems like air filters or flow meters, where you need to know how much pressure drops between two points.
Common Pressure Units
Gas pressure is expressed in several units depending on your country and industry. Here are the most common ones and how they relate to each other:
- PSI (pounds per square inch): Standard in the United States for tire pressure, gas cylinders, and plumbing. 1 psi equals about 6,895 Pascals.
- Bar and millibar: Widely used in Europe and in meteorology. 1 bar equals 100,000 Pascals, or roughly 14.5 psi.
- Pascals (Pa): The SI unit of pressure. 1 atmosphere equals 101,325 Pa. Because Pascals are small, you’ll often see kilopascals (kPa) instead.
- Atmospheres (atm): 1 atm is the average air pressure at sea level, equal to 14.7 psi or 760 mmHg.
- Torr or mmHg (millimeters of mercury): Common in medical and laboratory settings. 1 psi equals about 51.7 Torr.
- Inches of water column (in H₂O): Used for very low pressures, like in HVAC ductwork. 1 psi equals roughly 27.7 inches of water.
Most digital gauges let you toggle between units. For manual conversion, the NIST (National Institute of Standards and Technology) publishes reference tables covering all of these.
Bourdon Tube Gauges
The Bourdon tube gauge is one of the most widely used pressure instruments in the world, and it’s been around since the 1800s. It works on a purely mechanical principle with no electricity required.
Inside the gauge is a curved, flattened metal tube sealed at one end. When pressurized gas enters the open end, the tube tries to straighten out, much like a party horn uncoils when you blow into it. That tiny movement gets picked up by a link bar connected to a pivoting arm, which runs through a rack-and-pinion gear system. The gears magnify the tube’s small motion into a larger sweep of the needle across the dial face.
If the pressure inside the tube is higher than atmospheric pressure outside, the coil opens up. If it’s lower, the coil tightens. This makes Bourdon gauges capable of reading both positive pressure and vacuum. They’re reliable, affordable, and don’t need a power source, which is why you’ll find them on everything from welding regulators to industrial boilers. Their main limitation is that they contain moving parts, so they wear over time and need periodic calibration.
Digital Pressure Gauges
Digital gauges use electronic sensors instead of mechanical tubes. A pressure-sensitive element (typically a thin membrane) flexes when gas pushes against it, and that flex is converted into an electrical signal processed by a microchip. The result appears on a digital display.
Because there are no moving parts, digital gauges resist wear and vibration better than mechanical ones. They also offer higher precision for most applications and can store readings over time. Many models connect to data logging systems, making them valuable when you need to track pressure trends or document measurements for compliance. The tradeoff is cost (they’re generally more expensive) and the need for batteries or a power supply.
Manometers
A manometer is the simplest pressure measurement device conceptually. In its most basic form, it’s a U-shaped tube partially filled with liquid, usually water or mercury. You connect one end to the gas source, and the pressure pushes the liquid down on that side while raising it on the other. The height difference between the two columns tells you the pressure.
Manometers are excellent for measuring low pressures with high accuracy, which is why they’re still used in laboratory settings and HVAC work. The unit “millimeters of mercury” (mmHg) comes directly from mercury manometers. Their downside is that they’re bulky, orientation-sensitive, and impractical for high-pressure applications.
How to Take an Accurate Reading
The tool matters, but so does your technique. A few practical steps will keep your measurements reliable regardless of which gauge you’re using.
First, let the system stabilize. If you’ve just opened a valve or connected a line, give the pressure a moment to equalize before reading. Rushing the measurement while gas is still flowing can give you a transient spike rather than the true static pressure.
Second, read mechanical gauges straight on. Looking at the dial from an angle introduces parallax error, where the needle appears to point at a different number depending on your line of sight. Quality gauges include a mirrored band behind the needle to help you line up your view correctly: when the needle covers its own reflection, you’re looking at it head-on.
Third, check your gauge’s range. A gauge rated for 0 to 100 psi will give its most accurate readings in the middle third of that range (roughly 30 to 70 psi). If your expected pressure sits near the very bottom or top of the scale, you’ll get better accuracy from a gauge with a more appropriate range.
Fourth, account for temperature. Gas pressure changes with temperature. If you’re measuring a closed system that was recently heated or cooled, the reading reflects the current thermal state, not necessarily the “normal” operating pressure. For precise work, note the temperature alongside the pressure.
Calibration and Maintenance
Every pressure gauge drifts over time. Mechanical gauges are especially prone to this because their internal parts fatigue with repeated use. Industry recommendations for calibration frequency vary by application:
- Oil, gas, and chemical plants: every 3 to 6 months
- Pharmaceutical and medical settings: every 6 months
- Manufacturing, HVAC, and food processing: every 6 to 12 months
- General commercial use: every 12 months
For home use, formal calibration isn’t usually necessary, but it’s worth comparing your gauge against a known-good reference occasionally. If you have a tire pressure gauge that reads 5 psi differently from the one at a gas station, one of them is off. Replacing inexpensive gauges is often more practical than calibrating them.
Safety With Compressed Gas
Measuring pressure on compressed gas cylinders or high-pressure lines requires caution. OSHA regulations require that compressed gas cylinders have pressure relief devices installed and maintained, and that employers visually inspect cylinders to confirm they’re in safe condition before use.
Before connecting a gauge to any high-pressure source, verify the gauge’s maximum rated pressure exceeds the system’s possible peak pressure. Never use a gauge rated for 200 psi on a system that could spike to 300. Make sure connections are tight and compatible with the gas type, since some gases (like oxygen) require specific fittings to prevent contamination or reactions. When disconnecting from a pressurized line, bleed the pressure down first rather than simply pulling the fitting loose.