What is a pressure gauge, and how does it work?
Pressure gauges are mechanical or mechatronic instruments that measure and display the pressure of a liquid or gas using one of several measuring technologies: Bourdon tube, diaphragm element, capsule element, etc. The type of technology used depends on the medium being measured, the type of pressure to be measured, and the application. Pressure pushes against the elastic component, which in turn moves the pointer.
- In a Bourdon gauge, the medium enters a curved tube and causes it to stretch. This action is transmitted to the movement, and the corresponding deflection is displayed on the dial as a pressure reading. Bourdon gauges come in three tube types: C-shape (including coiled), spiral, and helical. The traditional C-type Bourdon gauge covers the majority of pressure applications from 10 psi up to 1,000 psi, while the coiled Bourdon tube covers ranges 1,500 psi and up.
- The spiral and helical types are found in direct drive gauges, with the tube being the only moving part; this makes direct drive gauges very shock resistant. Compare this to the movement of a traditional Bourdon gauge, which contains the pointer shaft gear, toothed segment, link, and other moving parts.
- In a diaphragm gauge, the measuring principle is the same except the medium pushes against a flexible corrugated metal disk (rather than entering a tube) to move the pointer. The benefits of diaphragm pressure gauges include high overload protection and the ability to measure low pressures and aggressive media. This video explains how pressure gauges work, comparing Bourdon tubes and diaphragm elements.
- In a low-pressure capsule gauge, the very sensitive mechanism consists of two corrugated discs fused together to create a capsule housed within the measuring chamber. The slightest pressure change causes the capsule to deflect; a pinion transfers this motion to the indicator. Note: capsule gauges are to be used only to measure dry gaseous media.
What pressures do pressure gauges measure?
Different gauges measure different pressures:
• Gauge pressure, or the pressure of a system above atmospheric pressure (positive gauge pressure) or below it (negative gauge pressure or vacuum pressure).
• Absolute pressure, or the sum of atmospheric pressure and applied pressure (gauge pressure).
• Differential pressure, or the difference between two applied pressures: Δp = p1 – p2.
What material should my pressure gauge be made of?
A pressure gauge has wetted parts and non-wetted parts. Wetted parts are components that come into direct contact with the media. In a Bourdon tube gauge, this includes the tube, movement, and pressure connection. The non-wetted parts are everything else: case, bezel (ring), window, pointer, etc.
• For standard applications with non-aggressive media such as air, water, or oil, we recommend gauges with a plastic or steel painted case and copper alloy wetted parts.
• For hydraulic applications, we recommend a stainless steel case for greater durability. A glycerin case fill dampens vibrations and eliminates pointer flutter, thus ensuring better readability of the dial.
• To measure aggressive/corrosive media and extreme temperatures, all-stainless steel gauges offer excellent performance.
What is meant by the accuracy class of a pressure gauge?
The accuracy class of a pressure gauge defines the permissible deviation of the display in percent of the full scale value. Example: For a gauge with a measuring range of 0 … 100 psi and an accuracy class of ±1.0%, if the pressure is 50 psi, the reading could range from 49 to 51 psi.
These are the accuracy classes per ASME B40.100:
• Grade B: ±3/2/3% (utility type gauges, models 11X.XX)
• Grade A: ±2/1/2% (2-1/2" size industrial grade gauges)
• Grade 1A: ±1.0% (4" size industrial grade gauges)
• Grade 2: ±0.5% of span (process type gauges, models 2XX.34)
• Grade 3A: ±0.25% of span (precision test gauge)
• Grade 4A: ±0.1% of span (precision test gauge)
For Grades B (±3/2/3%) and A (±2/1/2%), the accuracy varies depending on where the applied pressure falls in the measuring range. These gauges are more accurate in the middle 50% of the range and slightly less accurate in the first and last 25% of the range.
All pressure gauges, regardless of the accuracy class, require regular calibration to maintain their accuracy specification.
How do I prevent pressure gauge failure?
Even the most robust gauges aren‘t immune to failure. Here are the eight most common causes of gauge failure:
• Mechanical vibration
• Pulsation
• Extreme temperature
• Pressure spikes
• Overpressure
• Corrosion
• Clogging
• Mishandling and abuse
You can minimize or even prevent pressure gauge failure by using the right type of instrument for the application.
How do I choose a pressure gauge?
WIKA offers more than 120 models of pressure gauges, and that‘s not counting digital gauges or the variants of mechanical gauges, such as dry vs. liquid-filled, different materials and design features, etc. Some of the criteria to consider include:
• The gauge’s nominal size
• The media’s temperature
• The type of application the gauge will be used in
• The kind of media it will measure
• The type and range of pressure to be measured
• The process connections needed, such as NPT or G (metric)
• The connection location, such as lower (bottom) mount or back mount
• Any type of mounting options, such as panel mount or surface mount
• The required delivery time
• Cost
For more insights on how to select a pressure gauge, see this helpful article and contact our experienced product specialists.
