How Do Pressure Gauges Work? | Bourdon Tubes

How does a Bourdon tube pressure gauge measure pressure?

Fluid like air or water goes through the process connection and up into the Bourdon tube. As pressure inside increases, the tube straightens out. A gear transfers the motion to the dial pointer. In fancy engineering terms, the tube deflects, and the movement converts that deflection into a rotary movement of the pointer.

Why do pressure gauges come in standard ranges of 15, 30, 60, and 100 PSI?

A Frenchman named Eugene Bourdon invented the Bourdon tube in the mid-1800s. To this day, most Bourdon tubes are made in Europe where pressure is measured in bar or atmospheres. This is why the standard pressure gauge ranges in PSI tend to have equivalent bar denominations. For example, 15 PSI is equal to 1 bar, 30 PSI to 2 bar, and 60 PSI to 4 bar, and so on.

What about the Bourdon tube is different for different ranges of measurement?

A Bourdon tube’s size and wall thickness determine the range of a gauge. Larger, thin-wall tubes are used to measure lower pressures. Smaller, thicker-wall tubes are used to measure higher pressures. For very high pressures, thick-wall tubing is used to create a helical Bourdon tube within a gauge series. The mechanical movement is the same from as low as 10 PSI to as high as 10,000 PSI.

How long do Bourdon tube pressure gauges last?

The Bourdon tube is incredibly robust and can last millions of cycles and decades of operation under normal conditions. Conditions that lead to shorter lifespan include over-pressuring, extreme temperatures, shock, pulsation, vibration, corrosion, and clogging.

Can you recalibrate or replace the Bourdon tube?

Most pressure gauges can be calibrated by adjusting the pointer or gear inside. This can correct small changes in the movement or tube over time. However, if the Bourdon tube stretches, cracks, or breaks, the entire pressure gauge is not repairable.

Pressure Gauge Accuracy and Recalibration | FAQ

How do I read pressure gauge accuracy?

Gauge accuracies are often stated in one of two ways – full scale or low/middle/high. A full scale designation, such as 1% FS, means that the gauge maintains that accuracy throughout the entire dial range. When the accuracy is stated with three figures, such as 2-1-2%, then the gauge is +/- 2% in the first third of the gauge, 1% in the middle third (where it is most likely being operated) and 2% in the last third.

What determines a gauge’s accuracy?

It’s a combination of the bourdon tube, the movement, the time spent calibrating, and ultimately, the user’s ability to see the gauge. As gauges get larger they tend to get more precise because you can fit a larger bourdon tube and a larger movement into the case (both of which are more accurate than smaller tubes and movements). But most importantly the larger dial face allows for more precision in reading. A gauge is only as accurate as the human eye can interpret where the pointer is over the dial.   

How do you recalibrate a gauge?

First you need to determine whether the gauge reading error is an adjustment of your zero or your span. 

Zero Error / Pointer Adjustment:

When comparing to a reference, if the gauge is reading roughly the same amount high or low from the reference, then all you need to do is adjust the pointer. 

Span Adjustment / Gear Recalibration:

If the linearity of the reading is not consistent – such as the gauge is reading below the reference at the low end, then reading above the reference at the high end. Then you will have to adjust the gears on the movement to either “speed up” or “slow down” the movement of the pointer as it relates to the deflection of the tube.

Pressure Gauge Components & Functions | FAQ

What is a restrictor screw or throttle plug?

A restrictor screw or throttle plug serves to control pressure fluctuations within the gauge. By installing the screw or plug, users can regulate the flow of fluid into the gauge, stabilizing readings and mitigating the effects of rapid pressure changes.

What is a direct drive gauge?

 A direct drive gauge eliminates the need for gears or linkages by directly transmitting pressure to the sensing element. This design ensures immediate response to pressure changes and minimizes potential mechanical failures associated with complex transmission systems.

Why do pressure gauges need to be liquid filled?

Liquid-filled pressure gauges are essential for dampening mechanical vibrations and oscillations, particularly in environments prone to high levels of vibration or pulsation. Additionally, the liquid lubricates internal components, enhancing longevity and accuracy.

What is the difference between case fill fluids?

Case fill liquids, such as glycerin, silicone oil, or halocarbon oil, are selected based on their viscosity, temperature stability, and compatibility with the process medium. For instance, silicone oil is commonly used for its wide temperature range and resistance to chemical degradation.

What is the difference between lens materials?

Pressure gauge lenses are available in various materials, including acrylic, polycarbonate, and glass. Acrylic offers excellent clarity and impact resistance but may degrade when exposed to certain chemicals. Polycarbonate provides superior durability and resistance to impact and temperature fluctuations. Glass lenses offer high clarity and chemical resistance but may be more prone to breakage.

Are pressure gauge internals oil-free?

Pressure gauge internals can be oil-free, particularly in applications where oil contamination is undesirable. Dry gauges utilize mechanisms that do not require internal lubrication with oil, ensuring compatibility with sensitive processes or environments where oil presence is prohibited.

Pressure Gauge Maintenance | FAQ

How often should a gauge be recalibrated?

 Pressure gauges typically require recalibration at regular intervals, often annually, to ensure accuracy and reliability. However, the frequency of recalibration may vary depending on whether the gauge is experiencing:

• Vibration
• Pulsation
• Temperature Cycling
• Environmental Corrosion
• Other stress-inducing situations

There is black powder inside the case of my pressure gauge. What happened?

The presence of black powder inside the pressure gauge case is typically sign a vibration and pulsation stress on the gauge. The black powder is actually fine metallic shavings from the internal components of the pressure gauge – such as the sector gear, pinion, and pointer shaft –  grinding against each other at high frequency. Vibration and pulsation mitigation techniques should be used, such as liquid filling, remote mounting, and/or adding PulsePlus to dampen pulsation.

I need to recalibrate my liquid-filled gauge. What do I do?

A liquid-filled pressure gauge can be tested and recertified in the same way as a dry pressure gauge. However, if it requires a pointer adjustment or recalibration, then it will require the fluid to be drained from the gauge case prior to opening up the gauge for repairs.  Following repairs, the gauge can be closed up and refilled with the same fluid-type (typically glycerin or silicone) as was previously installed.  This can be a cumbersome task and occasionally results in the newly repaired gauge experiencing a case leak. Many pressure gauge users opt to replace a failed liquid filled gauge with a new one, rather than undergo the hassle of repairing.

I installed the gauge and it isn’t reading pressure. What’s going on?

For newly installed gauges, the most common failure is a clogging of the pressure orifice. This is especially common on gauges that have a restrictor screw installed.  If you suspect this may be the case, remove the gauge from the pipe fitting and inspect the inlet orifice for debris. If the gauge has a restrictor screw installed, remove the restrictor screw and then clean out the orifice.