Found in a range of different machines—from fire extinguishers to engine oil—pressure gauges are simple yet valuable contraptions. They measure the pressure of a liquid or gas within pipelines and containers, ensuring systems around the world function safely. But how do they work? We look inside the mechanism of a pressure gauge.
Pressure gauges work through a Bourdon tube, a hollow piece of metal. When a gas or liquid enters the tube, it expands and pushes a lever. The distance the lever moves is proportional to the gas or liquid pressure inside the hollow tube. Modern pressure gauges replace the Bourdon tube with electric sensors to take pressure readings more quickly and accurately.
Why Are Pressure Gauges Important?
Dangers of Compressed Liquids and Gases
Because pressure is related to force, it can be thought of as stored energy. If there is a buildup of pressure inside a container, the force can grow and cause a leak, or even an explosion. Even small canisters of compressed gases can present a hazard.
In manufacturing facilities, there can be hundreds of pressurized containers and pipes, transporting different chemicals and gases to sustain reactions. It’s necessary to constantly monitor the pressure at every point, ensuring the correct flow rates so that chemical reactions do not go out of control.
Pressure in Many Processes
Some processes, like gas chromatography, require a consistent supply of gas. Pressure gauges are essential for maintaining and monitoring the gas entering the machine so proper separation of compounds can happen.
In espresso machines, water is forced through coffee grounds under high pressure. A pressure gauge allows baristas to ensure the water pressure is sufficient and consistent between pours.
Another example is the oil pressure system in car engines. Oil is vital to lubricate all the moving parts of internal combustion (gasoline) engines, reducing wear and tear. If the oil pressure is too high or too low, pressure gauges alert the drivers through the car dashboard.
Looking Inside a Pressure Gauge
The Bourdon Tube Mechanism
A pressure gauge measures the pressure of a liquid or gas that passes through it. The simplest and most common type of gauge is the Bourdon tube pressure gauge.
Invented in the 1850s, the Bourdon tube is a circular, flat copper tube attached to the gauge. It connects on one end to the gas or liquid input, with the other end sealed.
When the chemical enters the Bourdon tube, it fills the hollow space within it and causes the structure to expand. Because it is wound up in a circular manner around the mechanism, introducing a gas or liquid at high pressures causes it to expand.
The Bourdon tube’s sealed end is attached to a level. When expansion happens, the lever is pulled up. The distance it moves is proportional to the pressure inside the Bourdon tube.
A series of gears and springs connect the lever to a needle on the face of the pressure gauge. By calibrating this mechanism, we can convert the lever’s movement to a known gas pressure.
The face of a gauge has a graduated scale behind a needle that indicates the pressure. You’ll often see different units of pressure (psi, bar, etc.), but we’re slowly replacing them with the SI unit—the Pascal.
Modifications to Improve Gauges
The entire gauge can also be sealed and filled with a lubricating liquid to reduce wear and tear in the moving parts. Just like in car engines, this helps to increase the longevity of the inner mechanism.
Brass and copper (the most common metals used to construct pressure gauges) are reactive to certain chemicals. Depending on the reactivity of the contents, it can be preferable to use stainless steel or a coated alloy instead.
Today, digital displays in gauges are common, although the operating mechanism remains the same. A digital display helps the operator read the numbers quickly; some displays can convert values from one unit of pressure to another (bar to psi, atm to kPa, etc.).
Diaphragm Pressure Gauges
Improves Sensitivity
Direct measurement of pressure is simple and effective, but sometimes we need to increase the sensitivity of our gauges to pick up small fluctuations in pressure.
Traditional Bourdon tube gauges struggle with precision measurements since the mechanism involves a direct expansion of the copper tube (which isn’t the most elastic).
To improve this, we can insert an elastic barrier (diaphragm) between the gas/liquid and the Bourdon tube mechanism.
The diaphragm increases the surface area in contact with the chemical, moving upward when pressure rises. This diaphragm is connected to the rest of the mechanism, allowing the gauge to capture minor adjustments to the pressure.
Reduces Corrosion and Contamination
Having chemicals flowing in and out of the Bourdon tube can damage it over time. Using a diaphragm prevents this direct contact.
Viscous liquids or those containing solid precipitates also benefit from this, as they are prone to collecting inside the hollow Bourdon tube with repeated measurements, affecting the instrument’s accuracy.
Furthermore, we can coat the diaphragm layer with an inert material, like PTFE (Teflon) or gold, to reduce its reactivity.
Electrical Pressure Sensors
From Bourdon Tube to Strain Gauge
Although the Bourdon tube pressure gauge is still widely used, electric sensors are available today. They work by a similar mechanism to the diaphragm gauge, where the gas/liquid pressure pushes against a mechanical diaphragm linked to a strain gauge.
Below are some strain gauges, also called force-sensing resistors.
The strain gauge sits within an electrical circuit, increasing resistance as more pressure is applied. We can then measure the current of the circuit to calculate the pressure of the system.
By converting the mechanical pressure to an electrical signal, electric pressure sensors are more sensitive and accurate than Bourdon tube gauges.
Measurements in Real Time
We sometimes require quick measurements. For example, monitoring reagent flow in dangerous chemical reactions, although these reactions are being phased out in line with green chemistry principles.
Electric pressure sensors are handy as they can measure pressure in real time and transmit it as an electric signal to monitoring systems. In Bourdon tube pressure gauges, there is often a delay as the tube expands.
Other sensors can be used, such as piezoelectric transducers (using a crystal to generate current) and optical sensors that detect pressure changes by measuring how much light passes through.
About the Author
Sean is a consultant for clients in the pharmaceutical industry and is an associate lecturer at La Trobe University, where unfortunate undergrads are subject to his ramblings on chemistry and pharmacology.
