Pressure measurement is essential in many industries, from chemical processing to food production and gas distribution. Two popular types of pressure instruments are diaphragm pressure gauges and capsule gauges. While they share some similarities in using elastic elements to detect pressure, they differ in design, application, and performance. Understanding these differences helps engineers and technicians select the right device for their process.
This article provides a detailed explanation of how diaphragm pressure gauges differ from capsule gauges. It examines their operating principles, construction, measurement ranges, applications, and performance factors.
Understanding Diaphragm Pressure Gauges
Principle of Operation
Diaphragm pressure gauges use a thin, flexible membrane called a diaphragm as the sensing element. This diaphragm can be made from materials such as stainless steel, Hastelloy, or elastomers, depending on the application. When pressure is applied to one side of the diaphragm, it deflects proportionally to the applied pressure.
This deflection is mechanically transmitted through a linkage to the gauge pointer, allowing a visual reading on the dial. The diaphragm’s movement is small but precise, enabling the gauge to measure relatively low to medium pressures accurately.
Design and Construction
The diaphragm is often clamped between two flanges. One side is exposed to the process medium, and the other side may be sealed with a reference pressure, such as atmospheric pressure or a vacuum. In many designs, the diaphragm is supported by a corrugated shape to improve sensitivity and repeatability.
For corrosive or viscous media, a diaphragm pressure gauge can be fitted with a chemical seal. This isolates the sensing element from direct contact with the process fluid while still transmitting pressure accurately.
Common Applications
Diaphragm pressure gauges are frequently used in industries where the medium is corrosive, viscous, or contains particulates. They are suitable for chemical plants, food processing lines, and wastewater treatment facilities. Their robust design and ability to be isolated from the process make them versatile and durable.
Understanding Capsule Gauges
Principle of Operation
Capsule gauges use a different type of elastic element: a capsule formed by joining two thin diaphragms at their edges to create a sealed chamber. The capsule is evacuated or filled with a reference gas, depending on the intended measurement.
When pressure is applied, the capsule expands or contracts. This movement is transferred via a mechanical linkage to the gauge pointer. Capsule gauges are highly sensitive to small pressure changes and are typically used for measuring low pressures.
Design and Construction
The capsule is usually made from thin metal such as stainless steel, brass, or phosphor bronze. Its low mass and thin walls allow it to respond to very small pressure differences. Unlike a single diaphragm, the capsule offers double-sided flexibility, which increases sensitivity.
Capsule gauges are often built into instruments intended for low-pressure measurement in clean and dry media. They do not require heavy-duty sealing systems, as they are not typically used for aggressive or viscous fluids.
Common Applications
Capsule gauges are ideal for measuring low pressures in applications such as laboratory experiments, air flow monitoring, gas distribution networks, and HVAC systems. They are often chosen for precision measurement where accuracy at the lower end of the scale is more important than withstanding harsh environments.
Comparing Measurement Ranges
Diaphragm Pressure Gauges Range
Diaphragm pressure gauges can measure from very low pressures up to several hundred bar, depending on the diaphragm material and thickness. They are versatile across a broad range of applications. Their construction can be adapted to handle both positive and negative pressures.
This flexibility makes them suitable for processes involving both high-pressure fluids and low-pressure gases, as long as the correct diaphragm design is chosen.
Capsule Gauges Range
Capsule gauges are primarily used for low-pressure measurement. Typical ranges are from a few millibars to a few bar. Their sensitivity is excellent in this range, but they are not designed to withstand high pressures.
Because the capsule element is delicate, it is unsuitable for applications where pressure surges or mechanical shocks are common.
Sensitivity and Accuracy Differences
Sensitivity of Diaphragm Gauges
While diaphragm pressure gauges can measure low pressures, they are generally less sensitive than capsule gauges at the very lowest ranges. The single diaphragm movement is smaller and may require more robust mechanical linkages.
However, diaphragm gauges maintain good accuracy across a much wider pressure spectrum, especially when fitted with high-quality mechanical components.
Sensitivity of Capsule Gauges
Capsule gauges excel in detecting minute pressure changes. The double-diaphragm construction allows greater movement for small pressure variations, resulting in higher sensitivity. This makes them ideal for laboratory or calibration work, where detecting fractional pressure differences is essential.
Media Compatibility
Diaphragm Pressure Gauges and Harsh Media
One major advantage of diaphragm pressure gauges is their ability to handle corrosive, dirty, or viscous media. The diaphragm can be coated or made from resistant alloys, and a chemical seal can be added to prevent damage.
This makes them suitable for oil, slurry, corrosive gases, and other challenging process fluids.
Capsule Gauges and Clean Media
Capsule gauges require clean, dry, non-corrosive gases to function reliably. Exposure to moisture, particulates, or aggressive chemicals can damage the thin capsule walls and affect performance.
They are therefore best suited to controlled environments where the gas composition is stable and free from contaminants.
Durability and Maintenance
Durability of Diaphragm Gauges
Diaphragm pressure gauges are generally more rugged than capsule gauges. Their design allows for thicker sensing elements, protective coatings, and mechanical sealing options. They can withstand pressure spikes better, especially when equipped with overload protection.
Maintenance usually involves periodic inspection of the diaphragm and seals. Replacement diaphragms can be fitted if necessary.
Durability of Capsule Gauges
Capsule gauges are delicate instruments. The thin walls of the capsule can deform if exposed to excessive pressure or mechanical shock. As a result, they require careful handling and installation in environments where sudden pressure changes are unlikely.
Maintenance focuses on keeping the media clean and preventing contamination inside the capsule.
Choosing Between the Two
When to Use Diaphragm Pressure Gauges
Diaphragm gauges are the best choice when the application involves harsh media, fluctuating pressures, or the need to cover a wide pressure range. They offer good accuracy and durability, making them reliable for long-term industrial use.
When to Use Capsule Gauges
Capsule gauges are preferable when the priority is extreme sensitivity at very low pressures in clean gas systems. They are suited to laboratory testing, air pressure monitoring, and gas flow measurement in stable environments.
Conclusion
While diaphragm pressure gauges and capsule gauges share a common purpose—measuring pressure—they differ significantly in design, sensitivity, durability, and application suitability. Diaphragm gauges use a single flexible membrane and are adaptable for a wide range of pressures and media, including harsh and corrosive environments. Capsule gauges use two diaphragms joined to form a sealed chamber, giving them superior sensitivity at low pressures but limiting their use to clean, non-corrosive gases.
Understanding these differences helps engineers and operators choose the right instrument for the job, ensuring accurate measurements and reliable performance in their specific application.
