A pressure gauge is a device that allows measuring the intensity of fluids in closed containers.
These instruments employ mechanical connections. Therefore, they are mainly useful for measuring static pressures or pressures that change slowly. This makes their application different from that of a pressure transmitter, which are used in cases where pressures change rapidly.
What is a pressure gauge for?
In machine settings
Pressure gauges are used for the setup and adjustment of fluid power machines. Without pressure gauges, fluid power systems would be unreliable and unpredictable.
In hydraulic systems
Pressure gauges help ensure that there are no leaks or pressure changes. In other words, they can affect the operating conditions of hydraulic systems.
These systems are designed to work in a set pressure range. Therefore, the pressure gauge must be rated for that range.
Hydraulic gauges are often installed in the pressure port of the pump to indicate system pressure. However, they can be installed anywhere on the machine where pressure monitoring is needed.
A pressure gauge is especially useful if the sub-circuits operate at a different pressure rate than the pump pressure. This is a clear application to use the pressure gauge after a reducing valve. Pressure reducing valves often have a metering port to take advantage of. This allows you to directly control your downstream pressure setting.
In fluid feeding systems
Pressure gauges have been used in fluid supply systems for many years. Therefore, the design of the pressure gauge for fluid supply systems is already highly refined.
The pressure gauge for fluid power applications has evolved towards an increase in application specific characteristics. For example, pressure gauges are now designed in a standardized way with hydraulically friendly pressure connections (such as straight SAE / metric threads) to prevent system leaks.
Analog meters with custom scales are more common. Digital pressure gauges with customizable firmware allow the measurement of the leak measurement process based on pressure or other parameters such as torque, load, force and hardness.
In compressed air and pneumatic systems
Compressed air and pneumatic systems are also filled with gauges, as pressure is also measured at many places in the system. Pressure is measured at the receiver, as well as at each FRL or self-contained regulator in the system. Sometimes pressure is also measured on pneumatic actuators.
Typically, pneumatic pressure gauges have a lower capacity than hydraulic pressure gauges.
How is pressure measured?
There are three ways to measure pressure with a pressure gauge: absolute, gauge, and vacuum.
Absolute pressure
It is based on a measurement of actual pressure, including ambient air. This means that it has zero reference with a perfect vacuum, but it can be as high as 10 bars at sea level. Absolute pressure readings are considered in applications that interact with ambient air, such as calculating compression ratio for flow requirements (cfm).
Gauge pressure
It has zero reference with respect to ambient pressure. It is used in most applications operating in ambient air, such as fluid power systems. It must be taken into account that disconnected from the equipment, the gauge pressure will indicate zero.
Vacuum pressure
It is expressed in Torr or refers to ambient pressure. Its units of measure “in.-Hg” (inches of mercury) measure pressure below ambient temperature.
Read also: LEARN TO RECOGNIZE SIGNS OF GAUGE FAILURE
The hydraulic pressure gauge can support different pressure ranges depending on the type of gauge and the material from which it is made. Because of this, gauge style and material make up two of the most important selection criteria for pressure gauges.
Types of pressure gauges
There are many types of pressure gauges, but the most common are Bourdon tube gauges and bellows gauge gauges.
Bourdon tube pressure gauge
Bourdon tubes take pressure and convert it into mechanical energy. This energy moves a dial on the gauge, showing the current amount of pressure in the system.
Bourdon tube pressure gauges are some of the most common gauges today. They have different configurations, such as curved, helical, and spiral. Differences in tube design, size, and material from which it is made vary depending on the pressure range.
An important feature to take into account is the cross section of the tubes. It changes with increasing pressure. Generally, as the gauge’s working pressure increases, the cross-sectional shape of the tube design will change. It normally goes from an oval shape to a circular shape.
The operation of the Bourdon tube is simple. They consist of a semicircular and flat metal tube. It is attached at one end and attached to a responsive lever mechanism at the other. As the pressure inside the tube increases, the force of the fluid attempts to straighten the bent tube. The tube is detached from the lever, which when connected to the needle on the screen, shows the pressure at the fluid port.
Bellows pressure gauge
While bellows gauges work in a similar way to Bourdon tubes, they differ in that they use a spring to calculate the amount of energy to be pressed. The spring expands and compresses from the pressure in the tubes. The energy created by that movement is transferred to gears that move the pressure dial.
The pressure range in which the gauge will be operating is the main selection factor in choosing the type of material used to make the gauge. Gauges that operate at higher pressures generally tend to be made of materials such as steel. When operating at lower pressures, they tend to be bronze.
The use of test point adapters at various locations in the hydraulic system allows measurement during troubleshooting. So you don’t need to buy dozens of pressure gauges. The test point connection connects to the meter, which can be screwed into the test points throughout the circuit. This allows you to connect under pressure to measure at various points in the system.
Most gauges are 21/2 inch in diameter. They can be top mount or panel mount. Meters are available in every imaginable size, material, and construction.
How to choose a good pressure gauge?
Used to test equipment or operate machinery, the correct pressure gauge helps reduce costly downtime.
In mechanical measurement applications for hydraulic systems, common threats to measuring reliability are vibrations, pulsations, and pressure spikes. Therefore, it is best to look for gauges specifically designed for hydraulic applications.
Its features should include: a forged bronze casing to prevent resonant frequencies from destroying internal components; a liquid-filled housing to protect the meter from extreme pressure cycling and vibrations; and a restriction to prevent damage to the meter from pressure spikes. Although the liquid used in the meter varies from application to application, glycerin is commonly used and works well in many conditions. The higher the viscosity of the liquid, the more it dampens vibrations.
When choosing between a dry, water or glycerin indicator, it is also important to consider the following: temperature range, required needle response time, pressure changes, and the amount of vibration expected from the application.
Finally, depending on the demands of the application, measurement accessories, such as specialized restrictions, Piston Snubber, or even diaphragm seals, may be needed to prevent premature failure of the pressure gauge.
