Hydraulic Flow Meters

What is the difference between Fluid Power, Hydraulics, and Pneumatics?

The term “Fluid Power” is a general catch-all reference to the fields of hydraulics and pneumatics where a pressurized fluid is used to create power. In the field of hydraulics, the fluid used to produce power is typically a liquid. The most common type of hydraulic liquid is oil. In the field of pneumatics, the fluid used to create power through pressure is typically a gas, most commonly air.

Typical hydraulic processes are found in applications like industrial, construction, and agricultural equipment. Common examples that are well known include machinery like cranes, backhoes, and forklifts.

What is the difference between liquids and fluids?

Although they are commonly used interchangeably, liquids and fluids are not the exact same thing. The term “fluid” applies to both gaseous and liquid states. A fluid is characterized by having no fixed shape and is influenced in shape by the application of external pressure. It moves and flows upon influence, as opposed to a typical solid that retains its shape when pressure is applied. Both gases and liquids are fluids. In essence, a “liquid” is a type of “fluid”. But not all fluids are liquid, some are gas.

What is the difference between a hydraulic fluid and a hydraulic oil?

Hydraulic oil is a subcategory of hydraulic fluids. All hydraulic oils are considered hydraulic fluids, but not all hydraulic fluids are oil. Other examples of non-oil based hydraulic fluids include synthetic high viscosity glycol mixtures.

What is the key principle of hydraulics?

The key operating principle to a hydraulic system is simply that pressure is applied to a liquid in a contained system, and this pressure creates a resultant force that can be converted to mechanical energy.

Which fluid is used in a hydraulic power system?

Typical hydraulic fluids are higher viscosity liquids that have inherent lubricating properties, like oil. Glycol and water-based liquids are also used, but oil is probably the most common fluid used in hydraulic systems due to its higher viscosity. Liquids with lubricating properties typically protect elements of a hydraulic system, such as seals. They also decrease system component wear by decreasing friction, and oils also provide some protection against parts susceptible to rust.

How does a typical hydraulic circuit work?

Typically, a simple hydraulic circuit in a process or application includes a pump that pushes oil or another high viscosity liquid within a process line or hose that then creates the force to create power. The system converts hydraulic energy into mechanical energy.

What is a hydraulic flow meter?

A hydraulic flow meter does more than verify that there is flow in pressurized systems. They typically can provide a measurement of the flow rate and can provide a variety of outputs and electronics that are not part of a simple flow sensor. They are typically part of high-pressure systems within an industrial process. The common characteristic of a hydraulic flow meter is the ability to operate under high pressures and to operate with high viscosity liquids like oil.

Where are hydraulic flow meters used and what does a hydraulic flow meter test?

Hydraulic flow meters are typically used in industrial applications. They are commonly used to help trouble shoot and to ensure optimum system performance. They can be used to verify that the flow of a liquid within a process is not impeded and can also determine whether pumps, that are part of a process, are producing an adequate amount of flow for the application.

They are commonly found in machinery that moves, lifts, presses, crushes, or creates some other mechanical energy. Common elements of a full hydraulic process can include pipes, hoses, flow meters, pumps, cylinders, actuators, motors, and the liquid being used.

What types of hydraulic flow meters are there?

Not all flow meter technologies are suitable for hydraulic applications due to media incompatibility and due to the inability to handle high pressures. One of the most common flow meter technologies used in hydraulics is positive displacement. Positive displacement flow meter types, also known as gear meters, are able to operate under high pressures and are able to properly measure and handle viscous media like hydraulic oil. Learn more about positive displacement technology by visiting our informational article.

Other hydraulic technologies include variable orifice meters and special variable area flow meters that are inherently viscosity-compensated, like our unique VKM Variable Area Flow Meter. We also offer a variety of different positive displacement technology types for hydraulics. Our DON-H Positive Displacement Flow Meter operates on the oval gear positive displacement principle and is designed specifically for high pressure applications. Our OME Positive Displacement Flow Meter operates on the helical gear positive displacement principle, while our ZDM Positive Displacement Flow Meter operates on the spherical gear principle.

How does a hydraulic flow meter work?

All types of hydraulic flow meters work by measuring the flow within the system line or pipe. The exact technology used to produce those measurements is specific to the exact technological principle that each particular flow meter uses to operate. Common technologies used in the field of hydraulics are mentioned in the previous paragraph. Each technology operates a bit differently.

How do you read a hydraulic flow meter?

Depending on the technology type your hydraulic flow meter uses, you will likely either read the measurement on a sight glass that is marked with a scale or read a dial type pointer indicator that also references a scale. Typically flow meters within the US will have scales that measure the flow in GPM, or gallons per minute. Other flow meters may also deliver a digital readout of the flow rate and any other flow parameters or provide an output that can be linked to an overall control area for the application.

How do I choose a flow meter for a hydraulic application?

The most important variables for a hydraulic process flow meter are the ability to handle the peak high pressures of the process and the ability to function correctly with high viscosity liquids. Each hydraulic application can have its own unique nuances that must also be taken into consideration. If you would like our expert help in selecting the best flow meter for your exact application, please do not hesitate to call or email us.

What are the pitfalls in hydraulic systems?

Leaks, leaks, leaks. It is hard to think of a hydraulic system, like those found in a tractor, without thinking about oil being present in abundance, especially as the tractor ages. Large industrial processes that employ hydraulic principles can benefit from the verification of proper flow in the system via a flow meter. They can be used to detect leaks and any system depressurization.

Do hydraulic processes require electricity?

Usually, hydraulic processes are largely mechanical in nature, however, some systems do have electricity as an integral part of the process. The danger in hydraulic systems that employ energy is the risk of sparks to a highly pressurized system which can result in explosions. These types of applications usually employ instrumentation that has been verified to be compatible with the risk.

Pneumatics vs Hydraulics

As mentioned, both hydraulics and pneumatics generate power based by pressure, but pneumatics utilize gas and hydraulics utilize liquids. Hydraulics typically employ pumps, while pneumatics utilize compressors to create pressure.

Hydraulics are typically used for applications that require a higher force to perform the necessary function as liquids are typically able to create more force because of the internal properties of resistance.

Pneumatic systems are generally much cleaner as they operate based on gases that cannot “spill”, make a mess, or contaminate. The nature of hydraulic liquids can also mean that the system may require more maintenance to verify that the media has not compromised the integrity of the system. As a result, pneumatic systems are generally preferable in applications involving sanitary concerns or clean space applications. They can also produce a faster “release” and higher speeds of movement. Hydraulic systems can be more complex and require higher cost process elements than pneumatic systems.

Learn more about why you should partner with KOBOLD for your application.

Featured Products
High Pressure Oval Gear Flow Meter
DON-H - Oval Gear Flow Meter for High Pressures

Stainless Steel | Optional Batching & Totalizing | Viscosities up to 1,000 cP & Higher | Up to 5,800 PSI | 0.13...10.6 GPM

Helical Gear Flow Meter
OME - Helical Gear Flow Meter

Advanced Technology for Oils | Quiet Operation with No Pulsation | Up to 580 PSI | 0.027...90 GPM

All-Metal Viscosity Compensating Flow Meter for Oils
VKM - All-Metal Viscosity Compensating Flow Meter and Switch

Made in the USA | Top Seller | 0.03...20 GPM | Patented Viscosity Compensation | Viscosities up to 540 cSt | Up to 5,000 PSI | Brass or Stainless

Positive Displacement Flow Meter
ZDM - Positive-Displacement Flow Meter

Radial/Spherical Gear Meter | Viscosities up to 100,000 cSt | Up to 6,500 PSI | Cast Iron or SS | 0.0005...138 GPM


KOBOLD USA is a subsidiary of KOBOLD Messring GmbH, a world-leading instrumentation engineering business founded in Germany in 1980 by Klaus J. Kobold. With patented technology and superior service, the company quickly established itself as one of the global leaders in sensor and control systems with high quality products. The KOBOLD brand name became synonymous with superior quality and technological advancement in instrumentation engineering.

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