Choose the Right Flowmeter for Your Application

Most closed liquid systems depend on the proper amounts of flow in each section. For example, a refrigeration system cools in direct proportion to the amount of coolant flow. Other systems require flow control to ensure that operational flows and the associated pressures do not exceed component specifications.

Design Requirements

Designers choose the proper flowmeter for their applications by making sure the device is capable of delivering the proper measurements under all system and environmental conditions. The proper measurements include limits on upper and lower flow ranges, accuracy, and granularity (for digital measurements).  System constraints include parameters for minimum and maximum expected flow and expected pressure levels. Physical constraints include the connection interface and the MTBF (mean time between failures) of the device. Certain corrosive materials as well as dangerous fluids require additional specifications for the base materials of the meter and the seals. Finally, environmental factors such as ambient temperature and vibration impose additional requirements on the flow specifications.

Measurement Requirements

Unlike a pressure meter, which is a probing device that does not perturb the system, a flowmeter is an inline device. The meter sensor adds resistance to the flow of fluid in the system. For systems with critical flows, the system designer must choose a sensor with minimum resistance to flow. Alternately, the designer can adjust the location of the sensor in the system to minimize the impact on system flow.

Turbine Flowmeters

A turbine flow meter, more accurately called an axial turbine, is appropriate for measuring either a liquid or natural gas. This meter is a transducer, which converts the rotational action of the turbine into an electrical signal proportional to flow. Common turbine meters generally display in units of gallons-per-minute or liters-per-minute.

The wheel of a turbine flowmeter is inserted into the liquid flow. The moving liquid impinges on the turbine blades, initiating the wheel rotation. Initially, the blades exert a great deal of pressure on the fluid, limiting the flow. As the wheel increases in speed, the resistance to fluid flow lessens, and the measurement system approaches a steady state. At the steady state, the flowmeter resistance is at a minimum, and the rotational speed of the wheel is proportional to the fluid flow.

A transducer converts the rotational speed to a proportional electrical signal. This transducer can count rotations by positional sensing, such as that provided by a fixed-point magnet on the shaft and a Hall Effect sensor. Manufacturers also implement this sensor by attaching magnets to the shaft and rotating the assembly in a coil. This generates an output signal in the same way a gas generator creates electricity.

Turbine flowmeters are not as accurate as other configurations such as displacement or jet meters at lower flow rates. However, it does affect the fluid flow less than the alternatives. These flowmeters are also less sensitive to quick changes in the flow rate because of their comparatively high rotational momentum.

Positive Displacement Flowmeters

A positive displacement flowmeter is a mechanical measurement device that accumulates liquid into a fixed volume, empties the filled volume, and counts the number of fill/empty cycles. The volume divided by the average time between cycles yields the flow rate.

Mechanical flowmeters must implement this process while maintaining the fluid in the pipe system. Some of the ways of performing this process include the following:

  • Reciprocating pistons – this system works by filling one piston cylinder while emptying a second. This action is similar to the operation of the pistons and cylinders in an automobile combustion engine.
  • Gear teeth – as a gear rotates, the teeth create a cavity with the cylinder wall. While one section of the cylinder fills the cavities, the section on the opposite side of the cylinder allows the fluid to escape. This system is more difficult to seal than the piston system, and so is more appropriate for use with viscous liquids than normal-viscosity liquids and gases.
  • Oval gears – oval gears are an alternate version of gear teeth, and work in a similar fashion.
  • Helical screws – helical screws are yet another version of gear teeth, which can provide larger cavities by extending the mechanism in the longitudinal direction.

 

Magnetic Flowmeters

Magnetic flowmeters, known in the industry as “mag meter”s or “electromag”s, take advantage of the effect on magnetic field of the moving liquid. The flowmeter creates a static magnetic field in the direction of the measurement tube. A pair of electrodes measures the voltage potential between two points along the tube, one at the start of the field and the end. Faraday’s Law of electromagnetic induction shows that the differential voltage will be proportional to fluid flow.

A strong advantage of this type of meter is the low impact on the fluid flow. A second benefit is the ability to measure very low fluid flow rates accurately. Designers must deal with the fact that the fluid flows over the electrode tips, which can create a problem with corrosive fluid materials. Some systems include automated electrode cleaners to deal with this challenge. A second challenge is the presence of stray voltages on the pipes. These voltages can be created in the same way as the flowmeter system, through induction. Pipes that travel close to high-current devices can pick up stray voltages. Magnetic flowmeters compensate for this effect with a pulsed electronic field. This cancels out the effect of any induced voltage on the measurement pipe.

Micro-Motion Coriolis Mass Flow

This type of meter takes advantage of the Coriolis Effect, which splits inertial force into centrifugal and coriolis forces through a change in the observation reference. Flowmeters do this by routing the fluid into a rotating lateral tube, and measuring the lateral displacement. This method measures mass flow rather than volumetric flow. The system yields a value proportional to volumetric flow by dividing the mass flow by the fluid density.

Like electromagnetic and ultrasonic flowmeters, the Coriolis flow meter can correct for varying fluid density conditions, created by changes in pressure or temperature. It can also compensate for non-linearities in the system transducers as well as variances in the characteristics of the transport fluid.

These are some of the examples of available flowmeters on the market. Designers choose the proper meters for their systems by comparing meter specifications to their system requirements