Flowmeter Selection Guide: Types For Application

• Flow measurement type - momentum (velocity), volumetric or mass flow measurement
• Media - type of media (liquid, gas or slurry) and any special condition such as particulates in the media and viscosity of the media
• Media conditions - pressure and temperature of media and whether media conditions are likely to remain constant or vary
• Flow range - required flow range of media (min and max readings required)
• Accuracy - required accuracy of the readings
• Environmental considerations - special installation considerations such as hygienic installation, installing in to an ATEX zone or requiring tamper proof readings

Here is an overview for some of the flowmeter options available to you:

Single-Use flowmeters (Figure 1) are used in high hygiene applications such as biopharma, food, or semiconductor fabrication when a CIP option is not optimal. Sensor types include invasive sensors that come in contact with the working fluid and are discarded after each use and non-invasive models that do not come in contact with the working fluid and are therefore reusable. Given the high value placed on typical working fluids, sensor reliability, accuracy, and compatibility with the workflow are key requirements for single-use flowmeters. If extremely tight accuracies are desired, <1%, then Coriolis type flowmeters are available for single-use applications. Single-use Coriolis flowmeters are at the high end of the price scale

Coriolis Snapshot: Extremely high accuracy and no pressure drop. The meters track mass flow and offer a very high turn-down ratio. Yet initial expense is high, clogging can occur, and meters are larger in overall size.

Coriolis flowmeters offer true mass flow measurement through two designs: a single tube or two parallel tubes. They operate via an oscillation which is induced in the tube(s) at a reference frequency. Based on Newton’s Second Law of Motion (F = m x a), the oscillation frequency will change with changes in mass flow rate. Among the most accurate of technologies available, Coriolis flowmeters are suitable for a wide and growing range of gas and liquid applications. These devices provide multiparameter data on mass, density, and temperature. They are used in pharmaceutical manufacturing, wastewater treatment facilities, nuclear facilities, natural gas measurement and custody transfer.

Differential Pressure Snapshot: Very high accuracy, multiple calibrations, outputs and size. At the same time, this model can be used with water or gases only and cannot handle particulates. It also requires power.

Differential pressure flowmeters (Figure 2) measure changes in pressure to determine flow velocity. They feature a flow-restrictive orifice or laminar flow element that evaluates the pressure drop through the restriction. The pressure drop between upstream and downstream points is proportional to the rate of flow. This technology works well when no moving parts are desired, or when an ultrafast response time is required. Differential Pressure flowmeters are typically found in more industrial applications, such as measuring the output of fuels (for example, benzines or jet fuel), in specialty chemical manufacturing, simple water measurement testing, or on aquafarms. They are also used in labs to measure and control the flow of gases when mixing them or separating them through chromatography.

Gear Snapshot: High accuracy; measurement is independent of fluid viscosity. No straight pipe runs required. Accuracy degrades slightly when measuring the flow of low-viscosity fluids.

Gear meters employ oval counter-synchronized rotors (gears) which are interlocked to rotate with the passing of liquid. The amount of fluid passing through the oval gears is well controlled, giving the meters a high level of accuracy. Designs are typically rugged and simple, allowing for installation in the most aggressive environments. In fact, gear meters are one of a few types which are suited to high-viscosity fluids. Used in hydraulics and other applications involving very viscous liquids, gear flowmeters work well in the pulp and paper industry, fuel or oil transfer, and manufacturing. Because the gears are stainless steel, they are ideal for the petrochemical industry or any application incorporating light to heavy oils.

Magnetic Snapshot: No obstruction of flow path, no pressure drop, no moving parts. These meters can handle heavy slurries. Yet, the fluid measured must be conductive or water-based, and the meter must be grounded.

Magnetic meters (or magmeters) are available in two design styles: insertion and full-bore. Coils in the meter produce a magnetic field. When a conductive fluid is passed through the field, a voltage is produced through an electrode in the meter wall or insertion probe; this generated voltage is proportional to the flow. Magmeters operate by measuring the electrical content of water or other fluids. The magnetic technology contains no moving parts, and the full-bore designs offer no intrusions into the flow stream. Magmeters are higher-end flowmeters and are used in the food & beverage industry, water purification, pulp and paper manufacturing, mining, chemical manufacturing, and petrochemical industries. They should not be used with low conductivity fluids such as de-ionized water.

Paddle Wheel Snapshot: Fast response time. Easy maintenance. Inexpensive. Some may be difficult to install. Uses moving parts and requires a full pipe.

Paddle wheel meters (Figure 3) include those with rotating paddle wheels, propellers, and oscillating disks (multi-jet types). The rotating component is designed to provide a pulse when passing either a magnetic or optical sensor. The frequency of the pulses is proportional to the velocity of the fluid at one point in the pipe or channel. These designs offer relatively high accuracy for their low cost. Some insertion versions are very easy to install while other styles are more difficult. The paddle wheel style of flowmeter is often seen in rural areas for irrigation, on aquafarms, in water/wastewater treatment, and for simple water measurement. These meters are also used in utilities and the oil and gas industries and can work with viscous fluids if a turbulent flow is present.

Thermal Dispersion Snapshot: No moving parts. Measures the mass of the gas, not the volume, so it is very accurate. However, the gas must be dry and free of particulates. Response time is fairly slow.

Thermal dispersion meters operate with a side-stream flow of gas which is directed through a capillary. The capillary includes two external heater-sensor coils, one downstream from the other. Gas flow carries heat from the upstream coil to the downstream coil. The resultant temperature-dependent resistance differential at each coil is then measured. The gradient at the coils is linearly proportional to the instantaneous flow rate.