Paddlewheel Flowmeters – Getting the Job Done

Paddlewheel Flowmeters
Getting the Job Done


Paddlewheel flowmeters are easy to install and operate, resulting in a very low overall cost of ownership. Relatively low cost paddlewheel style electronic flowmeters are proof that high cost doesn’t always mean better value.

The components of a system must be able to perform the required task – get the job done – and meet the other physical requirements of the application. Excess capability, features, and accuracy are a waste of money. You will get the most value for your money by purchasing system components that meet the demands of the system without being overkill. While expensive, high technology solutions may be available for your application; low cost paddlewheel flowmeters offer high accuracy solutions to many flow system applications – not just displaying the flow rate and the total accumulated flow amount.



Paddlewheel flowmeters are used to accurately measure and dispense preset volumes of water or other chemicals.

Water Dispensing System

Water dispensing systems commonly use preset cycle timers to dispense water. When the timer is activated, the system begins dispensing water until the preset time cycle times out. These preset (fixed) cycle timers can result in accuracy problems because they do not actually measure the flow rate! Any problem in the system that results in a change in the flow rate will result in an error in the amount of water dispensed. Some examples include worn pump components and changes in the system pressure, either of which can result in changes in the pump output. The cycle timer control cannot compensate for various flow rates because it is not measuring the flow rate.

Paddlewheel sensors actually measure the amount of water dispensed. When the dispensing system is activated, the electronic flow controller starts the pump and opens the correct dispensing valve. The sensor begins to output electrical pulses. These pulses are then counted by the electronic flow controller. Changes in the output flow rate of the pump will not affect the sensor count. When the correct amount of water has been dispensed, the dispensing valve is closed and the pump stopped.


Paddlewheel flowmeters can control chemical metering pump outputs.


Chemical metering pumps are used to inject chemicals such as chlorine and acids into water systems. The chemicalmust be injected into the system at the proper rate to achieve the correct water/chemical proportions. Depending on the application, too much or too little chemical can result in series problems. In a system that has changing water flow rates, a fixed feed rate chemical injection metering pump alone is not capable of reacting to changes in the flow rate of the system. Paddlewheel flowmeters can be used to start and stop inexpensive, fixed feed rate metering pumps resulting in the proper amount of chemical injection. When the meter has measured a specified volume of flow that has passed through the system, the chemical pump is turned on for a pre-programmed amount of time. This simple system results in a pump on time (chemical) per flow volume (water) ratio.

Variable speed pumps are used when near continuous injection of chemical is required. These pumps can be controlled directly by the sensor’s output signal. The high speed sine wave signal can be input directly into the pump’s electronic speed control. The pump speed, and therefore the amount of chemical injected, is programmed to react to changes in the frequency output by the flow sensor. A minimum pump speed is programmed for a frequency and a maximum pump speed is programmed for another frequency resulting in a speed:frequency ratio (pump output rate per flow rate).


Paddlewheel flowmeters can verify chemical injection has occurred.

Paddlewheel sensors are capable of measuring chemical metering pump output rates as low as 1 ounce per minute. When installed on the metering pump, the flow sensor can be used to alert the system operator that an error exists in the system. Some metering pumps include electronics that react to the paddlewheel output signal. If the metering pump should fail to inject chemical due to a pump malfunction, clogged fitting, exhausted chemical container, etc., an alarm output is triggered.

alarm graphic3


Paddlewheel flowmeters can monitor system flow rate.


When a system’s flow rate is critical, a paddlewheel flowmeter can be used to alert the system operator if the rate increases or decreases out of a programmed range. The electronic display can be programmed with a high and low rate amount which will trigger an alarm output signal if reached. The alarm can automatically reset or latch. Trigger and release values can be set, with hysteresis, which will eliminate “flickering” that can occur when the flow rate is at the alarm value.


Paddlewheel flowmeters work best with clean fluids. Particles and debris can prevent the paddle from spinning properly.

Install the pipe fitting in a location that includes a proper length of straight pipe before and after the meter. Because the paddle is inserted only a small distance into the flow stream, the flow stream must be a consistent velocity across the entire inside pipe diameter to obtain an accurate reading. The straight length of pipe will allow any swirl patterns in the flow stream to dissipate before contacting the paddlewheel. Swirl patterns can be caused by obstructions such as an elbow, tee, pump, etc. The minimum straight length of pipe required will depend on the type of obstruction before the paddlewheel. The absolute minimum is typically ten times the nominal pipe size before the meter and 5 times after. Thus, a 4” pipe would require a minimum of 40” (10 x 4) of straight pipe before the paddlewheel and 20” (5 x 4) after. Refer to the manufacturers instructions for specific requirements.

Flow disturbances4


Paddlewheel flowmeters may not function properly with high viscosity fluids. High viscosity fluids will tend to produce a laminar type flow profile. In a laminar flow profile, the center of the flowing fluid is moving faster than the outer edge. A turbulent flow profile, where the fluid velocity is the same across the entire pipe diameter, is required for accuracy. The fluid’s Reynolds Number must be greater then 4000 to ensure a fully developed turbulent flow profile. The Reynolds Number is a dimensionless number that combines the effects of viscosity, density, and flow velocity to identify either a turbulent or laminar flow profile.

Reynolds number5


The pipe must be full of water at all times. When the system starts and stops, any air in the line may lead to an erroneous reading.

Size the meter to work within the published operating range. Although the meter may read at flow rates other than published, the meter may not be accurate at these rates.

Be sure the saddle is properly installed. Saddle installation, pipe size, alignment and adjustment, is critical to an accurate reading.


Paddlewheel flowmeters consist of three primary components; the pipe fitting, the paddlewheel sensor, and the display/controller. These components can be purchased separately or as a package to meet the particular requirements of the application. The paddlewheel sensor is designed to be inserted into the pipe fitting. Approximately one half of the paddle protrudes into the flow stream. Fluid flowing through the pipe causes the paddlewheel to spin. As the magnets that are imbedded in the paddle spin past the sensor, electrical pulses are produced that are proportional to the rate of flow. The manufacturer publishes the number of output pulses produced, per volume of flow, for each specific pipe fitting. This number is called the K-factor.

PIPE FITTINGS – Various pipe fittings styles are available. Some fitting styles are designed to install directly into the pipeline using various connection methods such as male or female threads, socket weld, socket fusion, and butt fusion joints . These “in-line” fittings are available in a variety of materials such as PVDF, polypropylene, and stainless steel. They are available with and without union connections. Because the manufacturer can control the inside diameter of the fitting, in-line fittings are available in a variety of operating flow ranges to accommodate various applications.

Saddle style fittings are designed to mount directly on an existing pipe. The saddle is installed by simply drilling a hole in the pipe and clamping the saddle onto the pipe. Cutting the pipe and installing special adapters is not necessary. Saddles are available in a variety of materials.

PADDLEWHEEL SENSORS – The Paddlewheel sensor consists of the paddlewheel with its imbedded magnets and the electronic sensor. Manufacturers offer sensors in a variety of materials to meet most applications. Two types of sensor outputs are available, AC coil and Hall Effect.

AC Coil sensors generate an AC sine wave that is proportional to the flow rate. Because they generate their own power, these sensors do not require external input power. The signal range for an AC coil type sensor is limited to approximately 200 feet due to possible noise interference and voltage drop.

Hall Effect type sensors output a digital, current sinking, DC square wave that is proportional to the flow rate. Circuitry that is sensitive to magnetic fields is triggered by the spinning paddle. This circuitry requires external input voltage to operate. The signal range for a Hall Effect type sensor is approximately 1 mile.

DISPLAYS/CONTROLLER – Flow displays and controllers are used to receive the signal from the paddlewheel sensor, convert the signal into an actual flow rate or flow total value, and display the values. The processed signal can now be used to open and close valves, start and stop pumps, indicate high or low flow rate alarms in the system, or transmit 4-20mA and TTL level pulse signals to external equipment such as a PLC, chart recorder, metering pump, etc.

Paddlewheel flow sensors and display meters/controllers offer low cost solutions to a variety of water system applications.


Bill McDowell is a Sales Engineer with Blue-White Industries. He has been with Blue-White Industries for 20 years and has also held the position of Project Engineer and Director of Engineering. Bill resides in Garden Grove, California with his wife Jana and their two children Jillian and Sean.

For additional information, contact Blue-White Industries,
5300 Business Drive, Huntington Beach, CA 92649.
Phone 714-893-8529, Fax 714-894-9492,

Paddlewheel Flowmeters and why they are popular

paddleArticleCost of accuracy

Those of us in the instrumentation industry know there is a simple rule; if you want higher accuracy, it will usually cost more money. Paddlewheel Flowmeters come very close to dispelling that rule. Due to modern technology, advanced plastics and metals, solid-state circuitry, and good old capitalism the high cost of accuracy has been challenged.
With the use of computer-enabled testing facilities, paddle sensors are individually bench tested and are accurate to within 1% of the full-scale range reading. The sensor’s flow parameters are then printed and packed with that particular Flowmeter. The key is the use of technologies that were simply not available some years back. It is important to note that although the paddle sensor is tested, and accuracy confirmed, installation variables can affect the meter’s ultimate performance. But that would be the case with most types of Flowmeters. Dollar for dollar, the paddlewheel Flowmeter simply gives you hands down, more accuracy for your dollar.

Ease of installation

Comparatively speaking, paddlewheel Flowmeters have distinct advantages over many kinds of Flowmeters. Paddlewheel style Flowmeters can be installed either horizontally or vertically, with flow in either direction, without compromising overall meter performance. Paddlewheel Flowmeters can be purchased complete with a sensor, digital display and in-line meter body (pipe fitting) which can be easily installed into a piping system. You can also purchase the meters with saddle mount type pipe fittings which are very convenient for installation on existing, larger pipe sizes where in-line fittings are prohibitive.

Battery or AC operated

Today’s Paddlewheel Flowmeters use advanced solid-state circuitry which can operate on common batteries eliminating the need for electrical connections. The technology has advanced to the point where batteries last for a minimum of one year. When the batteries are replaced, no harm is done to the factory settings. Paddlewheel Flowmeters have considerable capabilities such as batch process control, analog output signals and remote readouts. These full-featured meters require a power source and are typically supplied with an AC/DC plug-in style transformer. Keep in mind the flexibility this meter offers the user.

Versatile meter

Paddlewheel Flowmeters are deserving of the attention they are receiving. While most applications are with water, paddlewheel meters also work remarkably well with viscous fluids, provided a fully developed turbulent flow profile exists. If the fluid’s Reynolds number is greater than 4000, the fluid we be a fully developed turbulent flow regardless of viscosity changes.



7741.92 x ID x V

ID = the pipe inside diameter in inches
V = the flow velocity in feet per second
Cst = the fluid viscosity in centistokes

To maintain accuracy, the Reynolds number must be above 4000 over the entire flow range. Unlike variable area meters, paddlewheel meters are not affected by changes in the fluid’s specific gravity. Special calibrations are not necessary.

Another compelling reason people are turning to paddlewheel style Flowmeters is their ability to have a remote readout. Paddlewheel Flowmeters easily permit the panel mount readout to be installed in the most desirous area, while the actual sensor is installed in an obscure area of the pipe system. For example, in any application, the paddle sensor can be installed in the proper location and the digital display can be remotely located in a more desirable location in the system such as on a centralized control panel.


The meter’s paddle and axle are in direct contact with the fluid. Since the paddle will spin at a velocity that is directly proportional to the rate of flow, these components will wear over time. Meters which are operated at the high end of their calibrated flow range will tend to wear more than units operated at the low end. Because every fluid has different characteristics, it is difficult to estimate the life expectancy of these components. Some water (DI water), may be very aggressive while some water may be exceedingly hard or full of abrasives. The resistance of the components to the chemical being measured should also be considered. Axles and paddles are easily replaceable. A neglected paddlewheel Flowmeter will in time have degraded accuracy.

            The maintenance factor should not in any way deter using paddlewheel meters, just remember, turbine meters have the similar problems; rotameters also have to be maintained.  This is a case where the benefits far outweigh the cost.

Maximize paddlewheel performance

If your flow falls below one foot per second you should expect some accuracy challenges.  Avoid using paddlewheel Flowmeters for measuring very dirty fluid, or liquids with rocks or pebbles that could break or damage the paddle or axle. Follow the manufacturer’s installation recommendations regarding straight lengths of pipe.  If you cut corners, do not expect optimum performance from your meter.

In summary

Paddlewheel Flowmeters should be considered by anyone who requires above average metering accuracy where cost is a consideration. These meters are also recommended because they are easier than most meters to install, can measure flow in either direction and serve the intended purpose ofmeasuring flow.  Considering the overall cost of ownership, it is very difficult not to consider the paddlewheel Flowmeter.

Following are some terms you might find useful. The more you understand how and why a Flowmeter works, the greater success you will have with your selection.

K-Factor – The number of signal pulses generated per unit volume of flow. Example: 225 pulses per gallon.

Reynolds Number – A dimensionless number that combines the effects of viscosity, density, and flow velocity which is used to identify either a turbulent or laminar flow profile.

Specific gravity- (relative density) The ratio of the density of a fluid, at it’s temperature, to the density of water at a specified reference temperature.

Feet per second of flow – The velocity of fluid flow in a pipe expressed as the number of linear feet of flowing fluid passing a given point in a pipe, per one second of time.

Robin Gledhill, President
Blue-White Industries
Huntington Beach, CA, USA

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Variable Area Flowmeters offer Economy and Value

There are many sophisticated ‘high-tech’ instruments available for flow measurement, but not all applications require the technology of those often costly instruments. In fact, in some applications simple flowmeters can offer many advantages over more costly and technical instruments. Flow rate is a result of the velocity of a fluid and volume. Although flowmeters may seem to be a modern invention, basic types were evident as far back as the early Roman era. They were often used for measuring water flow to households. The mathematical foundations of flow theory were evolved during the 17th century. Today there are many types of flowmeters, such as: variable area, positive displacement, ultrasonic, and mass flowmeters, just to name a few. Each type has its special features and there is no universal or perfect flowmeter that’s appropriate for all industries and applications. The right flowmeter for the job is the one that will perform effectively in that application, and at a reasonable expense. The variable area flowmeter offers many advantages over some more expensive and technologically advanced types, making it the meter of choice for many flow measurement applications.
Tapered Tube
The flowmeter is comprised of a vertically tapered tube through which the fluid to be measured is passed, from the smaller end of the taper and up to the larger end. As the fluid flows through the tube, it forces an indicator (float) upward. The clearance space between the float and tube increases as the float moves towards the top of the meter. This increasing area requires a larger amount of fluid to force the float higher. By varying the taper of the tube, the mass of the float, and the length of the tube, different flow ranges can be calibrated. The proper installation of the variable area flowmeter, as with all flow instruments, is critical to performance. It must be plumbed into a piping system correctly: that is, with the narrow part of the taper at the bottom.

Typically printed onto or next to the tube is a scale with the flow increments / measurements. The flow rate can be read by matching the increments on the tube with the edge of the float.


In recent years variable area flowmeters have become very competitively priced. Because these flowmeters have become more cost effective, more and more manufacturers of water treatment systems have begun including them as a standard accessory.

A few industries where use of variable area flowmeters is common include:

The makers of ultrafiltration equipment (reverse osmosis systems) use flowmeters to measure output through the membrane, and also to measure reject. Flowmeters play a critical role in helping to monitor the efficiency of the system.

Ultra pure water is used extensively in the manufacture of printed circuitry (cleansing of computer chips, particularly). Flowmeters are used both in the manufacturing and management of deionized water.

Solar-panel manufacturers recommend certain flow rates for optimum performance. Variable area flowmeters allow the installer and user to monitor flow economically, thus enabling critical process adjustments to be made.

Flowmeters offer an advantage over pressure gauges in filtration systems because they measure actual flow; as the filter becomes saturated (full), the flow rate drops. Just a glance at the flowmeter tells the operator if the filter needs cleaning, replacement of cartridges, or if there may be another problem, such as a tear in the filter material or a broken pipe.

In the commercial swimming pool industry flowmeters are used to measure flow through the pool’s filtration system so operators and health inspectors, can easily verify an adequate amount of water is passing through that filtration system to ensure clean swim water.


Be certain to check chemical compatibility. Don’t rely solely on compatibility charts, do your own testing. Many flowmeter manufacturers are willing to provide material sample kits so you can perform your compatibility test.

Specific gravity (‘weight’) of the liquid through the flowmeters affects flowmeter reliability and accuracy. Viscosity, the degree to which a fluid resists flow under applied force, also affects accuracy, as do elevated temperatures.

Accuracy & Repeatability

Without a good working knowledge of what these terms mean in the real world you can easily overbuy, or under buy a Flowmeter. Avoid being misled by the various terminologies used in the flow industry. Flowmeter manufactures may simply say 2% accurate, you need to question that statement, 2% of what? 2% of the indicated flow, or 2% of the full scale? These two seemingly similar accuracies are actually quite different, and could be costing you extra money. Repeatability is different than accuracy and in some industries may be more important.

Repeatability is the flowmeters’ ability to reproduce flow rates consistently under the same conditions. Repeatability is very important in the processing industry where tracking flow changes is so essential.

In summary, it is best to have a clear understanding of your flow measurement requirements and if you’re uncertain what the best flowmeter for your application is, consult an expert who can provide friendly assistance and accurate information. Blue-White has a full staff of in-house engineers to provide the assistance you may require.