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Getting the Job Done OVERKILL COSTS 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. A  FEW APPLICATIONS DISPENSING SYSTEMS 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. WATER TREATMENT SYSTEMS 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 chemical must 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).  FLOW VERIFICATION 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. FLOW RATE AND RANGE ALARMS Paddlewheel flowmeters can monitor  system flow rate.   FLOW RANGE ALARM 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.  INSTALLATION REQUIREMENTS 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 DISTURBANCES 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 NUMBER EQUATION 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.   HOW PADDLEWHEEL  FLOWMETERS WORK 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. ABOUT THE AUTHOR 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, For permission to reprint article, please send a request .