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Frequently Asked Questions - Flow Metering
  • What kind of flowmeter will work for gravity flow, partially filled pipe applications?
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    • An open channel flowmeter is the only truly reliable way to measure low velocity gravity flow discharges that are usually partially filled pipe applications. The use of an open channel flowmeter (OCF) requires some type of primary measuring device such as a flume or weir. In such a device, the rate of flow is directly proportional to the level of the water moving through the flume or weir. The OCF measures flow by being programmed to recognize the type and size of the flume or weir (usually done by simply selecting from a built-in menu) and then converting that level to a rate of flow.
      USA BlueBook offers two different brands of open channel flowmeters: Greyline & GLI International. We also offer stock Parshall, Palmer-Bowlus and trapezoidal flumes.
  • What are the advantages and disadvantages of an insertion-type flowmeter?
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    • Insertion meters typically have two parts: a flow sensor that is installed in the pipeline through a saddle, threaded inlet or special in-line fitting and the flow computer that displays the rate of flow and totalizes.
      The advantages are many. Insertion meters are very cost effective relative to pipe size, especially in 4" or larger pipes. This is because two sizes of insertion sensors can cover from 2" to 48" pipes. These meters are also very easy to install. The flow sensors can be mounted in vertical or horizontal pipes and normally only require a 1-1/2" or 2" NPT female inlet to the pipe. A hot-tap version of a flow sensor allows installation on a pressurized line as well as easy service without depressurization. Insertion meters also offer flexibility of use, since at any time a unit can be removed from one installation and reinstalled on another different size pipeline by simply reprogramming the flow computer. The various flow computer/display/transmitters offer many features such as 4-20 mA output, scaleable pulse outputs, programmable relays and multiple inputs. Finally, these meters are easy to service - the flow sensor can usually be completely rebuilt in a few minutes without special tools.
      The primary disadvantage is that insertion-type meters are not usually acceptable for billing purposes. Although these meters are very accurate, there is no AWWA standard and it is difficult to do precise testing and calibration as you would with a standard turbine meter that has test ports on it.
  • How can I measure the effluent from my plant, which flows by gravity to a lagoon through a pipe that is not full?
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    • There are two ways to measure gravity flows in partially full pipe. The most common is with an Open-Channel Flowmeter (OCF). This meter requires a primary measuring device such as a flume or weir. The OCF is essentially a level device that calculates flows based on the characteristics of a particular flume or weir configuration. If you already have a standard flume or weir on your pipeline the OCF is the best choice, otherwise you will have to consider the cost of installing such a primary device.
      The other option is the Area Velocity Meter or AVM that is newer technology and gaining popularity. The AVM has a sensor unit that installs in the pipeline (6" or larger) and calculates flow by determining the depth and the velocity of the water in the pipe along with the area of the pipe cross-section. The advantage of the AVM is that no primary device, flume or weir, is required.
      There are other pros and cons to each type of flowmeter, so evaluate your system and ask questions before purchasing.
  • How is an area velocity flowmeter (AVM) different from other flowmeters and where would I use one?
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    • First, an area velocity flowmeter is one of the few flowmeters that will measure flow in a partially filled pipe or channel. This is perfect for gravity flow systems with possible solids in the water. The AVM can be easily installed in the pipe at a manhole or even at the final discharge point. The AVM works by measuring two parameters: velocity and water depth in the pipe. The combination of these two measurements along with the pipe ID results in a fairly accurate flow measurement. The Greyline AVFM is unique in that an ultrasonic sensor rather than a pressure sensor does the level measurement. It is the pressure-sensing device that has plagued many of the earlier AVM’s offered by other manufacturers.
  • Can I install my positive displacement meter on a vertical pipe?
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    • Virtually all positive displacement meters require that they be installed on a horizontal plane. Installing these meters horizontally eliminates possible accuracy and excessive wear problems associated with a vertical installation. If the only pipe available is a vertical pipe, a meter in-setter is often used. There are a variety of pipe connection options available for the different pipe materials, sizes and types often used in household plumbing. Most can accommodate meters up to 1-inch in size. If you already have a meter installed on a vertical pipe, a vertical to horizontal meter re-setter is often the solution. These devices have meter threads on the vertical portion of the unit to fit in place of the existing meter. They also have meter couplings to install the meter horizontally.
  • How do I test my remote mechanical register to make sure it is working properly?
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    • The easiest way is test a mechanical remote is with a common 9-volt transistor battery. By removing the wires at the meter and gently brushing the two wires across the terminals of the battery, you are sending an electrical pulse through the wires energizing the remote and advancing the reading. In addition, you are testing the continuity of the wire as well. If the numerals do not advance, go to the remote and try the same test again attaching two short wires to the terminals on the remote. If the numerals advance, the wire from the meter to the remote will need to be changed. If the numerals do not advance, the remote will need to be changed.
  • What is the general rule to follow when determining the proper installation location of a flow meter to obtain the greatest accuracy?
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    • For the best accuracy results, the rule to follow is simple. Choose a place in the line to install the meter so that there is ten pipe diameters of straight pipe before (upstream) and five pipe diameters of straight pipe after (downstream) the meter location. These lengths of pipe provide adequate time for the water to straighten out and pass uniformly through the meter and beyond, eliminating turbulence that may affect accuracy.
  • What is the “K” factor when used with insertion-type paddlewheel meters?
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    • The “K” factor is used to properly process the pulses from the flow sensor to the monitor device. It is the number of pulses the sensor sends per unit of fluid passing through the pipe. The number is different for each pipe size and pipe material the sensor is used with. These numbers are determined through extensive testing by each meter manufacturer.
  • How do I determine the meter coupling size I need for my meters?
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    • In the smaller household meters such as 5/8" x 1/2" and 5/8" x 3/4" sized meters, many manufacturers have 5/8 in the casting of each of the meter sizes, which makes it a little confusing. The bore is 5/8", but the diameter of the threads is different between the two meter sizes. By measuring across the meter threads at the meter opening, you can easily determine the proper coupling size. The diameter of the meter threads of a 5/8" x 1/2" meter will be 1" and they are usually installed on a 1/2" water service line. The diameter of the meter threads on a 5/8" x 3/4" meter is 1-1/4", and they are usually installed on a 3/4" water service line.
  • I can’t or don’t want to cut my pipe. Which meter type can I use for dirty or wastewater and which meter can I use for drinking water?
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    • With the single sensor mounted on the outside of the pipe, the meter of choice for a dirty or wastewater application is a Doppler meter. The meter requires solids or bubbles in the liquid with a minimum size of 100 microns and a minimum concentration of 75 parts per million. The Doppler signal is bounced off of these particles to determine flow rate within the pipe. These meters work well on pipes made of PVC, carbon steel, cast iron, fiberglass and lined pipe. Doppler signals cannot transmit through pipe walls, which contain air pockets like wood or loose pipe liners with an air gap between pipe liner and pipe wall.
      For a drinking water quality application, the transit time flow meter is a good choice. They feature two non-intrusive sensors fixed to the pipe at pre-determined spacing based upon pipe diameter and liquid characteristics. The signal is transmitted between the sensors in the direction of liquid flow and then against the liquid flow. Since sound travels faster with the liquid flow than against it, a time differential is determined. The sound’s time of flight is accurately measured in both directions to calculate the flow rate. When the time of flight in both directions is the same, the flow meter reads zero flow. These meters work better in cleaner liquids where particulates do not interfere with the signals being transmitted. These meters work on a wide variety of pipe materials.