Is my Paddle wheel or Magnetic water meter giving me the correct Signal and ratio for flow gallons/liters flowing through it? We’ll walk you through water flow meter calculation.
Well, here is a test you can do in the field.
- You’ll need a multimeter with the “HZ” or Hertz (frequency) measurement feature (like the model 83 Fluke shown to the left) it is typically a specific button (arrow).
- You’ll need to know the “K-factor” of the meter being used.
- You’ll need to know the approx. gallons per minute flowing through the pipe (and meter) at the time the valve is open or flow is on.
The “K-Factor” of a water meter is the manufacturer’s way of describing liquid flow through their meter. A K-factor of “100” means that there are 100 “pulses” produced for each gallon of water. This means the little magnet in the paddle, or Toroidal shifting signal is creating that number of peak to peak waves for each gallon of water that passes through or over them.
That K factor will vary on the Paddlewheel, or magmeter, output depending on the pipe size, but the manufacturer does have a number that you use based on pipe diameter. It is also the number that you program into the controller to accurately measure it.
If you are having problems and the gallons rate/accumulation is significantly off (high or low) you can do some math and a test with the “HZ” equipped meter (shown above) to figure out if the signals are good.
Using the same K-Factor from above, do a fill test with a 5-gallon bucket (or two), and we might see that the pipe, wide open, produces 10 gallons in a minute. Now for the math.
- For ten gallons a minute, we should have produced 1000 pulses (100 per gallon*10) in a minute
- Divide that number by 60 to get actual pulses per Second (ppS) when in operation. That would be 16.6 in this case.
- Guess what? 16.7 ppS is the same as a 16.7 Hz signal (about a fourth of the frequency of the AC current that powers the unit and your home).
- To test that you are getting that, place the positive lead (+) of the “HZ” capable handheld on the “signal” terminal from the water meter on your controller terminals and the Negative lead (-) on the water meter signal ground connection in your controller.
- Open the valve, or whatever allows flow past the water meter, and keep it open while you go to the handheld meter.
- Press the HZ (Hertz) function on the meter and it should read 16.7.
Any variation greater than 5% could indicate an issue. The K-Factor could be off (Hi or low), or there could be interference if the water meter wire was run through conduit with 115/230 VAC. A really low number may indicate low signal strength and require the use of a “Pull up resistor” per the meter manufacturer’s specification. A really high number may indicate you are getting additional feedback, the pipe diameter calculation is wrong or there were not enough straight pipe lengths before the meter was installed.
Let’s do the math again with another K-Factor (and odd number).
This time, the manufacturers indicated K-Factor is 537.8 for the diameter of the pipe the paddlewheel water meter is installed into.
The “known” flow is 8 GPM.
|GPM * K factor= number of pulses per minute (ppM):||8 * 537.8=4,302.4 ppM|
|ppM/60 = pulses per Second (ppS) :||4,302.4 / 60= 71.7 ppS|
|ppS= Hertz:||71.7 ppS= 71.7 Hertz with the valve fully open|
If the Hertz reading is off by more than 5%, then the K factor is probably incorrect for that meter OR the installation is creating pulsing problems.
Now, the cool thing to do is follow the computations in reverse to determine a few other things. If we measure the Hertz and know the gallons, we can input the correct (or approximate) unknown K Factor for a meter. This may be helpful for when people use our Autotrol meters on Non-Lakewood equipment or those manufacturer’s equipment where they have not done the testing to “capture the curve”.
Have a couple of 5-gallon buckets ready that can capture the “operational flow” of the line the water meter is in. Time the amount of time to fill the two buckets. So, if it took 20 seconds to fill 10 gallons, then the GPM is actually 30 GPM. We get that by dividing the time it took to fill the buckets INTO 1 minute and then multiplying that result and the accumulated gallonage we used as a standard.
So: (( 60/20)*10 Gallons)= 30 GPM
So, we now know we flow at 30 GPM. Place the “HZ” capable meter on the terminals for the signal outputs from the meter. Open the flow and check the frequency. We will use that frequency to approximate the meter’s “K-factor”. As an example, if the frequency read with the valve open is “100 Hz” for the above example, we can reverse the math like this:
|Hertz= ppS:||100 HZ with valve fully open= 100 ppS|
|ppS *60=”pulses per Minute”:||100* 60= 6000 ppM|
|number of pulses per minute (ppM)/GPM= “K factor”:||6000/30= 200″K Factor” under full flow|
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