Tech Bulletin- Water Meters, Calibrating for Success and New Podcast
Water meters and those “Krazy K FaKtors”
Is my Paddle wheel or Magnetic water meter giving me the correct Signal and ratio for flow gallons/liters flowing through it?
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 manufacturers 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 Paddle wheel, 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 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 manufacturers specification. A really high number may indicate you are getting additional feedback, the pipe diameter calculation is wrong or there was 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 paddle wheel 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 manufacturers 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 on terminals for the signal outputs from the meter. Open the flow and check the frequency. We will use that frequency to approximate the meters “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|
Why am I chasing Conductivity calibration?
Well, first off, we’re guessing because you don’t trust your controller. Get over that. We’ve said it before and we’ll say it again:
“The days of transistors and vacuum tubes is long gone!”
There is no real warm-up time, or component variation, associated with the modern conductivity controller technologies ability to measure the Total Dissolved Solids (TDS) as conductivity accurately, and consistently, over time. The Microprocessors we use have advanced A/D (Analog to digital) inputs and accurate timers that put an end to that.
The conductivity sensors don’t “physically or electronically” vary significantly over time. The temperature compensation resistor is the only item that has a variable value in the sensor itself. Everything else is a straight ahead mechanical arrangement (Red and Black wires connect directly to the back side of either Carbon tip. White and green wire connect to Temperature Compensation thermistor buried in the body of the sensor).
So why are you chasing calibration then? It’s probably the ingrained thought processes and habits that are the enemies here. When a service tech visits an account once or twice a month, one of their service items is to verify that the controller readout matches the measured hand held TDS reading.
Easy enough. A quick tower water sample into the Myron-L (or whatever brand you use), compare it to the reading displayed, then immediately calibrate the controller to the handheld and gripe that it’s off “every time”. That is a cycle of failure.
To get off this spiraling path of chasing calibration, you will first need to see what the controller is really seeing. Do this by initializing the controllers calibration to clear out the old junk.
- Got to main menu (Press CLR button)
- Drop down in main menu system to “System Setup”.
- Press enter.
- Go to the “Initialization” option.
- Press enter.
- There are two sub options. “Calibration or Whole controller”. Choose calibration Initialization.
- When it asks are you sure, select “yes” (Press 1)
You will now see what the sensor is seeing in a raw state. All old calibrations are gone.
What does it read? Is it within 15% of your actual? If it is, you can adjust the calibration using the “PRO” button from the main process screen.
If it is not within that 15% limit, DO NOT JUST ENTER A NEW NUMBER to compensate for a huge error. Why, you might ask? Well, we need to talk about HOW conductivity is measured by the instrument.
The first thing that must be understood is that “TDS” is measured in conductance, or micromhos (μmhos). Conductance (how easily electrical signals conduct through a medium, like the dissolved salts in cooling tower water) is the inverse of resistance. So a High Conductance is a Low Resistance.
The second thing is that the probes are designed to allow a consistent TDS measurement of the fluid passing between their tips (What we call a cell constant). A fluids resistance value is generated by passing a known signal (amplitude and voltage) through the fluid. That read resistance value is inverted to be used as conductance for convenience, since TDS is the water treater’s controllable concern. Using the chart below, we can see that 1000 μmhos of conductance is actually the equivalent of 1000 ohms of resistance across the fluid from tip to tip. So, 4000 μmhos of conductance must be the same as 4000 ohms of resistance, right?
Actually, 4000 μmhos of conductance is the equivalent of 250 ohms of resistance. This chart is representative of the equivalencies for a standard Lakewood Cooling tower sensor.
|Conductivity μmhos =||Resistance (ohms)|
Why is this important? Look at the chart again. The difference between 3000 and 4000 μmhos conductance in the water is actually about 83 ohms. So, at that higher conductivity, 83 ohms = 1000 μmhos. Now look at the difference between 500 and 1500 μmhos. 1333 ohms =1000 μmhos. At a lower conductance, the resistance change needed to get the swing is quite a bit more.
What does that imply in how we end up chasing calibration? Look at the chart below. If you measured 4000 μmhos of TDS with your handheld, but the initialized Controller calibration (from the steps above) shows 2500 μmhos, you are inclined to enter the handheld value of 4000 μmhos as the gospel. But look what the equivalency did to the accuracy of the controllers reading across the scale.
As TDS concentration accumulated after the calibration, we can see a proportions problem develop. If the ACTUAL seen value by the sensor went up by 350 μmhos (a 50 ohm difference on the Factory Cond Curve) the altered value you entered in the “Cond calibration” curve would make the control value move to 4600 μmhos (+600 μmhos) for that same 50 ohm change.
Remember, you told it that its actual 2500 μmhos reading at the sensor was just over half what it should be reading, so now the multipliers and scale are moved to accommodate your change. You’re the human being, so you’re in charge. Even when you inject a problem.
This new calibration scale would mean that a slight swing in actual value would initiate blowdown WAY to early and for too long.
As you made up water, it would take less lower conductance (High resistance) to drop the reading to get it to shut off again. A few ohms looks like a big swing low, so the valve shuts early. You get hammer, valve action and are really not in control. Also, because the actual cause of the readings differential weren’t addressed, your customer will chase the calibration the next time they check the reading.
Now comes the bad news. If the problem were aeration, or a grounding issue, that hadn’t been identified/rectified, and that variation went away, the sample suddenly looks VERY conductive (High TDS) doubling the “read” conductivity value to 6000+ μmhos for that same 4000 TDS water. That means we would blow down the tower until it hit 4000 μmhos calibrated value (what’s really 2600 μmhos)…and that’s a lot of water and chemical down the drain.
How do you fix the underlying issue? Well, that’s a different (and site specific) conversation. PROPER and frequent Sensor cleaning (yes…every visit. Every time), reduce or eliminate sample line Aeration due to plumbing or cavitation, and be ready to chase electro interference from poorly grounded equipment. All are fixable, but must be identified. We hope that after reading this, you should have a better grasp of why “calibrating every time” is not a solution, it’s a band-aid… and not a very good one. The road to solving a long-term issue begins with initializing the calibration and SEEING the real issue for the first time.
Presented by Trace Blackmore
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