Chasing Calibration

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.