The Free Chlorine Equivalent Measurement:
A New Way to Monitor Free Chlorine

It's fast, accurate and easier to use than colorimeters and DPD test kits

By Richard Spahl, Myron L Company

The Ultrameter digital handheld instruments feature innovative, one-touch FCE free chlorine measurements, as well as a comprehensive array of other critical water quality parameter measurements.

Chlorine tests based on the measurement of free available chlorine (FAC) concentration under forced conditions can give a misleading picture of the effects of the chlorine. Changing pH or the addition of chemicals like cyanuric acid can radically alter its power as a disinfectant and its ability to destroy or degrade membranes. Accurately determining these effects therefore requires a test method based on the chemical reactivity of the chlorine, rather than on its concentration.

The Myron L Company has developed an innovative new way to report FAC quickly and accurately based on the measurement of ORP, the chemical characteristic of chlorine that directly reflects its effectiveness, cross-referenced with pH:

  • It provides an empirical measurement that does not require interpretation.
  • It is not affected by water color or turbidity.
  • It measures the true chemistry of the water, unaltered.
  • It accounts for changes in pH.
  • It reports the effects of CYA on disinfecting power.
  • It can be used to monitor non-chlorine oxidants.

FCE - How and Why it Works
The Myron L Company FCE feature cross-references ORP values with pH levels to automatically arrive at a concentration value for FAC that reflects the effect of pH on the ratio of HOCl to OCl-. This correlation is derived from a series of experiments in which exact amounts of chlorine (in the form of laboratory grade bleach: 5% NaOCl; 95% H2O) were added to de-ionized water in a closed system, thus controlling and excluding possible interferants. By using both a pH measurement and an ORP measurement, FCE can determine the relative contributions of HOCl and OCl- to the final ORP value and factors them into a final concentration calculation. (See Figure 1).

Similar experiments were performed using water to which precise amounts of calcium chloride (CaCl2) and sodium bicarbonate (NaHCO3) were added to slightly buffer the water. This allows the FCE feature to correlate low ORP values to the typically low FAC concentrations of tap water after it has been in the municipal water system for several days.

FCE - pH Included, Not Ignored
Unlike other FAC test methods that ignore the effect of pH on sanitizing power by artificially forcing the pH of their test sample to a single value, Myron L Company's FCE includes pH in its concentration calculation. This capability gives Myron L Company's FCE the ability to compensate for the effect of the changing ratio of chlorine species as pH changes, resulting in a FAC concentration value germane to the actual sanitizing power of the source water. OCl- is measured as OCl--, and HOCl is measured as HOCl. Those users who are primarily concerned with or who prefer free chlorine concentration levels have a reliable measurement that gives consistent and comparable results, reading to reading, without having to rigorously control or artificially manipulate the sample's pH.

Figure 1 – Sample Experimental Data Relating FAC ppm to ORP and pH

Figure 2 – Example of ORP Sensor Response Study

In addition, because the FCE function displays both the FAC concentration and a predicted, stable ORP value, the user can, by comparing these two values from successive measurements, track how ORP (and disinfecting power) falls as pH rises and how ORP rises as pH is lowered when concentration is constant.

FCE - Chemistry Measured
Both DPD kits and colorimeters may tell the user the FAC concentration of the sample in the test tube, but since the chemistry of that sample is quite different from the source water being analyzed, the results are imprecisely related to actual disinfection power. DPD kits and colorimeters only imply true disinfecting power; they do not measure it, and that is, after all, the whole point of the exercise.

The Myron L Company FCE method avoids these pitfalls and inaccuracies. FCE measures the real, unaltered chemistry of source water, including moment-to-moment changes in that chemistry.

The following controlled study shows exactly how differently the two methods respond, particularly at the high end where the effects of changes in pH are the greatest. In this study measurements were made with a digital colorimeter and a Myron L Company Ultrameter II 6P equipped with FCE. The solutions tested were made with various known concentrations of NaOCl in de-ionized water. The water was heated to above 80°C to remove any CO2 and, therefore, avoid interference from REDOX reactions between HOCL, OCl- and carbonates (HCO3).

In this study (See Table 1) as the pH rises and the ratio of OCl- to HOCL rises dramatically, the FCE is able to accurately track the changing concentration of FAC. The colorimeter's results do not.

Table 1 – Comparison of FCEto Digital Colorimeter

FCE - Handheld ORP Accuracy
The Myron L Company FCE function includes a pioneering feature that dramatically reduces the wait for stable ORP readings. This unique feature determines an extrapolated, final, stabilized ORP reading within 1 to 2 minutes rather than the typical 15 minutes or hours for other ORP systems.

The Predictive ORP feature's calculations are based on a model of sensor behavior developed through a series of experiments that measured the response time of a representative sample of ORP sensors over a range of controlled chlorine concentrations. The results of this set of experiments revealed that the shape of the curve is very similar for various ORP levels differing only in the initial starting point and the final stabilized reading. (See Figure 2).

Using a proprietary curve-matching algorithm, the Predictive ORP feature determines what point along the typical sensor response curve a measurement occurred and extrapolates an appropriate final reading. This extrapolated value is used to calculate the FCE ppm value without having to wait for the sensor to stabilize and is also reported directly to the instrument's display.

For more information contact:
Myron L Company
2450 Impala Drive | Carlsbad, CA 92010-7226
Tel: 1-760-438-2021