New Chlorine Sensor Operates for Up to a Year Without Calibration or Maintenance

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1 New Chlorine Sensor Operates for Up to a Year Without Calibration or Maintenance Most residual chlorine measurement systems require frequent calibration, maintenance and require constant flow, pressure and ph to maintain their calibrations. A new sensor technology platform overcomes these limitations. Background Two types of chlorine analyzers; those that use a colorimeter to measure the chlorine level optically and those are based on amperometry. A third type, Oxidation Reduction Potential (ORP) does not directly correlate with chlorine. It is discussed in another White Paper: Total Residual Oxidant Measurement: ORP or Amperometry. The colorimetric units require chromogenic reagents that must be disposed of and employ a waste stream of roughly 200 ml per minute. They require frequent maintenance and generally perform poorly in an application subject to biofouling. Note: the reactions that follow are for chlorine but could also apply to bromine. When chlorine is added to water it hydrolyzes to form: Cl 2 + H2O HOCl + H + + Cl HOCl (aq) OCl (aq) + H + (aq) Figure 1: 3 electrode amperometric cell In an amperometric sensor, application of a fixed voltage occurs between two electrodes and a reaction takes place at the working electrode (cathode) where reduction of chlorine (HOCl) back to chloride (Cl ) takes place (Figure 1). In an amperometric sensor, the current that flows because of this reduction is proportional to the chlorine presented to the sensor. The figure above shows the three electrode configuration. Most membrane sensors use the two electrode method. In general, the two electrode method readings are not as stable and electrodes will not last as long as the three electrode method. Problems with State of the Art Amperometric Chlorine Sensors Virtually all of the Reagentless amperometric chlorine sensors use a Johnson style membrane and electrolyte to control the reactions at these electrodes. These systems usually require elaborate pressure and flow control. In addition, recalibration of the membrane sensor is necessary if stretching or pressure spikes occur in the system. Membrane sensors also require frequent calibration, in some cases weekly or even daily due to changing conditions. They are also subject to the loss of electrolyte as it diffuses through the membrane. Membranes must also be frequently replaced, often quarterly.

2 A recent development in sensor technology is the use screen printed membranes. This eliminates the electrolyte replenishment but introduces another limitation in lifetime due to membrane degradation. PPM Cl 2 Amperometric Signal Vs. Temperature & DPD Samples Membrane DPD CL2 Temperature Figure 2: Relationship of temperature to signal on a typical membrane style sensor. (Malkov, 2008) These sensors have a short life (approximately 6 months). Additionally, Membrane sensors (screen printed or electrolyte style) generally will not work reliably at higher ph levels (7.5 to 8.5). There is also a complex temperature correlation as seen in Figure 2 and must be recalibrated if the temperature changes significantly. The membrane in an amperometric chlorine sensor restricts the electrode to measurement of only HOCl. In free chlorine applications, a ph of 5.0 to 7.0 is the ideal operation range for a membrane sensor, due to the high percentage of hypochlorous acid (HOCl) (>80%) in the sample and the steepness of the Free Chlorine Dissociation Curve Figure 3 in this range. The ph can move within this range and the chlorine concentration can drift without significantly diminishing the accuracy of the instrument. This ph range, however, is not naturally Figure 3:Chlorine dissociation curve present in drinking water facilities. A ph of 7.0 to 8.0 is typically the normal operating range for most drinking water facilities. The HOCl concentration is much lower versus the OCl (hypochlorite ion) in this range. Another type of amperometric sensor uses bare electrodes for the chlorine measurement. Depending upon the measurement potential selected, the bare electrode sensor can measure both species (HOCl and OCl ). Although the signal response is not equal for both species, this type of sensor can operate at a higher ph than a membrane sensor and is less ph dependent as seen in Table 1 (before ph compensation is applied).

3 Method ph 7.0 ph 7.5 ph 8.0 Uncompensated HSI (Bare) 1.0 ppm 0.95 ppm 0.85 ppm Uncompensated Membrane Sensor 1.0 ppm ~0.6 ppm ~0.2 ppm DPD 1.0 ppm 1.0 ppm 1.0 ppm Table 1: Compares the uncompensated chlorine signal with the HSI sensor, a typical membrane chlorine sensor versus the DPD method. Conventional bare electrode amperometric sensors have some disadvantages though. These sensors operate with the application of a fixed potential. If used without a reagent feed to waste (with a buffered solution), as the electrode is polarized, the measurement current (chlorine signal) drops significantly during the first 24 hours. This requires additional effort by a technician to calibrate the unit the following day. Frequent inspections are necessary since these units do not have an electrode cleaning system. A common problem encountered with online measurement of water chemistry in the field is fouled electrodes in the sensor system. When the working electrode is covered with either inorganic layers (salts such as calcium carbonate) or organic layers (biofouling) that inhibit electrode processes, the system will under report the chlorine level. Many sensor systems require a reagent feed of either an iodide solution or a buffer to lower the ph to 4.0. Other reagentless systems are dependent on flow or pressure and require controlled flow with a drain to waste to provide a constant flow rate to the sensor. This requirement further complicates the installation, maintenance and logistical requirements and results in excessive water loss and unnecessary consumption. Another problem with certain sensors is the lack of a flow independent measurement method that can installed in a process flow without adverse effects from changing flow rates on the sensor signal. Changing flow rates severely affect amperometric sensors. Preferably, a sensor system should provide accurate measurements with flow rates ranging from 0 to more than 7 feet per second without an appreciable change in the sensor output signal. Direct insertion into a pipe is impossible with most sensors without severely compromising accuracy. Frequent recalibration is necessary with most commercially available sensors due to changing electrode surface, fouling, electrolyte depletion, and membrane fouling or stretching. Maintenance costs often exceed the initial cost of the system (Instrument Testing Association, 1995). ORP Measurement Conventional ORP measurement suffers from some limitations, which make it generally impractical for monitoring of chlorine residuals. It responds very slowly to insult doses. Organics can easily poison the electrode surface. It often requires hours before accurate readings return. Its logarithmic response limits its value in accurately determining the chlorine residual but can serve to corroborate the chlorine sensor reading. It can also serve a purpose in identifying potable water contamination.

4 HSI s Existing Technology Platform HSI saw the need to develop a new Chlorine Sensor Platform that uses a different approach to residual measurement that eliminates many of problems described above. Its objective was to eliminate the many factors that require recalibration in conventional amperometric sensors, allowing much longer unattended operation, up to a year or longer. Its technology platform includes many other features that make it desirable in this application. This new sensor platform) enables the measurement of five parameters with only four electrodes and a temperature sensor. This lowers the cost, improves the reliability and accuracy of the sensor by using the same electrodes and much of the same circuitry for multiple sequential measurements (patent pending). Readings for Chlorine, Conductivity, ph and ORP update every 15 seconds. Temperature updates every second. The measurements performed by this highly integrated sensor are synergistic since they verify various properties of the electrodes to confirm the validity and accuracy of the reading. For example, the conductivity cycle detects faults in sensor connections or the presence of air. Zero chlorine and high ORP indicate a sensor fault condition. With flow independent operation, on board ph compensation, self cleaning electrodes and membrane less chlorine measurement, the sensor can function for very long periods without calibration. This novel design results in a radically lower cost system with minimal maintenance requirements and long life. Fabricated using an automated coining process, the solid platinum electrodes minimizes part cost, assembly labor and waste material. They will last the life of the system. An added benefit of bare electrode operation is that the sensor can operate at much higher ph levels than membrane sensors can (refer to Table 1). All components are on a single printed circuit board with stuffing options. Robust Operation HSI has developed a system that can operate for months without calibration under the harsh conditions of seawater monitoring of bromine residuals with ±5% accuracy. One of the reasons for its success is its electrode cleaning system. The system consists of a sensor with an integral magnetically coupled pump (patent pending). The motor is a three phase Brushless DC style with a rated life of 25,000 hours. The pump is seal less hence can operate at high pressures. Flow from the internal pump provides a uniform velocity across the electrodes to enable operation from zero to seven feet per second of water velocity in a pipe. The resultant flow also motivates captive Teflon balls that abrade the surface of the electrodes and the ph sensor, keeping them free of inorganic salts that build up in hard water as well as biologicals that can form during periods of extended low or no chlorine conditions. This ensures that the measurements will be accurate, further reducing the need to recalibrate. Salts, oils and biologicals removed continuously by the cleaning system also resist adhesion by nature of their Teflon construction. The ph and chlorine sensors both share the same reference electrode (eliminating an additional electrode). One of the chlorine electrodes is used as a solution ground for the ph measurement, which

5 eliminates another electrode and renders the noise sensitive ph system virtually immune to noise and stray currents (Issued Patent). The ORP and conductivity measurements require no additional electrodes or circuitry. The single digital temperature sensor compensates the conductivity, ph and chlorine measurements. Sensor calibrations are stored on board the sensor (lower portion) enabling either insitu or ex situ calibration. An on board memory chip can store over 14,000 measurements for later download. Since measurements are sequential, the potentials are constantly changing, many limitations of the existing technology are eliminated. This is especially true with ORP measurements. ORP can be measured rapidly (5 seconds) without the poisoning effect of organics. Since HSI uses bare electrode amperometry, temperature compensation is straightforward. Notice the difficulty of signal compensation over a wide temperature range as shown earlier in Figure 2 above. In contrast, without a membrane, HSI s sensor has a linear temperature coefficient. Consequently it will be accurate over a wide temperature range since the signal can easily and accurately be corrected for changes in temperature using its on board digital temperature sensor. As also noted earlier in Table 1, the ph effect on the uncompensated chlorine signal for the HSI sensor (since it a bare electrode style) is substantially less pronounced than on the membrane style sensor. Long Life Noise Immune Differential ph A major problem with conventional Combination ph Sensors is the loss of concentration of the electrolyte gel. This is an acute problem for drinking water applications where the source is surface water with low ionic strength. Gradual dilution of gel occurs by osmosis through the porous junction. A dilution of 100 to 1 can cause a possible measurement shift of two full ph units. This requires frequent recalibration and replacement of the electrode. Since this method does not depend on a reference electrode or KCl solution, the same dilution in a Differential Electrode will result in only a 0.05 ph shift. As a result, it can last much longer than conventional ph sensors. Figure 4: A schematic diagram of HSI s patented electrical isolation technique. An additional feature lowers maintenance even further. Most sensors of this type use a liquid electrolyte as the salt bridge requiring regular replenishment (usually quarterly). The HSI sensor requires no maintenance over its life due to its solid gel electrolyte. Replacement of the ph cartridge takes place when depleted (usually after two years or more). The HSI sensor uses a grounding method that results in relative noise immunity due to a patented method of electrical isolation (Figure 4). Stray currents in the water can alter a non isolated ph system.

6 Since the ground is common with the grounds of other electrical equipment, any other circuit that touches the water can inject current into the water that could pass by the ph signal, causing an offset. This will cause an error in measurement. Since the HSI system is isolated, other signals in the water have no electrical path to ground. Other circuits that touch the water have no return path near the ph electrode. If a DC offset is present in the water, then the entire circuit will drift up and down with that offset, since there is no current path. Rapid Response ORP By sequentially measuring ORP (for 5 seconds), a potential is impressed on the measuring electrode every cycle during the chlorine measurement that keeps the electrode free of poisoning. The circuitry rapidly equalizes when de energized and the open circuit ORP cycle begins. While the ORP reading does not exactly match the reading from a static ORP sensor system, it does correlate well over a wide range of chlorine levels. Since ORP use typically qualitative and readings from HSI s sensor are consistent, this is not a limitation in this application. In testing at HSI, commercial ORP sensors often differ by 30 to 50 mv in the same solution and flow even though they read within 5 mv in Zobell Solution. Direct Connection to a PLC Designed for the direct connection to a PLC or other OEM equipment, a 4 20 ma output for both chlorine and ph are included. Addition of Modbus over RS485 is also underway. The system operates on 24VDC power. (Figure 6) Other standard features include data logging with a Real Time Clock; Optional features include a display interface and USB connection for upgrading firmware.

7 Figure 5: Sensor and connectivity options for HSI s sensor. Bibliography Emerson Process Management, Rosemount Analytical. (n.d.). Application Note: Chlorine Measurement By Amperometric Sensor. Retrieved November 5, 2010, from Water Online: Measurement By Amperometric Sensor 0001 Eutech Instruments. (2004, September 28). Differential ph Measurements. Retrieved March 7, 2011, from Eutechinst.com: Malkov, J. H. (2008, January). Amperometric Probes or DPD Analyzers: Which Is Best For On Line Chlorine Monitoring? Retrieved November 8, 2010, from Waterworld: 25/issue 1/editorial feature/amperometricprobes or dpd analyzers which is best for on line chlorine monitoring.html

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