ORP Versus Amperometry
- 홈
- ORP Versus Amperometry
Total Residual Oxidant Measurement: ORP or Amperometry
There are two available methods for Total Residual Oxidant (TRO) measurement: Oxidation Reduction Potential (ORP) and amperometry. Until now, there have been no amperometric sensors that have been practical in a ballast water application. This paper will examine the relative benefits and limitations of both methods.
ORP 측정은 물 모니터링에 사용할 수 있는 가장 저렴한 방법입니다. 그것은 주로 수영장 산업뿐만 아니라 시안화물 파괴 (도금 및 광산 산업)와 같은 일부 틈새 응용 프로그램에서 사용됩니다. 그것은 자주 질적 지표로 사용됩니다. 상업용 수영장에서염소 수유 장비를 제어하는 데 사용되는 경우 대부분의 작업자는 종종 매일 염소 수준을 수동으로 측정합니다. 높은 수준의 유기 분자의 존재는 며칠 내에 센서를 파울할 수 있으며 청소가 필요합니다.
ORP stands for oxidation-reduction potential, which is a measure, in millivolts, of the tendency of a chemical substance to oxidize or reduce another chemical substance. Oxidation is the loss of electrons by an atom, molecule, or ion. The electrons lost by the atom in the reaction cannot exist in solution and have to be accepted by another substance in solution. So the complete reaction involving the oxidation will have to include another substance, which will be reduced Figure 1.
THE MEASUREMENT OF ORP
An ORP sensor consists of an ORP electrode and a reference electrode, as a Voltmeter measures the difference in potential (voltage). The principle behind the ORP measurement is the use of an inert metal electrode (platinum, sometimes gold), which, due to its low resistance, will give up electrons to an oxidant (chlorine in this case) or accept electrons from a reductant (sulfur dioxide in a dechlorination process) . The ORP electrode will continue to accept or give up electrons until it develops a potential, due to the build up charge, which is equal to the ORP of the solution. The typical accuracy of an ORP measurement is ±5 mV. This further complicated by the fact that different probes from the same manufacturer often will have a 20 to 50 mV difference in the same water sample.
An ORP sensor consists of an ORP electrode and a reference electrode, as a Voltmeter measures the difference in potential (voltage). The principle behind the ORP measurement is the use of an inert metal electrode (platinum, sometimes gold), which, due to its low resistance, will give up electrons to an oxidant (chlorine in this case) or accept electrons from a reductant (sulfur dioxide in a dechlorination process) . The ORP electrode will continue to accept or give up electrons until it develops a potential, due to the build up charge, which is equal to the ORP of the solution. The typical accuracy of an ORP measurement is ±5 mV. This further complicated by the fact that different probes from the same manufacturer often will have a 20 to 50 mV difference in the same water sample.
Note: manufactures test their sensors in Zobell Solution which contains a high level of redox couples. In this solution sensors will read very close to each other. That is not the case with real world samples in drinking water.
ORP Electrodes are Easily Poisoned
Figure 2 below illustrates the results on an experiment in 300 gallon spa using bromine as the sanitizer. An ORP Sensor was installed as a monitor (not controlling). A bromine generator with an amperometric sensor was also installed to control the sanitizer level. Synthetic perspiration was then added to the spa. The red line is the bromine level that the amperometric system measured throughout the test. The peaks in the blue line represent the time that the bromine generator was energized to satisfy demand. The synthetic perspiration (White, 1992)created a significant demand that persisted throughout the test, requiring the bromine generator to operate for about two hours at a time. After about 12 hours the ORP sensor, represented by the green line, registered negative values. The most likely cause was poisoning of the electrode. It did not recover from the condition for 29 hours. This would have caused a massive over chlorination or over bromination of the spa, had it been controlling the sanitizer.
In this test, synthetic perspiration was added to a hot tub with an electrolytic bromine generation unit. As can be seen above, the green line (ORP) dropped to a negative value before returning to normal after over 29 hours. (Silveri, 1999)
Baseline (zero chlorine) Level Varies with Different Water
As can be seen from the graph in Figure 3, five different water samples have a different ORP baseline that results in a higher ORP for the same chlorine level. The results vary by almost 200 mV. According to WHO, “there is a wide variation between 720 mV in different waters (1 ppm to 15 ppm chlorine) due to varying baseline ORP (zero chlorine)” (World Health Organization, 2006)
With amperometric sensors, zero current is always zero chlorine, so no zero calibration is needed. It should be noted that this comparison was in tap water. Baseline ORP of seawater ORP can range from -275 to 350 mV greatly exacerbating the baseline problem. (Cohrs, 2004)
The lower oxidation potential of bromine compared to chlorine means that ORP will not be as sensitive to the concentration as it will with chlorine. This also means that the potential from an ORP sensor will be closer to the baseline level (zero bromine level).
ORP를 갖춘 농도 측정
염소 농도 측정에 ORP를 사용하는 제한사항은 다음과 같습니다.
잠재적 인 측정에 ORP의 관계를 제어하는 Nernst 방정식에 따르면, 농도의이 logarithm을 곱하는 계수는 -59.16 mV와 동일하며 반 반응 (n)에서 전자의 수로 나뉩니다. 이 경우 n = 2; 따라서 계수는 -29.58입니다. Cl-, HOCl, H+의 농도가 10배 변경되면 ORP ±29.58mV만 변경됩니다. (에머슨 공정 액체 사업부, 2008)
ORP는 염화물 이온 (Cl-) 및 pH (H+)에 따라 달라집니다 그것은 하이포 클로산 (물에 염소)를 수행하는 만큼. 염화물 농도 또는 pH의 변화는 ORP에 영향을 미칩니다. 따라서 염소를 정확하게 측정하려면 염화물 이온 및 pH를 높은 정확도로 측정하거나 일정한 값으로 신중하게 제어해야 합니다.
측정된 밀리볼트로부터 저혈당 농도를 계산하기 위해 측정된 밀리볼트는 10의 지수로 나타납니다. ORP 측정의 일반적인 정확도는 ±5mV입니다. 이 오류만으로도 계산된 hypochlorous acid 농도가 ±30% 이상 감소하게 됩니다. 참조 전극 또는 ORP 분석기의 드리프트는 이 오류에만 추가됩니다.
온도가 있는 ORP의 모든 변화는 보상되지 않으며, 파생 농도의 오차를 더욱 증가시다.
사실상 모든 ORP 반 반응은 하나 이상의 물질을 포함하고, 대다수는 pH 의존성을 가지고 있습니다. 농도에 대한 ORP의 로그자리트 의존성은 측정된 밀리볼트의 오류를 곱합니다.
ORP 전극은 쉽게 독극되어 제거및 세척하지 않는 한 한 번에 몇 시간 동안 쓸모없게 됩니다.
바닷물 전기 염소화에서 ORP를 사용하면 대부분의 고유 문제를 확대합니다.
Zero calibration with ORP is difficult since different waters or contaminants with change the baseline. In seawater the baseline can range from -275 to 350 mV.
A logical conclusion from the foregoing is that ORP is not a good technique to apply to concentration measurements.
ORP를 갖춘 농도 측정
염소 농도 측정에 ORP를 사용하는 제한사항은 다음과 같습니다.
잠재적 인 측정에 ORP의 관계를 제어하는 Nernst 방정식에 따르면, 농도의이 logarithm을 곱하는 계수는 -59.16 mV와 동일하며 반 반응 (n)에서 전자의 수로 나뉩니다. 이 경우 n = 2; 따라서 계수는 -29.58입니다. Cl-, HOCl, H+의 농도가 10배 변경되면 ORP ±29.58mV만 변경됩니다. (에머슨 공정 액체 사업부, 2008)
ORP는 염화물 이온 (Cl-) 및 pH (H+)에 따라 달라집니다 그것은 하이포 클로산 (물에 염소)를 수행하는 만큼. 염화물 농도 또는 pH의 변화는 ORP에 영향을 미칩니다. 따라서 염소를 정확하게 측정하려면 염화물 이온 및 pH를 높은 정확도로 측정하거나 일정한 값으로 신중하게 제어해야 합니다.
측정된 밀리볼트로부터 저혈당 농도를 계산하기 위해 측정된 밀리볼트는 10의 지수로 나타납니다. ORP 측정의 일반적인 정확도는 ±5mV입니다. 이 오류만으로도 계산된 hypochlorous acid 농도가 ±30% 이상 감소하게 됩니다. 참조 전극 또는 ORP 분석기의 드리프트는 이 오류에만 추가됩니다.
온도가 있는 ORP의 모든 변화는 보상되지 않으며, 파생 농도의 오차를 더욱 증가시다.
사실상 모든 ORP 반 반응은 하나 이상의 물질을 포함하고, 대다수는 pH 의존성을 가지고 있습니다. 농도에 대한 ORP의 로그자리트 의존성은 측정된 밀리볼트의 오류를 곱합니다.
ORP 전극은 쉽게 독극되어 제거및 세척하지 않는 한 한 번에 몇 시간 동안 쓸모없게 됩니다.
바닷물 전기 염소화에서 ORP를 사용하면 대부분의 고유 문제를 확대합니다.
Zero calibration with ORP is difficult since different waters or contaminants with change the baseline. In seawater the baseline can range from -275 to 350 mV.
A logical conclusion from the foregoing is that ORP is not a good technique to apply to concentration measurements.
전류 측정
In an amperometric sensor, a fixed voltage is applied between two electrodes and a reaction takes place at the working electrode (cathode) in which chlorine is reduced from chlorine (HOCl) back to chloride (Cl-) Figure 4. This is reverse of what takes place in the chlorine generator where chlorine is generated at the anode. In an amperometric sensor the current that flows as a result of this reduction is proportional to the chlorine presented to the sensor. The figure above shows the “three electrode” configuration. Most membrane chlorine 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.
When chlorine is added to water it hydrolyzes to form:
Cl2 + H2O → HOCl + H+ + Cl-
HOCl (aq)OCl- (aq) + H+(aq)
It is usually hypochlorous acid (HOCl) that is measured by the amperometric membrane sensor
로지스믹 VS. 선형 신호 관계
As may be seen from Figure 5, in amperometric systems, the relationship to chlorine (or bromine) is linear vs. the logarithmic relationship for ORP. Any attempt to control bromine at 2 to 4 ppm will not result in very tight control since there is very poor resolution in that range when bromine is the oxidant.
Problems with use of orp in swimming pools
Cyanuric acid is widely used in virtually all outdoor swimming pools to prevent loss of chlorine due to ultraviolet light (sunlight). The most common form of stabilized chlorine is contains over 50% cyanuric acid by weight. As a result, in one season, cyanuric acid levels can increase rapidly, often exceeding 200 ppm. Levels above 300 ppm are often encountered in Arizona, California and Nevada. At levels of around 350 ppm, the ORP electrode is rapidly poisoned and must be cleaned every three days or so. There is no practical way to remove cyanuric acid other than draining some or all of the water. ORP systems cannot be used with high levels of cyanuric acid (over 40 ppm) while Amperometric systems can be used with Cyanuric Acid levels over 200. According to one ORP controller manufacturer swimming pools should not be run with more than 40 ppm of cyanuric acid even though most health departments allow up to 100 ppm and some even allow up to 200 ppm of cyanuric acid. This results in higher costs due to draining a portion of the water. The reason for this recommendation is, according to the company, that ORP increases when the sun goes down as the cyanuric acid. This allows the chlorine level to drop since the controller thinks there is more chlorine than there really is. The next morning when the sun rises the controller detects dangerously low levels of chlorine enters an alarm mode (when a condition of < 200 mV is detected) and shuts down the feed system, detecting a problem that requires intervention. As a result, commercial pools have to spend more money on water, in short supply in many areas, and suffer from needless control problems. HSI’s sensor is the only amperometric sensor that can be used in this application with high levels of cyanuric acid.
problems with orp in wastewater
A report comparing ORP to Amperometric sensors had the following comments regard ORP’s use in wastewater treatment plants:
“Ability to Provide Information to Meet Effluent Coliform Requirements” criterion, a score of two [out of 5] was determined. This score is lower than those given to the residual technologies for this same criterion because of questions raised in Chapter 9.0 that indicate ORP is not always correlated to microbial kill or may not operate successfully in all wastewater applications. Initial evidence suggests that: 1) some chemicals other than chlorine can elevate ORP levels but not produce effective microbial kill, 2) other chemicals can interfere with ORP function, and 3) that ORP may not be an effective solution for all wastewater applications based on the issues raised by points 1) and 2).
The authors point out, however, that some wastewater treatment plants have successfully used ORP and it does play a role in providing qualitative assessments of water quality.
“The ORP technology was given a criteria score of 2 for the criteria “Ability to Provide Process Control System Reliability”. As discussed in Chapter 7.0, there are concerns on how to check if ORP sensors are out of calibration. The inability to effectively predict or determine when an ORP analyzer goes out of calibration can impact process control stability.“
“Based on results from this work it was found that lab ORP probes couldn’t be used effectively to check field ORP calibrations because the lab units are easily fouled or poisoned by wastewater components (See Chapter 7.0).”
“Another concern is that ORP sensors from different manufacturers respond differently to variations in chlorine speciation.” (Damon S. Williams Associates, LLC, 2004)
In summary
In amperometric systems, the relationship to chlorine (or bromine) is linear vs. logarithmic for ORP. ORP will not be precise, require greater training, monitoring and could result in excessive corrosion of components or tanks, often defeating the purpose or its use in the first place.
Amperometric 시스템은 실제로 브롬을 측정 (TRO) 하지 다른 매개 변수 (redox), 따라서 더 정확 할 것이다.
염소 제로는 항상 0이므로 amperometric 시스템에서는 제로 보정이 필요하지 않습니다.
AMPerometric 시스템의 전극은 ORP 전극이 있기 때문에 유기체에 의해 쉽게 중독되지 않습니다.
ORP is both a good qualitative indicator and a poor quantitative method.
ORP 결론
The disadvantages of ORP all relate to routine equipment maintenance, calibration, electrode poisoning, presence of multiple redox couples, and very small exchange current that are logarithmically related to the analyte concentration. Or in other words, the assumption of a reversible chemical equilibrium, fast electrode kinetics, and the lack of interfering reactions are essential for chemical interpretations of the ORP potentials. Unfortunately, these conditions rarely, if ever, are met in real world systems (Kissinger, 1996). ORP can be used for qualitative analysis.
Halogen systems’ rapid response ORP
Halogen Systems included ORP measurement in its multi parameter sensor as an indicator for contamination. While the sensor’s chlorine measurement can detect chlorine from .05 to 15 ppm, it cannot detect the difference between contamination and water in which the chlorine residual was recently depleted. ORP can potentially fill in this gap by identifying a contamination event or cross connection by indicating a drop below the baseline ORP level. In addition, HSI’s measurement technique and self-cleaning system actually eliminates poisoning issues, providing a more reliable reading. While HSI’s ORP measurement may have a slight offset from other sensors but provides a reliable, qualitative measurement that overcomes many of the ORP problems.
References
Total Residual Oxidant Measurement: ORP or Amperometry
There are two available methods for Total Residual Oxidant (TRO) measurement: Oxidation Reduction Potential (ORP) and amperometry. Until now, there have been no amperometric sensors that have been practical in a ballast water application. This paper will examine the relative benefits and limitations of both methods.
ORP 측정은 물 모니터링에 사용할 수 있는 가장 저렴한 방법입니다. 그것은 주로 수영장 산업뿐만 아니라 시안화물 파괴 (도금 및 광산 산업)와 같은 일부 틈새 응용 프로그램에서 사용됩니다. 그것은 자주 질적 지표로 사용됩니다. 상업용 수영장에서염소 수유 장비를 제어하는 데 사용되는 경우 대부분의 작업자는 종종 매일 염소 수준을 수동으로 측정합니다. 높은 수준의 유기 분자의 존재는 며칠 내에 센서를 파울할 수 있으며 청소가 필요합니다.
ORP stands for oxidation-reduction potential, which is a measure, in millivolts, of the tendency of a chemical substance to oxidize or reduce another chemical substance. Oxidation is the loss of electrons by an atom, molecule, or ion. The electrons lost by the atom in the reaction cannot exist in solution and have to be accepted by another substance in solution. So the complete reaction involving the oxidation will have to include another substance, which will be reduced Figure 1.
THE MEASUREMENT OF ORP
An ORP sensor consists of an ORP electrode and a reference electrode, as a Voltmeter measures the difference in potential (voltage). The principle behind the ORP measurement is the use of an inert metal electrode (platinum, sometimes gold), which, due to its low resistance, will give up electrons to an oxidant (chlorine in this case) or accept electrons from a reductant (sulfur dioxide in a dechlorination process) . The ORP electrode will continue to accept or give up electrons until it develops a potential, due to the build up charge, which is equal to the ORP of the solution. The typical accuracy of an ORP measurement is ±5 mV. This further complicated by the fact that different probes from the same manufacturer often will have a 20 to 50 mV difference in the same water sample.
An ORP sensor consists of an ORP electrode and a reference electrode, as a Voltmeter measures the difference in potential (voltage). The principle behind the ORP measurement is the use of an inert metal electrode (platinum, sometimes gold), which, due to its low resistance, will give up electrons to an oxidant (chlorine in this case) or accept electrons from a reductant (sulfur dioxide in a dechlorination process) . The ORP electrode will continue to accept or give up electrons until it develops a potential, due to the build up charge, which is equal to the ORP of the solution. The typical accuracy of an ORP measurement is ±5 mV. This further complicated by the fact that different probes from the same manufacturer often will have a 20 to 50 mV difference in the same water sample.
Note: manufactures test their sensors in Zobell Solution which contains a high level of redox couples. In this solution sensors will read very close to each other. That is not the case with real world samples in drinking water.
ORP Electrodes are Easily Poisoned
Figure 2 below illustrates the results on an experiment in 300 gallon spa using bromine as the sanitizer. An ORP Sensor was installed as a monitor (not controlling). A bromine generator with an amperometric sensor was also installed to control the sanitizer level. Synthetic perspiration was then added to the spa. The red line is the bromine level that the amperometric system measured throughout the test. The peaks in the blue line represent the time that the bromine generator was energized to satisfy demand. The synthetic perspiration (White, 1992)created a significant demand that persisted throughout the test, requiring the bromine generator to operate for about two hours at a time. After about 12 hours the ORP sensor, represented by the green line, registered negative values. The most likely cause was poisoning of the electrode. It did not recover from the condition for 29 hours. This would have caused a massive over chlorination or over bromination of the spa, had it been controlling the sanitizer.
In this test, synthetic perspiration was added to a hot tub with an electrolytic bromine generation unit. As can be seen above, the green line (ORP) dropped to a negative value before returning to normal after over 29 hours. (Silveri, 1999)
Baseline (zero chlorine) Level Varies with Different Water
As can be seen from the graph in Figure 3, five different water samples have a different ORP baseline that results in a higher ORP for the same chlorine level. The results vary by almost 200 mV. According to WHO, “there is a wide variation between 720 mV in different waters (1 ppm to 15 ppm chlorine) due to varying baseline ORP (zero chlorine)” (World Health Organization, 2006)
With amperometric sensors, zero current is always zero chlorine, so no zero calibration is needed. It should be noted that this comparison was in tap water. Baseline ORP of seawater ORP can range from -275 to 350 mV greatly exacerbating the baseline problem. (Cohrs, 2004)
The lower oxidation potential of bromine compared to chlorine means that ORP will not be as sensitive to the concentration as it will with chlorine. This also means that the potential from an ORP sensor will be closer to the baseline level (zero bromine level).
ORP를 갖춘 농도 측정
염소 농도 측정에 ORP를 사용하는 제한사항은 다음과 같습니다.
잠재적 인 측정에 ORP의 관계를 제어하는 Nernst 방정식에 따르면, 농도의이 logarithm을 곱하는 계수는 -59.16 mV와 동일하며 반 반응 (n)에서 전자의 수로 나뉩니다. 이 경우 n = 2; 따라서 계수는 -29.58입니다. Cl-, HOCl, H+의 농도가 10배 변경되면 ORP ±29.58mV만 변경됩니다. (에머슨 공정 액체 사업부, 2008)
ORP는 염화물 이온 (Cl-) 및 pH (H+)에 따라 달라집니다 그것은 하이포 클로산 (물에 염소)를 수행하는 만큼. 염화물 농도 또는 pH의 변화는 ORP에 영향을 미칩니다. 따라서 염소를 정확하게 측정하려면 염화물 이온 및 pH를 높은 정확도로 측정하거나 일정한 값으로 신중하게 제어해야 합니다.
측정된 밀리볼트로부터 저혈당 농도를 계산하기 위해 측정된 밀리볼트는 10의 지수로 나타납니다. ORP 측정의 일반적인 정확도는 ±5mV입니다. 이 오류만으로도 계산된 hypochlorous acid 농도가 ±30% 이상 감소하게 됩니다. 참조 전극 또는 ORP 분석기의 드리프트는 이 오류에만 추가됩니다.
온도가 있는 ORP의 모든 변화는 보상되지 않으며, 파생 농도의 오차를 더욱 증가시다.
사실상 모든 ORP 반 반응은 하나 이상의 물질을 포함하고, 대다수는 pH 의존성을 가지고 있습니다. 농도에 대한 ORP의 로그자리트 의존성은 측정된 밀리볼트의 오류를 곱합니다.
ORP 전극은 쉽게 독극되어 제거및 세척하지 않는 한 한 번에 몇 시간 동안 쓸모없게 됩니다.
바닷물 전기 염소화에서 ORP를 사용하면 대부분의 고유 문제를 확대합니다.
Zero calibration with ORP is difficult since different waters or contaminants with change the baseline. In seawater the baseline can range from -275 to 350 mV.
A logical conclusion from the foregoing is that ORP is not a good technique to apply to concentration measurements.
ORP를 갖춘 농도 측정
염소 농도 측정에 ORP를 사용하는 제한사항은 다음과 같습니다.
잠재적 인 측정에 ORP의 관계를 제어하는 Nernst 방정식에 따르면, 농도의이 logarithm을 곱하는 계수는 -59.16 mV와 동일하며 반 반응 (n)에서 전자의 수로 나뉩니다. 이 경우 n = 2; 따라서 계수는 -29.58입니다. Cl-, HOCl, H+의 농도가 10배 변경되면 ORP ±29.58mV만 변경됩니다. (에머슨 공정 액체 사업부, 2008)
ORP는 염화물 이온 (Cl-) 및 pH (H+)에 따라 달라집니다 그것은 하이포 클로산 (물에 염소)를 수행하는 만큼. 염화물 농도 또는 pH의 변화는 ORP에 영향을 미칩니다. 따라서 염소를 정확하게 측정하려면 염화물 이온 및 pH를 높은 정확도로 측정하거나 일정한 값으로 신중하게 제어해야 합니다.
측정된 밀리볼트로부터 저혈당 농도를 계산하기 위해 측정된 밀리볼트는 10의 지수로 나타납니다. ORP 측정의 일반적인 정확도는 ±5mV입니다. 이 오류만으로도 계산된 hypochlorous acid 농도가 ±30% 이상 감소하게 됩니다. 참조 전극 또는 ORP 분석기의 드리프트는 이 오류에만 추가됩니다.
온도가 있는 ORP의 모든 변화는 보상되지 않으며, 파생 농도의 오차를 더욱 증가시다.
사실상 모든 ORP 반 반응은 하나 이상의 물질을 포함하고, 대다수는 pH 의존성을 가지고 있습니다. 농도에 대한 ORP의 로그자리트 의존성은 측정된 밀리볼트의 오류를 곱합니다.
ORP 전극은 쉽게 독극되어 제거및 세척하지 않는 한 한 번에 몇 시간 동안 쓸모없게 됩니다.
바닷물 전기 염소화에서 ORP를 사용하면 대부분의 고유 문제를 확대합니다.
Zero calibration with ORP is difficult since different waters or contaminants with change the baseline. In seawater the baseline can range from -275 to 350 mV.
A logical conclusion from the foregoing is that ORP is not a good technique to apply to concentration measurements.
전류 측정
In an amperometric sensor, a fixed voltage is applied between two electrodes and a reaction takes place at the working electrode (cathode) in which chlorine is reduced from chlorine (HOCl) back to chloride (Cl-) Figure 4. This is reverse of what takes place in the chlorine generator where chlorine is generated at the anode. In an amperometric sensor the current that flows as a result of this reduction is proportional to the chlorine presented to the sensor. The figure above shows the “three electrode” configuration. Most membrane chlorine 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.
When chlorine is added to water it hydrolyzes to form:
Cl2 + H2O → HOCl + H+ + Cl-
HOCl (aq)OCl- (aq) + H+(aq)
It is usually hypochlorous acid (HOCl) that is measured by the amperometric membrane sensor
로지스믹 VS. 선형 신호 관계
As may be seen from Figure 5, in amperometric systems, the relationship to chlorine (or bromine) is linear vs. the logarithmic relationship for ORP. Any attempt to control bromine at 2 to 4 ppm will not result in very tight control since there is very poor resolution in that range when bromine is the oxidant.
Problems with use of orp in swimming pools
Cyanuric acid is widely used in virtually all outdoor swimming pools to prevent loss of chlorine due to ultraviolet light (sunlight). The most common form of stabilized chlorine is contains over 50% cyanuric acid by weight. As a result, in one season, cyanuric acid levels can increase rapidly, often exceeding 200 ppm. Levels above 300 ppm are often encountered in Arizona, California and Nevada. At levels of around 350 ppm, the ORP electrode is rapidly poisoned and must be cleaned every three days or so. There is no practical way to remove cyanuric acid other than draining some or all of the water. ORP systems cannot be used with high levels of cyanuric acid (over 40 ppm) while Amperometric systems can be used with Cyanuric Acid levels over 200. According to one ORP controller manufacturer swimming pools should not be run with more than 40 ppm of cyanuric acid even though most health departments allow up to 100 ppm and some even allow up to 200 ppm of cyanuric acid. This results in higher costs due to draining a portion of the water. The reason for this recommendation is, according to the company, that ORP increases when the sun goes down as the cyanuric acid. This allows the chlorine level to drop since the controller thinks there is more chlorine than there really is. The next morning when the sun rises the controller detects dangerously low levels of chlorine enters an alarm mode (when a condition of < 200 mV is detected) and shuts down the feed system, detecting a problem that requires intervention. As a result, commercial pools have to spend more money on water, in short supply in many areas, and suffer from needless control problems. HSI’s sensor is the only amperometric sensor that can be used in this application with high levels of cyanuric acid.
problems with orp in wastewater
A report comparing ORP to Amperometric sensors had the following comments regard ORP’s use in wastewater treatment plants:
“Ability to Provide Information to Meet Effluent Coliform Requirements” criterion, a score of two [out of 5] was determined. This score is lower than those given to the residual technologies for this same criterion because of questions raised in Chapter 9.0 that indicate ORP is not always correlated to microbial kill or may not operate successfully in all wastewater applications. Initial evidence suggests that: 1) some chemicals other than chlorine can elevate ORP levels but not produce effective microbial kill, 2) other chemicals can interfere with ORP function, and 3) that ORP may not be an effective solution for all wastewater applications based on the issues raised by points 1) and 2).
The authors point out, however, that some wastewater treatment plants have successfully used ORP and it does play a role in providing qualitative assessments of water quality.
“The ORP technology was given a criteria score of 2 for the criteria “Ability to Provide Process Control System Reliability”. As discussed in Chapter 7.0, there are concerns on how to check if ORP sensors are out of calibration. The inability to effectively predict or determine when an ORP analyzer goes out of calibration can impact process control stability.“
“Based on results from this work it was found that lab ORP probes couldn’t be used effectively to check field ORP calibrations because the lab units are easily fouled or poisoned by wastewater components (See Chapter 7.0).”
“Another concern is that ORP sensors from different manufacturers respond differently to variations in chlorine speciation.” (Damon S. Williams Associates, LLC, 2004)
In summary
In amperometric systems, the relationship to chlorine (or bromine) is linear vs. logarithmic for ORP. ORP will not be precise, require greater training, monitoring and could result in excessive corrosion of components or tanks, often defeating the purpose or its use in the first place.
Amperometric 시스템은 실제로 브롬을 측정 (TRO) 하지 다른 매개 변수 (redox), 따라서 더 정확 할 것이다.
염소 제로는 항상 0이므로 amperometric 시스템에서는 제로 보정이 필요하지 않습니다.
AMPerometric 시스템의 전극은 ORP 전극이 있기 때문에 유기체에 의해 쉽게 중독되지 않습니다.
ORP is both a good qualitative indicator and a poor quantitative method.
ORP 결론
The disadvantages of ORP all relate to routine equipment maintenance, calibration, electrode poisoning, presence of multiple redox couples, and very small exchange current that are logarithmically related to the analyte concentration. Or in other words, the assumption of a reversible chemical equilibrium, fast electrode kinetics, and the lack of interfering reactions are essential for chemical interpretations of the ORP potentials. Unfortunately, these conditions rarely, if ever, are met in real world systems (Kissinger, 1996). ORP can be used for qualitative analysis.
Halogen systems’ rapid response ORP
Halogen Systems included ORP measurement in its multi parameter sensor as an indicator for contamination. While the sensor’s chlorine measurement can detect chlorine from .05 to 15 ppm, it cannot detect the difference between contamination and water in which the chlorine residual was recently depleted. ORP can potentially fill in this gap by identifying a contamination event or cross connection by indicating a drop below the baseline ORP level. In addition, HSI’s measurement technique and self-cleaning system actually eliminates poisoning issues, providing a more reliable reading. While HSI’s ORP measurement may have a slight offset from other sensors but provides a reliable, qualitative measurement that overcomes many of the ORP problems.
