I predict that in 5 years ORP probes will be more or less obsolete (apart from spares for old units) and everyone will change over to amperometric for true chlorine readings.learthur said:
Ideal ORP level?
- Thread starter JasonLion
- Start date
For those that want to know what teapot's amerometric sensor is, http://www2.emersonprocess.com/siteadmi ... 3-6063.pdf
The 1st one I saw when I googled it was over $2000, just for the sensor. I suspect it will take more than 5 years for that to come down, but hey, you never know.
Ozone and ORP are not a good combo. The ozone will tell the ORP that the level is very high and cause a turning down of the chlorine feed system. This will lead to low levels of FC in the pool and we all know that means.
Scott
The 1st one I saw when I googled it was over $2000, just for the sensor. I suspect it will take more than 5 years for that to come down, but hey, you never know.
Ozone and ORP are not a good combo. The ozone will tell the ORP that the level is very high and cause a turning down of the chlorine feed system. This will lead to low levels of FC in the pool and we all know that means.
Scott
I think it's Hayward that have begun using them, a special gold tipped sensor probe, in the UK that's about £500 more than the standard units also Prominent and Topline have units in production again street price around £500-600 more than the ORP equivalent. The more accurate sensing means less fluctuation in chemical so a cost saving can be made although probably not that much on a domestic pool commercially the extra cost should be recouped over a year.
I have a Polaris watermatic system and love it but the learning curve is a little steep. Cya levels over 30 really affect the orp sensor. Also if calcium level too low it also affects sensor. For me I run an orp level of 700 in the hot summer. I use about 1 1/2 gal of 6% bleach per day. I have had my system for 3 seasons now and it it really a low maintaince way to keep a clear pool. You also must adjust feed rates and wait times to give the sensor time to read. 25,000 gal pool.
That's about 3.7 ppm FC per day which is pretty high. It's probably because of your low 30 ppm (or lower) CYA. If this is a residential pool, you would likely be able to operate at a higher CYA level (with a higher corresponding FC target) and use a timer with a peristaltic pump and maintain a fairly consistent FC using only 2 ppm per day at lower cost. Unless your weather varies a lot with sunny and cloudy days or unless you have large pool parties, daily chlorine demand doesn't change very much. That's how most people with either peristaltic pumps or with saltwater chlorine generators are able to manage their pools without having to check the chlorine every day.jfpetesn said:
By the way, it's not the CYA that directly affects the ORP sensor, but rather the FC/CYA ratio that determines the active chlorine level. ORP sensors don't do well measuring lower active chlorine levels (i.e. lower ORP levels). For a Chemtrol sensor, 700 mV would be around 4 ppm FC with 30 ppm CYA; for an Oakton, it would be around 8 ppm FC with 30 ppm CYA. For Aquarius and Sensorex, it would be even higher. So the idea of any sort of absolute ORP meaning anything is ridiculous; in theory perhaps, but not with real-world sensors.
I use sensorex sensors and they have worked pretty well. Their customer support is great. From my experience (residential pool now, commercial pool experience past) increasing the CYA blinds the sensor. The higher the CYA the less responsive my sensor becomes which results in overfeeding. I run calibrations on my sensor monthly. I have ORP reference solution that I dip my sensor in and it results in a specific mv reading. When it varies I know my sensor is getting dirty.
I can tell you from personal experience that maintaining a 2-3ppm chlorine in the hot summers here is near impossible. I get surface alge blooms that I can see on the bottom every 3-4 days. I can easily brush them off but if I maintain a 4-5 ppm level I do not get these. I just find I have to worry less if I run it a little higher.
Using continuous ORP feeding has worked out well for me but until one understands the chemical relationships between pH, CH temp, UV, & ORP it can be a real challenge. Definately not for the novice. According to the studies I have read an CYA level of 20-25 results in 80% effectiveness of a CYA level of 50. In europe they are banning CYA in commercial applications. Cant imagine trying to maintain a chlorine level in a commercial application without it.
My chlorine costs are approx $2.50/day sometimes less depending on the weather but that is the max.
I can tell you from personal experience that maintaining a 2-3ppm chlorine in the hot summers here is near impossible. I get surface alge blooms that I can see on the bottom every 3-4 days. I can easily brush them off but if I maintain a 4-5 ppm level I do not get these. I just find I have to worry less if I run it a little higher.
Using continuous ORP feeding has worked out well for me but until one understands the chemical relationships between pH, CH temp, UV, & ORP it can be a real challenge. Definately not for the novice. According to the studies I have read an CYA level of 20-25 results in 80% effectiveness of a CYA level of 50. In europe they are banning CYA in commercial applications. Cant imagine trying to maintain a chlorine level in a commercial application without it.
My chlorine costs are approx $2.50/day sometimes less depending on the weather but that is the max.
Thanks for the info. That's interesting that you need a significantly higher active chlorine level to keep algae from growing in your situation -- more than double what is normally required. Of course, that assumes the CYA reading is accurate and that can vary +/-15 ppm which at the low CYA levels is a heck of a lot of error (perhaps the error isn't constant in the range since it's a turbidimetric test). Anyway, whatever works for you is fine.
Yes I agree with you that pools with no CYA will have a difficult time maintaining chlorine levels, but that's what they do with German DIN 19643 used in Europe. Between the bather load and for outdoor pools the sunlight they try and maintain 0.3 to 0.6 ppm FC but usually run on the higher side for easier process control. It obviously can be done since they are doing it, but if they used 4 ppm FC with 20 ppm CYA they would get the equivalent of 0.2 ppm with a far easier time of maintaining the chlorine level consistently everywhere, even with local bather "incidents" (such as urination).
If by CYA effectiveness you mean protection from sunlight, the 80% can be a bit misleading because it's compared against no CYA at all which has a lot of loss from sunlight. With no CYA at all, half the FC is lost every hour in a pool of typical depth. So over an equivalent 8 hour day (more than 8 hours of sunlight, but angled during the day) that's a 99.6% loss -- essentially all the chlorine is lost even before the day ends (since there are additional losses other than from sunlight). Roughly speaking, the following table gives a typical chlorine loss rate over a day at various CYA levels.
CYA ... % FC Loss ... FC Loss @ FC/CYA = 0.1
. 30 ....... 75% .................... 2.25
. 50 ....... 40% .................... 2.00
. 80 ....... 22% .................... 1.76
100 ....... 15% .................... 1.50
Yes I agree with you that pools with no CYA will have a difficult time maintaining chlorine levels, but that's what they do with German DIN 19643 used in Europe. Between the bather load and for outdoor pools the sunlight they try and maintain 0.3 to 0.6 ppm FC but usually run on the higher side for easier process control. It obviously can be done since they are doing it, but if they used 4 ppm FC with 20 ppm CYA they would get the equivalent of 0.2 ppm with a far easier time of maintaining the chlorine level consistently everywhere, even with local bather "incidents" (such as urination).
If by CYA effectiveness you mean protection from sunlight, the 80% can be a bit misleading because it's compared against no CYA at all which has a lot of loss from sunlight. With no CYA at all, half the FC is lost every hour in a pool of typical depth. So over an equivalent 8 hour day (more than 8 hours of sunlight, but angled during the day) that's a 99.6% loss -- essentially all the chlorine is lost even before the day ends (since there are additional losses other than from sunlight). Roughly speaking, the following table gives a typical chlorine loss rate over a day at various CYA levels.
CYA ... % FC Loss ... FC Loss @ FC/CYA = 0.1
. 30 ....... 75% .................... 2.25
. 50 ....... 40% .................... 2.00
. 80 ....... 22% .................... 1.76
100 ....... 15% .................... 1.50
I agree that the CYA test is fairly subjective and relatively inaccurrate. I say I am running a 25 ppm CYA now and all is well. I get a reasonable response time with my sensor. If I run much over 30 my ORP of say 700 could be a FC of 7-9 ppm. With the CYA at 25 my ORP of 700 (daytime) is a FC of 4-5. I dont object to higher CYA but I find to obtain a 650-700 ORP I need much more chlorine. I suppose I could raise my CYA and just adjust my FC to 2-4 and note my ORP at that level and use that number as my adequate ORP even if it were 450-500.
After reading many studies I felt that an ORP of 700 was giving me an accepted sanitation level. What I have found is that with a 2-3 ppm FC in the 104 degree days we are having is that I get a gradual algae growth on the bottom of the pool. It starts on day 1 or 2 with a greenish cast in the corners where the bottom meets the sides and progresses to a greeenish cast on the bottom particularily in the shady areas. It is geen algae and by the 4th or 5th day is noticable. It easily brushes off and I have to brush the bottom about every 5th day. I do not get a real bloom and none in the open water, infact my water looks great but for the small amount in the bottom. I really do not regularily use algacide. I keep my chemicals to a minimum. I have fantastic looking water that is very clear and polished.
The only regular problem I have is with the Dang paint dusting after 2-3 years. Looking for a solution for that considering pebble tech or tileing the bottom but lots of $$$$$.
I appreciate your comments.
After reading many studies I felt that an ORP of 700 was giving me an accepted sanitation level. What I have found is that with a 2-3 ppm FC in the 104 degree days we are having is that I get a gradual algae growth on the bottom of the pool. It starts on day 1 or 2 with a greenish cast in the corners where the bottom meets the sides and progresses to a greeenish cast on the bottom particularily in the shady areas. It is geen algae and by the 4th or 5th day is noticable. It easily brushes off and I have to brush the bottom about every 5th day. I do not get a real bloom and none in the open water, infact my water looks great but for the small amount in the bottom. I really do not regularily use algacide. I keep my chemicals to a minimum. I have fantastic looking water that is very clear and polished.
The only regular problem I have is with the Dang paint dusting after 2-3 years. Looking for a solution for that considering pebble tech or tileing the bottom but lots of $$$$$.
I appreciate your comments.
If the only reason you are running the higher FC level is to get the 700 mV reading from your sensor, then that is absolutely not necessary and I don't know where you get that 700 mV for one particular manufacturer's sensor is an "acceptable sanitation level". I was thinking you were saying that the sensor became unstable at higher CYA, getting more wild or varying in its readings. If that is not the case, then you should choose an FC/CYA target and then set the ORP to whatever mV reading achieves that, even if its 650 mV or even lower. As I have tried to explain in numerous posts on ORP, the absolute mV ORP reading for any given sensor is completely meaningless since there is NO consistency between the sensors from different manufacturers measuring identical pool water. As described in this post, different sensors measuring the same water varied by more than 100 mV in 23% of the pools.
Even the manufacturer's own tables of FC (with no CYA) vs. mV ORP not only vary in absolute magnitude (i.e. a bias difference), but also in slope (change in mV for every doubling of FC). Look at this post for example and see in this link where your Sensorex sensor presumably shows an 83 mV increase for every doubling in FC which is absolutely ridiculous. At a pH of 7.5, they claim that 700 mV is at an FC of around 1.1 ppm with no CYA. That is certainly not what you are seeing since at 4-5 ppm FC with 20-25 ppm CYA (assuming these values are accurate) this has the same active chlorine (hypochorous acid) that is what ORP is indirectly measuring of only 0.15 ppm to 0.27 ppm FC with no CYA, though this is more in line with what other sensors report. For example, at a pH of 7.5, temp of 80ºF, 4 ppm FC with 25 ppm CYA would be 707 mV on Chemtrol, 670 mV on Oakton, 609 mV on Aquarius, and only 474 mV on Sensorex if one were to believe their table which is obviously wrong (which begs the question of what in the heck are they publishing?).
So from what you describe it sounds like you may need to run a higher FC/CYA ratio to prevent algae from growing, but you could also try to see what 4 ppm FC at 40 ppm CYA does just in case it's error in measurement of CYA that is the main problem. If that prevents the algae from growing, then you will use a lot less chlorine. Of course doing such experiments isn't easy since you'd have to dilute the water to lower the CYA level if you wanted to go back to where you were or you could run at 8 ppm FC with 40 ppm CYA or something like that and still use less chlorine than you are currently.
Even the manufacturer's own tables of FC (with no CYA) vs. mV ORP not only vary in absolute magnitude (i.e. a bias difference), but also in slope (change in mV for every doubling of FC). Look at this post for example and see in this link where your Sensorex sensor presumably shows an 83 mV increase for every doubling in FC which is absolutely ridiculous. At a pH of 7.5, they claim that 700 mV is at an FC of around 1.1 ppm with no CYA. That is certainly not what you are seeing since at 4-5 ppm FC with 20-25 ppm CYA (assuming these values are accurate) this has the same active chlorine (hypochorous acid) that is what ORP is indirectly measuring of only 0.15 ppm to 0.27 ppm FC with no CYA, though this is more in line with what other sensors report. For example, at a pH of 7.5, temp of 80ºF, 4 ppm FC with 25 ppm CYA would be 707 mV on Chemtrol, 670 mV on Oakton, 609 mV on Aquarius, and only 474 mV on Sensorex if one were to believe their table which is obviously wrong (which begs the question of what in the heck are they publishing?).
So from what you describe it sounds like you may need to run a higher FC/CYA ratio to prevent algae from growing, but you could also try to see what 4 ppm FC at 40 ppm CYA does just in case it's error in measurement of CYA that is the main problem. If that prevents the algae from growing, then you will use a lot less chlorine. Of course doing such experiments isn't easy since you'd have to dilute the water to lower the CYA level if you wanted to go back to where you were or you could run at 8 ppm FC with 40 ppm CYA or something like that and still use less chlorine than you are currently.
I got the 700mv from a WHO study I read somewhere. From what I read at 700mv any bacterium introduced would be killed in less than 30 sec. At 650 I believe it was within 1.5 minutes.
What I see from my sensor with higher CYA levels is that after a 20 minute feed time I will not see an increase in ORP for 20-30 min. Where at a CYA level of 25 I will see a rise in ORP in say 10 min. What this does is cause my controller to run through another feed cycle even though the correct ORP level has already been achieved (overfeed). The higher the CYA the longer the delay for the probe to see the ORP change. At 40-50ppm CYA I see an attenuation of ORP readings above 650mv (ie I could have a FC level of 20ppm and still only see an ORP reading of 670). At that CYA level I think I could pump 25 gal of 6% and never see 700. It is almost like something is coating the probe. If I keep my CYA below 30 I do not see any of these issues.
Take a look at this article, particularily page 2. Let me know what you think because this is what I see with my probe.
Attachment removed. Please do not post third party content. Providing a link to the original at another site is appropriate. JasonLion
What I see from my sensor with higher CYA levels is that after a 20 minute feed time I will not see an increase in ORP for 20-30 min. Where at a CYA level of 25 I will see a rise in ORP in say 10 min. What this does is cause my controller to run through another feed cycle even though the correct ORP level has already been achieved (overfeed). The higher the CYA the longer the delay for the probe to see the ORP change. At 40-50ppm CYA I see an attenuation of ORP readings above 650mv (ie I could have a FC level of 20ppm and still only see an ORP reading of 670). At that CYA level I think I could pump 25 gal of 6% and never see 700. It is almost like something is coating the probe. If I keep my CYA below 30 I do not see any of these issues.
Take a look at this article, particularily page 2. Let me know what you think because this is what I see with my probe.
Attachment removed. Please do not post third party content. Providing a link to the original at another site is appropriate. JasonLion
ORP sensors have all kinds of complex problems and quirks. For example, there is no such thing as an absolute ORP reading, there is no standard for how the sensors respond. Therefore it is impossible to have a single number that works as a target ORP reading. Further, a variety of things can cause the readings to vary that have nothing at all to do with sanitation.
Each sensor/pool/water chemistry is different and you need to calibrate your target ORP level based on FC readings from a good test kit in your pool. Appropriate FC target levels depend on your CYA level as described in Pool School. Find some suitable FC level for your pool, adjust everything so your levels are appropriate based on test kit readings and then note the current ORP reading and use that as your ORP target.
The ORP response curve varies with the CYA level, with the PH, with the water temperature, with sunlight shining on the pool or not, with dissolved hydrogen gas, and with many other factors. You can't assume that you will be able to reach an ORP level of 700 in all circumstances, nor is it appropriate to try and do so. This does not reflect a problem with your specific sensor, nor is anything "coating" the probe.
Higher CYA levels do narrow (and shift) the range over which ORP varies for interesting FC levels. As the CYA level goes up this eventually narrows the response so much that the noise in the system overwhelms the signal. With care it is generally possible to use ORP up to about a CYA of 50, sometimes a little higher, but usually it is more reliable at lower CYA levels, say around 20 to 30. However, lower CYA levels greatly increase the amount of chlorine required, despite the lower FC levels, you need to add more total chlorine to maintain a reasonable level. For an outdoor residential pool it is nearly always simpler, less expensive, and more reliable to just give up on ORP automation and use time or percentage based chlorine feed.
Each sensor/pool/water chemistry is different and you need to calibrate your target ORP level based on FC readings from a good test kit in your pool. Appropriate FC target levels depend on your CYA level as described in Pool School. Find some suitable FC level for your pool, adjust everything so your levels are appropriate based on test kit readings and then note the current ORP reading and use that as your ORP target.
The ORP response curve varies with the CYA level, with the PH, with the water temperature, with sunlight shining on the pool or not, with dissolved hydrogen gas, and with many other factors. You can't assume that you will be able to reach an ORP level of 700 in all circumstances, nor is it appropriate to try and do so. This does not reflect a problem with your specific sensor, nor is anything "coating" the probe.
Higher CYA levels do narrow (and shift) the range over which ORP varies for interesting FC levels. As the CYA level goes up this eventually narrows the response so much that the noise in the system overwhelms the signal. With care it is generally possible to use ORP up to about a CYA of 50, sometimes a little higher, but usually it is more reliable at lower CYA levels, say around 20 to 30. However, lower CYA levels greatly increase the amount of chlorine required, despite the lower FC levels, you need to add more total chlorine to maintain a reasonable level. For an outdoor residential pool it is nearly always simpler, less expensive, and more reliable to just give up on ORP automation and use time or percentage based chlorine feed.
Well stated and I think you are probably correct. I have the TFP test kit so I am assuming my results are close. I will seriously consider your opinion and work on implementing it. Hey I can always save some money. So if you had to pick the perfect FC/CYA level for a residential pool what would you choose? 40/6? I was probably too fixated on some "number" and would drive myself crazy trying to get there.
There are so many chemical interactions going on in pool water. Sometimes ignorance is bliss. One other interesting observation demonstrating the effects of UV on sanitation. I can literally watch the ORP reading increase as the sun goes down.
I will say that the automated systems maintain much tighter pool chemistry ranges and overall better water quality. I do not miss the wild swings in FC or pH resulting in less wear and tear on equipment and the pool itself.
There are so many chemical interactions going on in pool water. Sometimes ignorance is bliss. One other interesting observation demonstrating the effects of UV on sanitation. I can literally watch the ORP reading increase as the sun goes down.
I will say that the automated systems maintain much tighter pool chemistry ranges and overall better water quality. I do not miss the wild swings in FC or pH resulting in less wear and tear on equipment and the pool itself.
If you want the minimum amount of chlorine needed to prevent algae growth while still killing pathogens reasonably quickly, then it would be the Chlorine / CYA Chart level with an FC that is at least 7.5% of the CYA level. However, in your pool notwithstanding measurement error, you found you needed an FC that was around 15-20% of the CYA level. So you might just need to experiment a little at the higher CYA level, but with proportionately higher FC levels. Technically, those should give the same ORP reading.jfpetesn said:
As for what happens with UV, this just shows how ridiculous the ORP sensor reading is because only a very small amount of the chlorine gets broken down by sunlight and produces hydroxyl radicals that should report much higher ORP levels while the chlorine that is depleted is replenished quickly from that attached to CYA where half of the FC can be released in 1/4 second (through a double pathway; 4 seconds through a single pathway) which for the very slow drop in FC is essentially replenished immediately. Outdoor pools exposed to the UV in sunlight have far higher oxidation rates yet the ORP sensor from what you describe indicates that it is lower. How ludicrous! As Jason mentioned, hydrogen gas bubbles can also interfere yet have virtually nothing to do with oxidation rates from chlorine. Again, ludicrous! You need to separate in your mind any association of ORP with sanitation. There is a rough correlation only because of the correlation with active chlorine level, but it is obviously influenced and fooled by things that aren't related to sanitation or goes in the opposite direction.
By the way, the seconds versus minute or two kill times are for 99.9% or higher kill rates. You don't need anything close to seconds rates in a residential pool. The risk for person-to-person transmission is far lower in a residential pool. One mainly sets the chlorine rate to be able to kill algae and that rate allows for quite reasonably fast killing of most pathogens.
This thread is a GEM for anyone who is having ORP/pH sensors! Learnt a LOT from this thread. Kudos to the contributors!
In my case, I am noticing that maintaining a 5 FC - 7.5pH - 50 CYA - 50 TA, I have to dial in the ORP to be around 570~600.
Setting ORP to 650 causes continuous chlorination and FC raises rapidly to SLAM levels!
In my case, I am noticing that maintaining a 5 FC - 7.5pH - 50 CYA - 50 TA, I have to dial in the ORP to be around 570~600.
Setting ORP to 650 causes continuous chlorination and FC raises rapidly to SLAM levels!