Can weather affect icp test

[TriggerFinger]

I bought an ATI ICP test for the first time. Does anyone know if outside temperatures when mailing them off affect the water samples?

 

[Lasse]

According to the ICP part - nope - because ICP analyse atoms - not compounds

Sincerely Lasse

 

[Randy Holmes-Farley]

I also asked them in detail about freezing since I had some concerns about calcium carbonate precipitating and not redissolving from frozen water, possibly taking out many trace elements with it.

They did some studies to show it was ok, but it is for a tremendously interesting reason to a chemist. It is an oddity that the form of calcium carbonate that forms from freezing seawater (ikaite) is an odd one that will redissolve, while aragonite and calcite would not.

We discuss it more here:

Frozen sample issue

I know you may not want to post in another sponsors forum, so we can discuss it here, but here's the background and how you got mentioned as suggesting freezing is not an issue: https://www.reef2reef.com/threads/concern-about-frozen-samples.357285/ My starting discussion (which also seems to...

www.reef2reef.com

 

[Dan_P]

I bought an ATI ICP test for the first time. Does anyone know if outside temperatures when mailing them off affect the water samples?

Most likely it will, but what we don’t know is a) how much it will affect the sample, and b) whether it will matter to the decisions you make about aquarium maintenance.

You have to seriously consider the fact that ICP companies catering to the aquarium hobby do not provide scientific data nor statistics to the costumer about the reliability of their ICP method. I am not talking up a conspiracy theory, just pointing out that reliable analytical companies are transparent about what they do and how they follow standard quality procedures. Analytical procedures also include the steps for collecting and preserving the sample. How much detail did you receive from the vendor about the variability In the results these steps can cause?

I would view these tests as approximations of what is in your aquarium water.

 

[TriggerFinger]

Thanks @Lasse and @Randy Holmes-Farley
@Dan_P I bought the icp tests to compare the water between my two tanks. One has coraline growing out the wazoo and the other only gets it on the powerheads and overflow box. They have the same type of rock, maintenance, same source water and salt mix. Only differences are size (90 vs 29 gallons) and lighting. Chinese black box vs lumi lite pro.
I’m mostly just curious about the differences between the two for all the stuff I can’t test for.

 

[Dan_P]

Thanks @Lasse and @Randy Holmes-Farley
@Dan_P I bought the icp tests to compare the water between my two tanks. One has coraline growing out the wazoo and the other only gets it on the powerheads and overflow box. They have the same type of rock, maintenance, same source water and salt mix. Only differences are size (90 vs 29 gallons) and lighting. Chinese black box vs lumi lite pro.
I’m mostly just curious about the differences between the two for all the stuff I can’t test for.

It will be interesting to compare the two, although here is where my ranting comes in. If we don’t know what the standard deviation is in the measurement, the 95% confidence interval is another term you might see, how do we know that any difference we see is just noise in the data or a real difference in the two setups?

I hope you publish the two sets of data so we can hear what forum members think.

Dan

 

[Lasse]

Most likely it will, but what we don’t know is a) how much it will affect the sample, and b) whether it will matter to the decisions you make about aquarium maintenance.

This means that we must stop all testes we do with start with all hobby tests. As I have seen - there is no method description, no interference tables or no information at all in most hobby tests . There is devices with a accuracy of ± 0.02 that we use in order to withhold total concentration around 0.03 ppm. Devices that says to be useful for use by not analytic trained persons and give valid results in such a low concentrations that even professional labs have difficulties with it.

If we should talk about reliability in testing and forget the elephant in the room - the hobby tests and all the money going that way. As an example - the nitrite interference in nitrate tests - how many nitrat tests give a warning for that, especially in the low range?

If we don’t know what the standard deviation is in the measurement, the 95% confidence interval is another term you might see, how do we know that any difference we see is just noise in the data or a real difference in the two setups?

See above

I have done - during the last 4 years - 17 ICP test of my own aquarium and at my work we have done +200 tests in different aquariums. These results have been very useful in order to tune in the aquariums and help checking our own measurements of the 4 "big". Both at home and at work we have achieved success during a long period without any serve crashes. When an aquarium is tuned in - can take a year or two - the results from test to test does not differ very much if you not actively dose things.

Of cause - you can't rely on one single test - these tests - lika all others we use - its the trend that is important. If we can get people to understand that a test - even if the result is 100% correct - is only a snapshot of the picture at the test moment it will be more helpful compared with rants about things we really do not know about.

ICP test have help me to hinder a crash in my aquarium. My Hanna shows 0.1 ppm - my Triton test show 0,018 ppm. This was not a issue only for me. R2R was full of threads speculation that the P content in the sample was altered during transportation and that Hanna Checker testing was the most accurate method to rely on. However - all my other observations shows the opposite - I decide to start dosing PO4 in spite of the fact that my Hanna rise to around 0.14. Meaningless to say but all things turned to the better after this change - everything indicate that the ICP test was more near the real concentration compared with my Hi-774. After this - I have purchase a Red Sea Pro PO4 test and even this indicate a concentration around 0.08 ppm lower than the Hi-774

This thread will be interesting to follow - in post #92 Ehsan from Triton steps in

Sincerely Lasse

 

[TriggerFinger]

Thanks @Lasse. In the aquariums that you work with (200+ tests), are regular tests performed with Red Sea, Hanna checkers etc. also or do they solely use ICP? An ICP test seems more convenient and a lot easier in that situation.

 

[Rick Mathew]

This means that we must stop all testes we do with start with all hobby tests. As I have seen - there is no method description, no interference tables or no information at all in most hobby tests . There is devices with a accuracy of ± 0.02 that we use in order to withhold total concentration around 0.03 ppm. Devices that says to be useful for use by not analytic trained persons and give valid results in such a low concentrations that even professional labs have difficulties with it.

If we should talk about reliability in testing and forget the elephant in the room - the hobby tests and all the money going that way. As an example - the nitrite interference in nitrate tests - how many nitrat tests give a warning for that, especially in the low range?


See above

I have done - during the last 4 years - 17 ICP test of my own aquarium and at my work we have done +200 tests in different aquariums. These results have been very useful in order to tune in the aquariums and help checking our own measurements of the 4 "big". Both at home and at work we have achieved success during a long period without any serve crashes. When an aquarium is tuned in - can take a year or two - the results from test to test does not differ very much if you not actively dose things.

Of cause - you can't rely on one single test - these tests - lika all others we use - its the trend that is important. If we can get people to understand that a test - even if the result is 100% correct - is only a snapshot of the picture at the test moment it will be more helpful compared with rants about things we really do not know about.

ICP test have help me to hinder a crash in my aquarium. My Hanna shows 0.1 ppm - my Triton test show 0,018 ppm. This was not a issue only for me. R2R was full of threads speculation that the P content in the sample was altered during transportation and that Hanna Checker testing was the most accurate method to rely on. However - all my other observations shows the opposite - I decide to start dosing PO4 in spite of the fact that my Hanna rise to around 0.14. Meaningless to say but all things turned to the better after this change - everything indicate that the ICP test was more near the real concentration compared with my Hi-774. After this - I have purchase a Red Sea Pro PO4 test and even this indicate a concentration around 0.08 ppm lower than the Hi-774

This thread will be interesting to follow - in post #92 Ehsan from Triton steps in

Sincerely Lasse

Lasse, Thanks for your insights. I completely understand your point. I have a different way of looking at it. Using any of the hobby grade tests I can evaluate their performance as it relates to accuracy, precision and test repeatability. I can prepare known standards in the desired test range and perform the test multiple times. Using this information I am able to gage the reliability of the results I get…Good or Bad by using the statistical tool of Gage R&R. I have done this using my selected test methods (See NO3 Example below). If my testing error exceeds 20% of my allowable range of the parameter I can then refine my procedures to bring it in or find another test or at least understand the reliability of the measurement. This is not possible for me to do with the outside testing services ICP or otherwise. I do not have access to their instruments or other outside testing labs procedures. This means I must take the value they report as the true representative value of my tank water without knowing the standard error of the test actually is….It would be nice to know there Gage R&R for their tests.

As you said we actually use devices that report ± .02 instrument errors to measure targeted values of ± .03. But this is only part of the story. The way the test is done…The total procedure, from taking the sample to making the measurement and everything in between…This is the error we need to know. I will tell you this error value is often much greater than the reported instrument error. This is true for hobby test kits as well as outside testing services. I some cases it is so large we might as well pull a number out of a hat as run the tests…Saves time and money J… That is why I use the Gage R&R tool to refine the test and find out at what level to trust the results. There are some tests; PO4 is a good example, where in order to get what I would consider a reliable result I need to do multiple measurements to have any confidence in the results….Using Gage R&R helped me to do this. I have no experience with the HI-744 so I cannot comment on its accuracy or precision. I use the Hi736 Phosphorous checker and do the conversion to PO4. The best I have been able to achieve is ± .017ppm, which is just slightly higher than the reported instrument error of ± .015ppm. This means that I need to be aware that if I get a measurement of .04 ppm it could be .057ppm or .023ppm then I need to decide if I am ok with that or not. Because I use test results to look for tends as you do, I don’t so much worry about any single measurement (unless it is really whacked out) but I react to the trends as you mentioned. So if I get a measurement that concerns me I retest on subsequent days to see the trend and then make my decision whether to take corrective action or not.

I cannot comment on whether a Hanna Checker or outside testing is more accurate because I have no solid data to make that judgment on and I don’t know of any extensive data that would answer this question….Only mostly anecdotal stuff. Having some real data could be very valuable to our hobby. Like you I can only go by the results and health of my system and how I manage it. Like you I have done hundreds of tests with hobby grade kits and have for the most part managed my system based on them. I have had good success doing this…The fish are well…coral color is good…and growth, well it is actually out of hand…

Because I have limited funds I can only do periodic outside testing (every 3 months currently), which also like you I use it to calibrate my test procedures and evaluate elements that I cannot test for. This means for me that I must rely on my own testing to manage my system. This is why I put my efforts into doing my tests the very best that I can using the tools I have available…Hobby grade test kits. So when I get a test result back from an outside testing service that does not match my results (which I do), like your PO4 results with the HI-774, I find it interesting and would like to know why but I need to manage my day to day parameters with what I have.

I have used the results of outside serves to get better results with my tests. For example my calcium tests were consistently 22-25 points lower than outside services reported so now when I measure my calcium I adjust the value up by 22 points. The issue with doing this with some of the other parameter the variation reported by multiple vendors was large and I did not know which one to choose.

Respectfully Rick

GAGE R&R FOR NO3 AT 3 PPM
​

 

[Lasse]

@Rick Mathew We are inline with each other. You have found ways around the limitation of the hobby tests and you are able to get results that you can relay on. But you are a skill statistician and know your math. 99 % of people using these test do not have your skill or my scepticism to my own technique. The relay on the figures and make decisions that can be catastrophic. i can´t count all threads that start - high ammonia readings what to do. after that all answers - change water, do that or do that - your fish is going to die. Reading the thread often shows up a total ammonia reading of 0.2 - 0.25 ppm. The truth of this reading is that many total ammonium hobby test often show 0.25 even if it zero. and if the result was the right - it is no danger because a reading of 0.25 NH3/NH4 only will give a concentration of 0.02ppm of the toxic form NH3 at pH 8.2 - and its a safe value.

The Hi 774 is basically the same as you use but convert the reading into PO4 directly. And I have worked out a procedure there I analyse the same sample 10 times in a row and after that can take an average - it means that I with rather high accuracy comes around the ±0.02 limit. But - in my case - it shows that my meter probably have a systematic error of 0.08 ppm and therefore I - as you - use my outside tests to get a reading that I rely on. I was ranting in my last post because there is a tendency that when outside tests and hobby tests show different - it is always the outside tests that you can´t rely on. For me - the outside test is the ones I adapt my use of hobby tests too. I know that my Ca readings always was - as yours 15 - 20 ppm lower compared with the ICP test. After seen this for many tests - I adapt may way of reading my hobby test and now - they are nearly the same.

One thing is the nitrate tests. I have had contact with one of the ICP testers here in Europe that the same time do advanced NO3 and NO2 tests. Their database show that it is very common with NO2 levels around 0.02-0.05 ppm - but there is aquariums with zero upt to 0.15 NO2. Already levels around 0.02 - 0.05 will give NO3 readings below 5 ppm a huge error.

Today - I use Tritons N-DOC in order to know what maximal my NO3 can be. And if I am below max 10 ppm - I fine with that because it probably indicate that my NO3 levels is around 5.

Sincerely Lasse

 

[Lasse]

Thanks @Lasse. In the aquariums that you work with (200+ tests), are regular tests performed with Red Sea, Hanna checkers etc. also or do they solely use ICP? An ICP test seems more convenient and a lot easier in that situation.

There is daily tests taken of PO4 and alkalinity in most of the systems. Salifert and Hanna Hi 713. ICP test every 2 months as a checkup. We use calcium reactors and Triton Core 7 - both are balanced and we need only to analyse alkalinity on a daily base. @Sallstrom knows more of this. Both alkalinity and PO4 can change quickly in these heavy coral populated tanks

Sincerely Lasse

 

[Dan_P]

This means that we must stop all testes we do with start with all hobby tests. As I have seen - there is no method description, no interference tables or no information at all in most hobby tests .

Correct! Strictly speaking, there is little value in data collected with a method of which you know nothing about. Accepting the numbers from such a test is simply an act of faith in the vendor. A step towards being informed about a method is periodic testing of a reference standard. Replicate measurements will provide information about variability. Another best practice would be creating a calibration curve. There are things you can do rather not using these tests

There is devices with a accuracy of ± 0.02 that we use in order to withhold total concentration around 0.03 ppm. Devices that says to be useful for use by not analytic trained persons and give valid results in such a low concentrations that even professional labs have difficulties with it.

And worse, some aquarists are relying on ICP measurements of phosphorous and have no idea what the ± is for P!!

If we should talk about reliability in testing and forget the elephant in the room - the hobby tests and all the money going that way. As an example - the nitrite interference in nitrate tests - how many nitrat tests give a warning for that, especially in the low range?

Another good example of faith in the vendor.

I have done - during the last 4 years - 17 ICP test of my own aquarium and at my work we have done +200 tests in different aquariums. These results have been very useful in order to tune in the aquariums and help checking our own measurements of the 4 "big". Both at home and at work we have achieved success during a long period without any serve crashes. When an aquarium is tuned in - can take a year or two - the results from test to test does not differ very much if you not actively dose things.

Unfortunately, successes with aquaria add very little to our knowledge about the accuracy of your ICP results or even whether they were actually useful.

Of cause - you can't rely on one single test - these tests - lika all others we use - its the trend that is important. If we can get people to understand that a test - even if the result is 100% correct - is only a snapshot of the picture at the test moment it will be more helpful compared with rants about things we really do not know about.

Totally agree! Trends are more informative than single point measurements.

ICP test have help me to hinder a crash in my aquarium. My Hanna shows 0.1 ppm - my Triton test show 0,018 ppm. This was not a issue only for me. R2R was full of threads speculation that the P content in the sample was altered during transportation and that Hanna Checker testing was the most accurate method to rely on. However - all my other observations shows the opposite - I decide to start dosing PO4 in spite of the fact that my Hanna rise to around 0.14. Meaningless to say but all things turned to the better after this change - everything indicate that the ICP test was more near the real concentration compared with my Hi-774. After this - I have purchase a Red Sea Pro PO4 test and even this indicate a concentration around 0.08 ppm lower than the Hi-774

Again, we have no data whatsoever to say that the ICP test provided a more accurate measurement of the PO4 than the Hanna Checker. And as far as switching to another test kit and the actual PO4 concentration, unless you are testing both test kits against the same reference standard at the same time, all we have is conjecture building on conjecture building on conjecture as to the actual PO4 value in the aquarium water.

Dan

 

[Rick Mathew]

@Rick Mathew We are inline with each other. You have found ways around the limitation of the hobby tests and you are able to get results that you can relay on. But you are a skill statistician and know your math. 99 % of people using these test do not have your skill or my skepticism to my own technique. The relay on the figures and make decisions that can be catastrophic. i can´t count all threads that start - high ammonia readings what to do. after that all answers - change water, do that or do that - your fish is going to die. Reading the thread often shows up a total ammonia reading of 0.2 - 0.25 ppm. The truth of this reading is that many total ammonium hobby test often show 0.25 even if it zero. and if the result was the right - it is no danger because a reading of 0.25 NH3/NH4 only will give a concentration of 0.02ppm of the toxic form NH3 at pH 8.2 - and its a safe value.

The Hi 774 is basically the same as you use but convert the reading into PO4 directly. And I have worked out a procedure there I analyse the same sample 10 times in a row and after that can take an average - it means that I with rather high accuracy comes around the ±0.02 limit. But - in my case - it shows that my meter probably have a systematic error of 0.08 ppm and therefore I - as you - use my outside tests to get a reading that I rely on. I was ranting in my last post because there is a tendency that when outside tests and hobby tests show different - it is always the outside tests that you can´t rely on. For me - the outside test is the ones I adapt my use of hobby tests too. I know that my Ca readings always was - as yours 15 - 20 ppm lower compared with the ICP test. After seen this for many tests - I adapt may way of reading my hobby test and now - they are nearly the same.

One thing is the nitrate tests. I have had contact with one of the ICP testers here in Europe that the same time do advanced NO3 and NO2 tests. Their database show that it is very common with NO2 levels around 0.02-0.05 ppm - but there is aquariums with zero upt to 0.15 NO2. Already levels around 0.02 - 0.05 will give NO3 readings below 5 ppm a huge error.

Today - I use Tritons N-DOC in order to know what maximal my NO3 can be. And if I am below max 10 ppm - I fine with that because it probably indicate that my NO3 levels is around 5.

Sincerely Lasse

That is very interesting information on the NO3 tests...Will have to follow up on that one...Thanks

I also believe for the most part we are in line.

I have no idea about the percentage of reefers who have the skills necessary to improve their test results. I also have observed individuals "panicking" because they get a measurement that indicates "trouble ahead" and make decisions to correct the situation and cause major issues with their systems...When in reality the test results we incorrect and the corrections they made caused major problems.....Loss of live stock and even complete tank crashes...

What I will say is having mathematical and statistical skills are really not necessary to get more reliable test results...It does start with a healthy dose of skepticism about the results you are getting....But from there it it takes a desire to improve the outcomes. The Gage R&R looks complicated but there are spreadsheets available that all you need to do is multiple tests. You can do this vs a known value or just do multiple tests to see the test variance...plug in the results and it does all the math for you....But like anything else it take some work and that desire to improve...You obviously have put a great deal of effort into understanding the results of not only your own tests but the results of outside testing services so you are very familiar with what I am saying...considerable effort is required.

That is interesting about the HI-774. I also have the same tester and I have also observed that when I compare my result from the HI-736 I get higher values on the HI-774 testing the same sample (.02-.03 higher)...I stopped using the 774 because I could not repeatability I was looking for...I did not put a lot of effort into it because I was satisfied with my HI-736 results but I have a lot of data that I will take a look and and share.

Thank you for your response and insights...You bring great value to this forum!!

rick

 

[Rick Mathew]

@Rick Mathew We are inline with each other. You have found ways around the limitation of the hobby tests and you are able to get results that you can relay on. But you are a skill statistician and know your math. 99 % of people using these test do not have your skill or my scepticism to my own technique. The relay on the figures and make decisions that can be catastrophic. i can´t count all threads that start - high ammonia readings what to do. after that all answers - change water, do that or do that - your fish is going to die. Reading the thread often shows up a total ammonia reading of 0.2 - 0.25 ppm. The truth of this reading is that many total ammonium hobby test often show 0.25 even if it zero. and if the result was the right - it is no danger because a reading of 0.25 NH3/NH4 only will give a concentration of 0.02ppm of the toxic form NH3 at pH 8.2 - and its a safe value.

The Hi 774 is basically the same as you use but convert the reading into PO4 directly. And I have worked out a procedure there I analyse the same sample 10 times in a row and after that can take an average - it means that I with rather high accuracy comes around the ±0.02 limit. But - in my case - it shows that my meter probably have a systematic error of 0.08 ppm and therefore I - as you - use my outside tests to get a reading that I rely on. I was ranting in my last post because there is a tendency that when outside tests and hobby tests show different - it is always the outside tests that you can´t rely on. For me - the outside test is the ones I adapt my use of hobby tests too. I know that my Ca readings always was - as yours 15 - 20 ppm lower compared with the ICP test. After seen this for many tests - I adapt may way of reading my hobby test and now - they are nearly the same.

One thing is the nitrate tests. I have had contact with one of the ICP testers here in Europe that the same time do advanced NO3 and NO2 tests. Their database show that it is very common with NO2 levels around 0.02-0.05 ppm - but there is aquariums with zero upt to 0.15 NO2. Already levels around 0.02 - 0.05 will give NO3 readings below 5 ppm a huge error.

Today - I use Tritons N-DOC in order to know what maximal my NO3 can be. And if I am below max 10 ppm - I fine with that because it probably indicate that my NO3 levels is around 5.

Sincerely Lasse

Lasse: Below is some (not all) of the data I have on comparing my measurements of PO4 on the HI-736 and HI-774. The tests were done on the same water samples taking at least 3 measurements and getting the average...As you can see my HI-774 measures (on Average) .02 higher than my Hi-736...Have no idea why ... More to explore!...so I just thought I would share it with you.

HI736	HI-774	DIFFERENCE
0.05	0.08	0.03
0.07	0.09	0.02
0.09	0.12	0.03
0.06	0.08	0.02
0.08	0.09	0.01
0.06	0.06	0
0.06	0.08	0.02
0.07	0.09	0.02
0.07	0.09	0.02
0.06	0.09	0.03
0.02	0.05	0.03
0.04	0.08	0.04
0.05	0.05	0
0.03	0.05	0.02
0.05	0.07	0.02
0.05	0.07	0.02
0.03	0.05	0.02
0.04	0.06	0.02
0.04	0.05	0.01
0.04	0.05	0.01
0.03	0.04	0.01
0.04	0.08	0.04
0.05	0.06	0.01
0.04	0.06	0.02
0.04	0.04	0
	AVG	0.02

 

[Dan_P]

Lasse: Below is some (not all) of the data I have on comparing my measurements of PO4 on the HI-736 and HI-774. The tests were done on the same water samples taking at least 3 measurements and getting the average...As you can see my HI-774 measures (on Average) .02 higher than my Hi-736...Have no idea why ... More to explore!...so I just thought I would share it with you.

HI736	HI-774	DIFFERENCE
0.05	0.08	0.03
0.07	0.09	0.02
0.09	0.12	0.03
0.06	0.08	0.02
0.08	0.09	0.01
0.06	0.06	0
0.06	0.08	0.02
0.07	0.09	0.02
0.07	0.09	0.02
0.06	0.09	0.03
0.02	0.05	0.03
0.04	0.08	0.04
0.05	0.05	0
0.03	0.05	0.02
0.05	0.07	0.02
0.05	0.07	0.02
0.03	0.05	0.02
0.04	0.06	0.02
0.04	0.05	0.01
0.04	0.05	0.01
0.03	0.04	0.01
0.04	0.08	0.04
0.05	0.06	0.01
0.04	0.06	0.02
0.04	0.04	0
	AVG	0.02

Could the 0.02 ppm shift be a result of forcing one of the Hanna Checker regression lines through zero? Didn’t @taricha find something odd about the near zero readings in the ULR P Checker?

 

[Rick Mathew]

Could the 0.02 ppm shift be a result of forcing one of the Hanna Checker regression lines through zero? Didn’t @taricha find something odd about the near zero readings in the ULR P Checker?

Now that you mention it I seem to recall a discussion around forcing the slope to go through the zero. I think it was in the development of the nitrate test using the 736 I'll have to go back and look and see....... good memory

 

[Dan_P]

Now that you mention it I seem to recall a discussion around forcing the slope to go through the zero. I think it was in the development of the nitrate test using the 736 I'll have to go back and look and see....... good memory

I was also thinking that Hanna might have forced a regression through zero forone of their Checkers and produced the 0.02 difference between their two Checkers. It might require a spectroscopy study to investigate.

 

[Rick Mathew]

I was also thinking that Hanna might have forced a regression through zero forone of their Checkers and produced the 0.02 difference between their two Checkers. It might require a spectroscopy study to investigate.

Could you find it using standards from...say .01 to .04 measured on both...and plot the slope. ..or am I confused

 

[taricha]

Could the 0.02 ppm shift be a result of forcing one of the Hanna Checker regression lines through zero? Didn’t @taricha find something odd about the near zero readings in the ULR P Checker?

What I noticed was that the amount of color required above zero in order to get a reading is more than the color difference error at other concentrations. It requires about 2-3 times the normal uncertainty to get a reading above zero. In other words, they built in a buffer against false positives.

Could you find it using standards from...say .01 to .04 measured on both...and plot the slope. ..or am I confused

If you want to probe that for your checkers, then use a constant color solution - like drops of tea or koolaid or something, don't bother with the hassle of a chemical test just to check the low range color linearity.

 

[Lasse]

Nearly all analyses I know of is not linear near zero - it is often a error in measurement equipment. High quality gears have compensations to this. To expect this for a equipment that cost $50 - $60 - probably not.

HI-774 and Hi-736 is - IMO - the same meter - it is only the reading thats differ. As mine HI-774 looks like it overestimate my result with 0.08 - IMO - it is probably a fault of individual meters instead of differences between models.

Sincerely Lasse