Hanna low range nitrate accuracy, nitrite interference

[Dan_P]

Dan's being modest. He was filtering zinc out of the red sea NO3 test to improve color formation at least a couple of years before hanna's kit said we should.
Sometimes I think he DIY's all his own tests, and just buys hobby kits to check his work.


it's an impressive amount of color, right!? And they do it with a first step dilution of 7 to 11mL.
I assume the nitrite to color step is pretty simple and efficient, with most kits turning all the NO2 into the color forming compound. Hence the supposition that the NO3->NO2 is vastly improved.

Forgot to mention that the Hanna chemistry is reducing NO3 with Zn at pH 10.5. I believe (never measured) all othe kits perform this reduction at low pH. I wonder about the benefit of this approach.

 

[Rick Mathew]

I am playing with the chemistry, cutting the sample size by half, centrifuging instead of filtering and using the Hanna Cl2 Checker to measure the color intensity. The color intensity surprised me and I immediately checked my NaNO3 for the presence of NO2-. None detected.

If I get a chance this week, I will dope Instant Ocean with NaNO2 to see what happens. If I get really industrious, I will attempt to isolate the metal from reagent B to reduce NO3 to NO2 and measure the nitrite level.

I have some NaNO2 on the way and am going to start poking around...Interesting rabbit hole

 

[syilk1]

Bought one of these today before reading on it as went to a new reef store to buy food. Saw it and was like " I didnt know hanna made that. " Guy said it has great reviews LOL. I went home and opened and was like what the heck.. Need a Major in Chemistry to do this test. I sighed and sat down and watched a bunch of you tube videos which did teach me about the disc and filter and how to cut the packets and pour the powder which I always struggled with the Phosphate checker. Now I at least know hoe to use the packets LOL. I have not checked Nitrates in 6 plus months and have mostly SPS so I usually just let them tell me how they are doing. I did the dilution method as I have a broken skimmer that I have been waiting for an impeller for two weeks so figured Nitrates may be a bit high. I got a result of 11ppm. It was cumbersome and LONG as others have said. MAKE SURE YOU USE FRESH SALT WATER to dilute if you need to. I originally used RO/DI... WRONG. I then decided to see if my well water RO/DI saltwater batch may have Nitrates itself as never have had low nitrates in tank... It read 0.00.. Which is good I guess. But man is this test a pain. I guess I would say stick with Red Sea unless you like to spend 20 minutes doing a test. It will probably go up on shelf for another 6 months

 

[Dan_P]

so, anybody got any sodium nitrite laying around and want to tell us how much spiking tank water with 1.0ppm NO2 raises the hanna NO3 reading?

If the hanna NO3 test were 100% efficient at the step of converting NO3->NO2 before color formation, then 1ppm NO2 would raise the NO3 reading by 1.35.
If it's only 50% efficient at the NO3->NO2 step, then 1ppm NO2 gives you +2.7ppm higher NO3 reading.
Other hobby test kits are way less than 50%, more like 10% ballpark I think. (That's why tiny NO2 reads as big NO3.)

The data below is based on N=1 tests. @Rick Mathew will be doing a more thorough evaluation.

I used the Hanna Total Cl2 Checker to measure color intensity. The NaNO2 came from Loudwolf. I am also running half the test volume (chemicals halved as well). I tested Instant Ocean spiked with 0.1 ppm NO3 with the Hanna Marine NO3 and RedSea Pro NO3 test chemicals. I found that the test color intensity for Hanna is higher than for RedSea, while the “corresponding nitrate level“ for Hanna is lower than that for RedSea.

I think this means 1) Hanna chemistry does a more complete reduction of nitrite to nitrate (corresponding NO3 ppm is slightly higher than NO2 ppm), and NO2 will interfere less as Jonathan surmised, and 2) Hanna chemistry will have a lower limit of detection (Color intensity is higher at the corresponding nitrate level).

 

[Righteous]

The data below is based on N=1 tests. @Rick Mathew will be doing a more thorough evaluation.

I used the Hanna Total Cl2 Checker to measure color intensity. The NaNO2 came from Loudwolf. I am also running half the test volume (chemicals halved as well). I tested Instant Ocean spiked with 0.1 ppm NO3 with the Hanna Marine NO3 and RedSea Pro NO3 test chemicals. I found that the test color intensity for Hanna is higher than for RedSea, while the “corresponding nitrate level“ for Hanna is lower than that for RedSea.

I think this means 1) Hanna chemistry does a more complete reduction of nitrite to nitrate (corresponding NO3 ppm is slightly higher than NO2 ppm), and NO2 will interfere less as Jonathan surmised, and 2) Hanna chemistry will have a lower limit of detection (Color intensity is higher at the corresponding nitrate level).

Thanks for this analysis Dan... super awesome.

 

[taricha]

The NaNO2 came from Loudwolf. I am also running half the test volume (chemicals halved as well). I tested Instant Ocean spiked with 0.1 ppm NO3

I think this is a typo and you spiked with 0.1ppm NO2, right?

I think this means 1) Hanna chemistry does a more complete reduction of nitrite to nitrate (corresponding NO3 ppm is slightly higher than NO2 ppm), and NO2 will interfere less as Jonathan surmised, and 2) Hanna chemistry will have a lower limit of detection (Color intensity is higher at the corresponding nitrate level).

oooh. So you added 0.1ppm NO2 - this would be the same N content as 0.135 ppm NO3.

ok, so Red Sea Pro would need ~1.0ppm NO3 to generate that much (0.1ppm) NO2.
0.135/1.00 = 13.5% efficiency (at turning NO3->NO2)

Hanna would need 0.15 ppm NO3 to generate that much NO2.
0.135/0.15 = 90%(!?!?) efficiency (at turning NO3->NO2)

There's a subtler point here too, maybe.
that you found the raw color to be greater in the hanna vs red sea with just NO2 being tested (0.17 vs 0.13 in the same colorimeter) could be Red Sea "losing" NO2 that was already there.
If the reaction reduces NO3->NO2, it might also reduce it further, to things that don't form color.
We see this if you let the red sea zinc step go too long, you lose color. If you wait many minutes until all the bubbles stop, it forms no color at all. I wonder if you tested the NO2-spiked sample and left out the zinc powder in each product, would we see that Red sea reads higher (because it was losing NO2 in the zinc step) and hanna would read nearly the same (maybe the high-pH version of the zinc reduction stabilizes it from going "too far")?

This is a guess based on the fact that the hanna reaction seems to have essentially eliminated bubbling.

 

[Dan_P]

I think this is a typo and you spiked with 0.1ppm NO2, right?



oooh. So you added 0.1ppm NO2 - this would be the same N content as 0.135 ppm NO3.

ok, so Red Sea Pro would need ~1.0ppm NO3 to generate that much (0.1ppm) NO2.
0.135/1.00 = 13.5% efficiency (at turning NO3->NO2)

Hanna would need 0.15 ppm NO3 to generate that much NO2.
0.135/0.15 = 90%(!?!?) efficiency (at turning NO3->NO2)

There's a subtler point here too, maybe.
that you found the raw color to be greater in the hanna vs red sea with just NO2 being tested (0.17 vs 0.13 in the same colorimeter) could be Red Sea "losing" NO2 that was already there.
If the reaction reduces NO3->NO2, it might also reduce it further, to things that don't form color.
We see this if you let the red sea zinc step go too long, you lose color. If you wait many minutes until all the bubbles stop, it forms no color at all. I wonder if you tested the NO2-spiked sample and left out the zinc powder in each product, would we see that Red sea reads higher (because it was losing NO2 in the zinc step) and hanna would read nearly the same (maybe the high-pH version of the zinc reduction stabilizes it from going "too far")?

This is a guess based on the fact that the hanna reaction seems to have essentially eliminated bubbling.

Yeah, that was a typo. I spiked NO2- NOT NO3- .

I was surprised at the Zn mediated NO3 reduction yield with the Hanna chemistry. A closer look at this will sharpen up the yield number.

I am looking forwards to even more interesting information from the work you and @Rick Mathew are doing with the Hanna NO3 test chemistry.

 

[Rick Mathew]

Yeah, that was a typo. I spiked NO2- NOT NO3- .

I was surprised at the Zn mediated NO3 reduction yield with the Hanna chemistry. A closer look at this will sharpen up the yield number.

I am looking forwards to even more interesting information from the work you and @Rick Mathew are doing with the Hanna NO3 test chemistry.

Getting closer

 

[Rick Mathew]

HANNA HI-781 NITRATE TESTER EVALUATION​

I was excited to see that Hanna Instruments came out with a Nitrate tester so as soon as there were available I picked one up and gave it a “Test Drive” (Pun Intended). What follows are the results of that “Drive”

Some time ago (2017) I started working on a method to measure Nitrates with more precision. Here is a link to the work.

https://www.reef2reef.com/ams/test-meter-for-testing-nitrates.599/

From that time on I have been measuring my NO3 using this method. So in this evaluation I used it as the comparison. I also used the Nyos Nitrate test as kind of a “reality check.”

OUTLINE

• Key Performance Factors Evaluated:
• Ease of Use and Some Variables
• Mixing ​
• Salinity
• Accuracy
• Precision (Repeatability)
• Conclusions

EASE OF USE

Just like any other new test that I have gotten, it took me several test runs to get the hang of it and to work out any “kinks” that could cause variability in the measurement results. I have found that with almost every new test method there are generally some techniques that I have to work out to better perform the measurement. The particular ones in this test for me were:

1) Putting Reagent “C” into the vial without dumping it all over. There was quite a bit of Reagent “C” and if I wasn’t careful it would all dump out at once…This just took a bit of practice and I had it down pretty solid.

2) The extraction and filtering of the sample after the addition and mixing of Reagent “B”. The extraction part was just a matter of figuring out the best way to hold the vial and the syringe. After a few tries I found a way that worked well for me. The filtering part was a bit more difficult. I would frequently get leaks. These leaks would reduce the amount of filtrate I would have for the 10 mL vial and would not bring the volume up to the 10 mL mark. After many tries and discussing the issue with other Reefers, I came to the conclusion I had a faulty filter holder…After all it couldn’t be me!!... I know it is a poor workman who blames his tools J …Nonetheless I was not able to get it to work consistently so I went to plan B. I got some Nylon syringe filter .45 micron filters from Amazon….Problem solved!

3) The third one had to do with the shaking of the sample after the addition of Reagent B. The instruction say to add Reagent B and shake “vigorously”. In some communications with @Dan_P and @taricha about my first measurements and the variability I was seeing, It was suggested through some experiments Dan_P was doing that the “shaking” might be a variable in the measurements…The intensity of the shaking that represented my interpretation of “vigorous” might not be enough and or could vary from measurement to measurement depending on how “vigorous” I was feeling at the moment. I conducted a simple experiment to see if there was any “significant” variation related to the shaking of the sample. You can see the results in Table 1 and Chart 1

TABLE 1

CHART 1
​

The reference in Method 4 to the Auto-Shaker can be seen in the link below. I have been using this shaker for quite a while now and it does a good job…a bit noisy but very helpful. https://www.youtube.com/watch?v=sBlGgNaLMq8

In looking at the data and the chart one can see that the Mixing Intensity can produce variability in the measurement and attention should be paid to this step in order to reduce the variability of the measurement.

SALINITY EFFECT

I looked at the salinity effect at the low end the measurement scale. Table 2 and Chart 2 show the results. There is clearly a salinity effect with this test which adds an additional variable to the measurement outcome. I could not find any literature from Hanna to indicate at what salinity level the test was developed around, but given my results in testing the accuracy I would guess it was around 34-35ppt, but that is just a guess. This was not an extensive evaluation of the salinity effect but nonetheless it is an indicator that we need to pay attention to the salinity as a potential variable in the measurement.

TABLE 2

CHART 2
​

Before I actually did the Accuracy and Precision experiments I worked to reduce the variability from measurement to measurement in these three areas. What follows are the results of the evaluation for Accuracy and Precision.

ACCURACY

THE STANDARDS

I made standards in two different ways. The first one was using a 35ppt saltwater made from Reagent Grade Sodium Chloride and a HACH Nitrate Standard. Concentrations of 1, 3, 5 and 20 ppm were prepared. The second “standards” were made from tank water (35 ppt) spiked 2 ppm with HACH Nitrate Standard. The tank water was tested using my standard procedure and the HI-781 Tester. Three tests were performed using each method and the averages of the 6 values value were used as the “base” amount to calculate the Standard Concentration.

MEASUREMENT PROCEDURE

The standards made from the HACH Nitrate Standard and the sodium chloride was measured 5 times at each concentration level. The HACH spiked tank water was measured 3 times at each level. Both the HI-781 and the HI-764 + the Red Sea Chemistry were evaluated. The resulting measurements were then used to calculate the Relative Accuracy and the Margin of Error at a 95% Confidence Interval. The Results can be seen in Tables 3 & 4. Table 5 is the data used to establish the Standard values for the spiked tank water.

​

TABLE 3

TABLE 4

TABLE 5

Looking at the accuracy results the HI-781 performed well: From a low of 81% to 98% in the tank water spiked samples. I would consider those results to be respectable and defiantly functional for hobby use. They were also closely comparable to my current Red Sea and HI-764 procedure and I would feel confident in replacing my current test with the HI-781 as far as accuracy is concerned.


PRECISION

The precision is the measure of the repeatability of the measurement. I have expressed it as the margin of error at a 95% confidence level. This value is ± value that represents the expected deviation from the true mean of the measurements. I think the best way to see the precision is to look at a plot of the data with the error bars reported by the instrument manufacture and observe the overlap of the individual sample measurements….Large overlap…Good precision…Low overlap…Low precision. In this experiment 5 individual samples were measured at 4 concentration levels (1, 3, 5, 20ppm). Both the HI-781 and the HI-764 + Red Sea Chemistry were evaluated. Below are the charts of the data.

HI-781 Results​

Hi-764 and Red Sea Chemistry Test Results​

The above data indicates good Precision in both procedures. The reported variation one could expect, per the Hanna literature for the HI-781, is ± .25 ppm ± 2% of the reading. These values are represented in the error bars in the above charts. As can be seen from both the data tables as well as the bar charts the performance of the HI-781 was better than the published variation.



CONCLUSIONS

The Hanna HI-781 Nitrate tester is a great addition to the Hanna Checker line. For those of us who have need for a more precise Nitrate measurement, it provides an accurate method for measurement. It does take some effort and time to perform the test as well as some practice to get the protocol down, but if you value the need for the precision it is worth the effort. (IMO). The filter setup appears to be a problem with regard to leaking. I have noticed several posts that mentioned this. The workaround is easy, although it adds to the cost of the test. I got some 25 mm .45 micron syringe filters and they worked fine. I also looked at another solution which was to get a different reusable filter holder. I found this one but never evaluated it. https://www.sterlitech.com/polyprop...tA4n6fB_n9dMPx7bQyfcJcTRZmaBSTbUaAjySEALw_wcB

I hope you find this information useful. Feel free to ask any questions.

Rick

 

[Dan_P]

HANNA HI-781 NITRATE TESTER EVALUATION​

I was excited to see that Hanna Instruments came out with a Nitrate tester so as soon as there were available I picked one up and gave it a “Test Drive” (Pun Intended). What follows are the results of that “Drive”

Some time ago (2017) I started working on a method to measure Nitrates with more precision. Here is a link to the work.

https://www.reef2reef.com/ams/test-meter-for-testing-nitrates.599/

From that time on I have been measuring my NO3 using this method. So in this evaluation I used it as the comparison. I also used the Nyos Nitrate test as kind of a “reality check.”

OUTLINE

• Key Performance Factors Evaluated:
• Ease of Use and Some Variables
• Mixing ​
• Salinity
• Accuracy
• Precision (Repeatability)
• Conclusions

EASE OF USE

Just like any other new test that I have gotten, it took me several test runs to get the hang of it and to work out any “kinks” that could cause variability in the measurement results. I have found that with almost every new test method there are generally some techniques that I have to work out to better perform the measurement. The particular ones in this test for me were:

1) Putting Reagent “C” into the vial without dumping it all over. There was quite a bit of Reagent “C” and if I wasn’t careful it would all dump out at once…This just took a bit of practice and I had it down pretty solid.

2) The extraction and filtering of the sample after the addition and mixing of Reagent “B”. The extraction part was just a matter of figuring out the best way to hold the vial and the syringe. After a few tries I found a way that worked well for me. The filtering part was a bit more difficult. I would frequently get leaks. These leaks would reduce the amount of filtrate I would have for the 10 mL vial and would not bring the volume up to the 10 mL mark. After many tries and discussing the issue with other Reefers, I came to the conclusion I had a faulty filter holder…After all it couldn’t be me!!... I know it is a poor workman who blames his tools J …Nonetheless I was not able to get it to work consistently so I went to plan B. I got some Nylon syringe filter .45 micron filters from Amazon….Problem solved!

3) The third one had to do with the shaking of the sample after the addition of Reagent B. The instruction say to add Reagent B and shake “vigorously”. In some communications with @Dan_P and @taricha about my first measurements and the variability I was seeing, It was suggested through some experiments Dan_P was doing that the “shaking” might be a variable in the measurements…The intensity of the shaking that represented my interpretation of “vigorous” might not be enough and or could vary from measurement to measurement depending on how “vigorous” I was feeling at the moment. I conducted a simple experiment to see if there was any “significant” variation related to the shaking of the sample. You can see the results in Table 1 and Chart 1

TABLE 1

CHART 1
​

The reference in Method 4 to the Auto-Shaker can be seen in the link below. I have been using this shaker for quite a while now and it does a good job…a bit noisy but very helpful. https://www.youtube.com/watch?v=sBlGgNaLMq8

In looking at the data and the chart one can see that the Mixing Intensity can produce variability in the measurement and attention should be paid to this step in order to reduce the variability of the measurement.

SALINITY EFFECT

I looked at the salinity effect at the low end the measurement scale. Table 2 and Chart 2 show the results. There is clearly a salinity effect with this test which adds an additional variable to the measurement outcome. I could not find any literature from Hanna to indicate at what salinity level the test was developed around, but given my results in testing the accuracy I would guess it was around 34-35ppt, but that is just a guess. This was not an extensive evaluation of the salinity effect but nonetheless it is an indicator that we need to pay attention to the salinity as a potential variable in the measurement.

TABLE 2

CHART 2
​

Before I actually did the Accuracy and Precision experiments I worked to reduce the variability from measurement to measurement in these three areas. What follows are the results of the evaluation for Accuracy and Precision.

ACCURACY

THE STANDARDS

I made standards in two different ways. The first one was using a 35ppt saltwater made from Reagent Grade Sodium Chloride and a HACH Nitrate Standard. Concentrations of 1, 3, 5 and 20 ppm were prepared. The second “standards” were made from tank water (35 ppt) spiked 2 ppm with HACH Nitrate Standard. The tank water was tested using my standard procedure and the HI-781 Tester. Three tests were performed using each method and the averages of the 6 values value were used as the “base” amount to calculate the Standard Concentration.

MEASUREMENT PROCEDURE

The standards made from the HACH Nitrate Standard and the sodium chloride was measured 5 times at each concentration level. The HACH spiked tank water was measured 3 times at each level. Both the HI-781 and the HI-764 + the Red Sea Chemistry were evaluated. The resulting measurements were then used to calculate the Relative Accuracy and the Margin of Error at a 95% Confidence Interval. The Results can be seen in Tables 3 & 4. Table 5 is the data used to establish the Standard values for the spiked tank water.

​

TABLE 3

TABLE 4

TABLE 5

Looking at the accuracy results the HI-781 performed well: From a low of 81% to 98% in the tank water spiked samples. I would consider those results to be respectable and defiantly functional for hobby use. They were also closely comparable to my current Red Sea and HI-764 procedure and I would feel confident in replacing my current test with the HI-781 as far as accuracy is concerned.


PRECISION

The precision is the measure of the repeatability of the measurement. I have expressed it as the margin of error at a 95% confidence level. This value is ± value that represents the expected deviation from the true mean of the measurements. I think the best way to see the precision is to look at a plot of the data with the error bars reported by the instrument manufacture and observe the overlap of the individual sample measurements….Large overlap…Good precision…Low overlap…Low precision. In this experiment 5 individual samples were measured at 4 concentration levels (1, 3, 5, 20ppm). Both the HI-781 and the HI-764 + Red Sea Chemistry were evaluated. Below are the charts of the data.

HI-781 Results​

Hi-764 and Red Sea Chemistry Test Results​

The above data indicates good Precision in both procedures. The reported variation one could expect, per the Hanna literature for the HI-781, is ± .25 ppm ± 2% of the reading. These values are represented in the error bars in the above charts. As can be seen from both the data tables as well as the bar charts the performance of the HI-781 was better than the published variation.



CONCLUSIONS

The Hanna HI-781 Nitrate tester is a great addition to the Hanna Checker line. For those of us who have need for a more precise Nitrate measurement, it provides an accurate method for measurement. It does take some effort and time to perform the test as well as some practice to get the protocol down, but if you value the need for the precision it is worth the effort. (IMO). The filter setup appears to be a problem with regard to leaking. I have noticed several posts that mentioned this. The workaround is easy, although it adds to the cost of the test. I got some 25 mm .45 micron syringe filters and they worked fine. I also looked at another solution which was to get a different reusable filter holder. I found this one but never evaluated it. https://www.sterlitech.com/polyprop...tA4n6fB_n9dMPx7bQyfcJcTRZmaBSTbUaAjySEALw_wcB

I hope you find this information useful. Feel free to ask any questions.

Rick

Rick, great post, and yeah, I find this information useful. I am looking forwards to the next episode of “Rick Clears The Fog Of Analytical Methods”. Give us a hint

 

[Rick Mathew]

Rick, great post, and yeah, I find this information useful. I am looking forwards to the next episode of “Rick Clears The Fog Of Analytical Methods”. Give us a hint

Thanks Dan...Appreciate it ...Thanks for all your help

"COMMING SOON" IN A POST NEAR YOU---" SAMPLE STRORAGE EXPERIMETS PART 3"---TRIP 3 TO WONDERLAND

Rick

 

[AcanthurusRex]

Thanks for the great work @Rick Mathew.
I have been using the HI-764 method. I have made my own standards and been good with the results. Based on your comparison I don't think I need to switch the the "official" tester.

 

[Rick Mathew]

Thanks for the great work @Rick Mathew.
I have been using the HI-764 method. I have made my own standards and been good with the results. Based on your comparison I don't think I need to switch the the "official" tester.

Thank you....Appreciate it

I too am satisfied with the other method....don't intend to make the switch

 

[taricha]

This is so great, I love the visualization of the variability. This is one hanna kit where the stated error "+-0.25ppm +-2%" is quite confusing to interpret.

 

[Rick Mathew]

This is so great, I love the visualization of the variability. This is one hanna kit where the stated error "+-0.25ppm +-2%" is quite confusing to interpret.

Thank you...appreciate it.

Yes...I know exactly what you mean...It would be helpful if we could get clarity from Hanna as to exactly what they are reporting and how they arrived at that...I will ask.

 

[Randy Holmes-Farley]

This is so great, I love the visualization of the variability. This is one hanna kit where the stated error "+-0.25ppm +-2%" is quite confusing to interpret.

What is confusing about it? You mean the fact that the % and the straight ppm error did not scale up the same amount diluted vs not diluted?

Accuracy @ 25°C/77°F

±0.25 ppm ±2% of reading @ 25 °C (77 °F) ±2.5 ppm ± 5% of calculated reading using dilution

 

[taricha]

What is confusing about it? You mean the fact that the % and the straight ppm error did not scale up the same amount diluted vs not diluted?

Because since I saw this statement from a Hanna rep on these boards a few years ago, I've always interpreted the hanna error as +- constant OR +-%, take whichever is larger.

"...the accuracy statement = Accuracy @ 25°C/77°F ±0.3 dKH ±5% of reading
...
If 5% of the total reading is great than 0.3 dkH then the use the 5% as your accuracy. If 5% of your total reading is equal to or less than 0.3 dKH then use 0.3 dKH as your accuracy statement."

But the Nitrate reading only goes up to 5, so +-2% will never be bigger than +-0.25ppm. Which means either the "OR" interpretation isn't valid - it should always be "AND" , or the +-2% is meaningless window dressing.

 

[Dan_P]

This is so great, I love the visualization of the variability. This is one hanna kit where the stated error "+-0.25ppm +-2%" is quite confusing to interpret.

The unorthodox reporting of test accuracy might be thought of as Hanna’s way of telling you that the test has a limit of quantitation, in this case 0.25. The +/- 2 % is the normal way to report instrument reading error. I wonder if the detector poops out at high percent transmission or the chemistry just does not produce detectable color at low concentration of the analyte. The color intensity range is quite high in this test. Maybe there is a linearity issue over the entire range. You might resolve this with a spectrometer.

 

[Rick Mathew]

The unorthodox reporting of test accuracy might be thought of as Hanna’s way of telling you that the test has a limit of quantitation, in this case 0.25. The +/- 2 % is the normal way to report instrument reading error. I wonder if the detector poops out at high percent transmission or the chemistry just does not produce detectable color at low concentration of the analyte. The color intensity range is quite high in this test. Maybe there is a linearity issue over the entire range. You might resolve this with a spectrometer.

Yes this is not real clear what they are saying....Accuracy is the ability to measure and report the "True" value of that thing that is being measured...In this case the level of NO3. So if it really means Accuracy then that would mean the reported value would be within ± .25 ppm + 2% of the meter reading of the true value in the range of 0-5 ppm....At least that is my understanding. I have asked Hanna to help clarify this but have not yet received a response.

 

[Dan_P]

Yes this is not real clear what they are saying....Accuracy is the ability to measure and report the "True" value of that thing that is being measured...In this case the level of NO3. So if it really means Accuracy then that would mean the reported value would be within ± .25 ppm + 2% of the meter reading of the true value in the range of 0-5 ppm....At least that is my understanding. I have asked Hanna to help clarify this but have not yet received a response.

My understanding too, with the caveat that a meter reading of 0.25 and below means 0 ppm Is a possibility right? That’s why I am thinking in terms of LOQ.

If I get a little free time, I will look at the spectra of a Hanna test and see if there is detectable color when Hanna reads zero.

@taricha did quite a detailed look at the response of one of the Checkers and thought it look “strange” near zero.