Nitrate- cannot get it to fall

[areefer01]

At low nutrient levels, the checkers are basically very expensive random number generators.

Probably why we have stressed to stop chasing numbers be it nitrate, phosphate, ph, alkalinity, PAR, or any other number that a hobbyist can generate these days.

Question remains though - are the numbers good enough if we are not trying to cure cancer?

 

[Dan_P]

Why then just nitrate and phosphate? Should we not included any of the other hobby grade results or ICP for that matter?

Traditional hobbyist are somewhat limited to what we can use as you are very much aware of. We are not trying to reach low earth orbit or fly on mars. We have to have faith in how we conducted the test and its result - ratio or not.

Interesting thread never the less.

The issue is not the test results but what we try to infer from those results.

 

[Dan_P]

I didn't know how crude the measurements actually are, so I took the time to calculate it through and see how badly we would actually be approximating the ratios... (the math nerd in me kept me from going to bed. It's 01:00am here)

If you just want the results, ignore the colored part.

The Hanna LR Nitrate checker (HI781) reads 0.00 - 5.00ppm NO₃ with an accuracy of "±0.25ppm ±2% of reading".
The ULR Phosphate checker (HI774) reads 0.00 - 0.90ppm PO₄ with an accuracy of "±0.02ppm ±5% of reading".
Let's say the true values are in the middle of each checker's range: 2.50ppm NO₃ and 0.45ppm PO₄.

The expected readings are then within the following ranges:
NO₃: [2.21; 2.81]
because
2.21ppm + 2.21ppm ⋅ 2% + 0.25ppm ≈ 2.50ppm, and
2.81ppm - 2.81ppm ⋅ 2% - 0.25ppm ≈ 2.50ppm

PO₄: [0.41; 0.49]
because
0.41ppm + 0.41ppm ⋅ 5% + 0.02ppm ≈ 0.45ppm, and
0.49ppm - 0.49ppm ⋅ 5% - 0.02ppm ≈ 0.45ppm

Now convert the readings to N and P masses:
[2.21; 2.81]ppm NO₃ ≈ [0.499; 0.634]ppm N (NO₃ to N by dividing by 4.427)
[0.41; 0.49]ppm PO₄ ≈ [0.134; 0.161]ppm P (PO₄ to P by dividing by 3.066)
True masses:
2.50ppm NO₃ ≈ 0.565ppm N
0.45ppm PO₄ ≈ 0.147ppm P

So we would expect an N:P mass ratio between 0.499 / 0.161 ≈ 3.1:1 and 0.634 / 0.134 ≈ 4.7:1
while the true mass ratio is 3.85:1

Converting to molar/atomic ratios (multiply the mass ratio by 30.974 / 14.007 ≈ 2.211)
we get a molar N:P ratio between 6.8:1 and 10.5:1
while the true molar ratio is 8.5:1
(Shortcut: molar N:P = ppm NO₃ / ppm PO₄ × 1.532)

That's roughly -20% / +23% in the worst case, where both checkers are off in opposite directions. If the errors are random, the error will typically be smaller, though.
However, this isn't the Hanna Checkers' worst-case scenario. The fixed error terms (0.02ppm & 0.25ppm) will make it really messy when you have low nutrient levels. If we had 1.0ppm NO₃ and 0.05ppm PO₄ (true molar ratio ≈ 31:1), the readings could produce much larger errors: anything from ≈ 15:1 to ≈ 68:1 or so.

All that work, just to say: You're right, Dan. At mid-range levels we get a ballpark figure. At low nutrient levels, the checkers are basically very expensive random number generators.

Good night and pleasant dreams. I hope you aren’t too excited to sleep now. And thanks for the calculations. They clarify the point.

 

[EnterName]

At low nutrient levels, the checkers are basically very expensive random number generators.

Probably why we have stressed to stop chasing numbers be it nitrate, phosphate, ph, alkalinity, PAR, or any other number that a hobbyist can generate these days.

Question remains though - are the numbers good enough if we are not trying to cure cancer?

Judging from the different opinions in this thread, you can't make everyone happy at once. But you can get close

To reasonably approximate the N:P ratio and stay within Delbeek's recommendations, you will need to stay at the upper limit of Randy's phosphate target range. And even then it's a gamble. This is also above the study's "safety threshold" of 0.03ppm phosphate, so Randy, the study, and Delbeek are all happy . Not happy are those who prefer low nutrient levels closer to nature, and those who say to stop worrying about ratios all together. Also unhappy will be the people who know that the detectable nitrate and phosphate are just a fraction of the available N and P, but Delbeek made the same simplification.

If you want to keep phosphate levels low, you are lost when it comes to ratios. The readings are just too random, so all you can really do is look at absolute values. This makes those who don't want you to look at ratios happy. If you stay above 0.03ppm phosphate (above 0.06ppm with the Hanna Checker, including its error), the study will also say you are probably fine. But those who believe Delbeek will be very angry with you, since your ratios could be all over the place. Whether 0.06ppm phosphate is low enough for those who like low/natural levels will depend on the individual, but at least you are within Randy's target ranges

You can't make everyone happy, that's just life

 

[areefer01]

The issue is not the test results but what we try to infer from those results.

That is a fair point. Is that not the case though when a hobbyist uses said results to inquire about a issue or cause? I don't believe we are saying the number can be used for A but not B so maybe I am nit picking.