Not sure if i missed the feature, but if Fertilator could calculate a Target ppm for each chemical according to Tank Volume, or its not as simple as that

Thanks!

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Not sure if i missed the feature, but if Fertilator could calculate a Target ppm for each chemical according to Tank Volume, or its not as simple as that

Thanks!

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When selecting the volume of the aquarium it shows the units as gallons, however the calculation seems to be using Liters.

For example, when I add 10g of KNO3 to my 82gal aquarium it raises the nitrate level about 5.2 ppm (actual / measured) but this fertilator says it should raise the ppm 19.8 ppm. If I multiply my 82gal x 3.79 (the conversion factor from gallons to liters) = 310L. Using 310 as the volume (in "US gallons") the fertilator gives me 5.2ppm, exactly what I measured.

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Further still, when I select the Liters unit the calculations are off by another 3.79x. Using the same numbers from above, ( 82gal = 310L ) + 10g of KNO3 = 5.2ppm. However when I input 310L + 10g KNO3 it says I should get 19.8ppm, which is the same percentage error. The result should be exactly the same as above, 5.2ppm.

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Another calculation in question... The product I bought from GLA labeled "mono-potassium phosphate" gives me a measured .75ppm increase of phosphate in 82gal per .44g dose. This corresponds to the fertilator's calculation for "K2HPO4", which is actually Dipotassium phosphate. I'm not sure who's wrong, the Fertilator, or GLA but there's definitely a discrepancy here.

On a positive note, the US gallons function appears to be working correctly for this calculation.

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I'm not going to do UK gallons but I think those calculations, including calculations for each chemical are worth checking as well at this point.

Had me scratching my head on this for a while.

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For the record: I started into this trying to calculate the exact volume of water in my aquarium system (Considering concrete background, lower then normal water level, plumbing & filter volume, etc.) for more accurate fert dosing. I figured if I knew the dose size of a fert, and the before / after nutrient concentration I could work backward to find the exact water volume. But the calculations using the fertilator were wildly off from reasonable expectations & my own measurements. Once I figured out the errors above, 82gal clicked for both nitrate and PO4 calculations exactly in line with measured results. Further, I have a high degree of confidence in my measurements because I am using a quality milligram scale to measure dry fert doses, and a Hach DR2000 spectrophotometer to measure actual nutrient ppms in my aquarium water.

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The correct answer is 19.8 ppm of NO3-I have noticed what I believe are errors in the calculations.

When selecting the volume of the aquarium it shows the units as gallons, however the calculation seems to be using Liters.

For example, when I add 10g of KNO3 to my 82gal aquarium it raises the nitrate level about 5.2 ppm but this fertilator says it should raise the ppm 19.8 ppm. If I multiply my 82gal x 3.79 (the conversion factor from gallons to liters) = 310L, I get the correct 5.2ppm.

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10g KNO3 x 62g per moleNO3 / 101.1 g per mole KNO3 * 1/82 gal * 1/3.78L per gal *1000 mg per g = 19.8 mg/L = 19.8 ppm

Since you do not show any of your calculations it is not possible to say why you are wrong but maybe you are finding ppm's of N not ppm's of NO3. If you divide PPM's of NO3 by 4.4 you get ppm's of N. Post your calculations!

The problems are simple programming errors. Someone started with liters, then forgot and labeled the pull down menu "gallons". That error got perpetuated for the calculation associated with the "liters" label in the pull down menu when the same formula was used for the "liters" calculation but multiplied by the 3.79 conversion factor. That explains why the "Liters" calculation is off by the same amount in the same direction. The "US Gallons" calculation is actually the correct "liters" calculation.

I'm not sure what's going on with the PO4 stuff. Like I said, it's possible GLA sold me mis-labeled fertilizer, but based on what I've seen already, my money is on the fertilator being programmed wrong on that too.

If you're looking for PPM N. In one mole of KNO3 you have only 14.01g.

In 10g you have about 1/10 of the mass of N since the molar mass of KNO3 is 101.

So you can say in 10g of KNO3 you have 1.4g of N.

1400 mg/310L = 4.5ppm. These are all very rough estimates. I think the discrepancy is coming from the fact that there is a moles ppm and a mass ppm. The held convention in chemistry I believe is mass ppm.

The measured concentrations I'm getting don't correspond "roughly" to the errors I've pointed out. They correspond EXACTLY. It's a predictable error. If I was getting numbers that didn't fit an easy pattern I wouldn't be so certain but in this case it's obvious.

If you want the fertilator to give correct nitrate results, enter your aquarium volume in liters but select "US gallons" in the fertilator. Doing that will give you the correct KNO3 dosage to ppms of nitrate calculation.

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But, you're aware ppm is mg/L and that the manual calculation by ray-the-pilot only makes the following assumptions:

1) The purity of the solute (KNO3)

2) The purity of fluid (water)

3) The solubility and rules of Chemistry

Since your very nice equipment is significantly (~400%) different from that calculation, you have to wonder about 1, or 2 (this includes, perhaps, your tank having flow such that the location of your sample has a lower concentration of NO3 ions relative to the rest of the tank, or the calibration of your scale, your estimate for water volume is way off, maybe that your plants are N limited and uptook super duper fast, a combination of some of any of that, or something else). Regardless, you should be calibrating the tools vs a known sample of RO/DI and KNO3. The steps ray took above is exactly how you'd calibrate vs the sample.

It is not a question of the niceness of the equipment. In this case, I'm gambling on the math because of assumption 3 above.

I don't know what else to say here guys. Test it for yourself & see.

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I'm really not being a jerk, man: you've actually done the 1 gallon distilled test above, yeah? Also calibrated the scale vs APC's community sample weights for KNO3 teaspoons? Does a US dollar bill weigh 1 gram on that scale? I ask because in my line of work I've been *sure* of things based on years of experience and many metrics only to be proven wrong by something I took for granted. It's good to reset and look at all the assumptions and compare vs a method with the least assumptions.

I'll see about video recording the test on a 1gal bottle of distilled water & posting that video to youtube. Not sure if I can pull all that off but we'll see.

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I have done the arithmetic. :banplease

- I found all conversions within rounding errors.
- I found no significant difference between 82-US gallons and 310 liters.
- When Fertilating KNO3 for 82 US gallons and 310 liters the difference is 0.02-ppm NO3 and 0.01-ppm K.

- 10g KNO3*14.01g/molN/101.1MolN*1/82gal*1/3.78*1000mg/g=4.5ppm N.
- Some may notice when we multiply 4.5 by 4.4 the resultant is 19.8.
- No surprise when 10gKNO3*62g/molNO3/101.1*1/82gal*1/3.78*1000mg/g=19.8ppm NO3.

I cannot say what Oreo is doing, but I can say the Fertilator is within reason arithmetically correct.

I am indeed using a cadmium reduction method, and upon re-reading the summary of method provided by Hach, the units are indeed displayed in NO3-N. (Multiply that by 4.4 to get ppm of NO3) That was my mistake, and a good catch by JoeRound for noticing the difference.

I wonder if the same thing is causing the discrepancy with my PO4 readings.... time for me to go back & re-check things.

Wonder if I've been massively overdosing my ferts this whole time by mistaking ppm of N for ppm of NO3? Sheesh.

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732 Posts

Well I've only been doing this since 1970; so, I defer to your experience. However in my laboratory we do it this way:I'm not doing calculations. I'm dosing & testing using high quality professional lab equipment. I'm a professional with nearly 10 years of water treatment experience.

You need at least three measurements: the sample, a reference standard and a blank.

The sample is your aquarium water.

The blank is distilled water ( no nitrates present)

The reference standard is a solution that you make up with a known concentration of NO3-

You could take 10.12 g of KNO3 and dilute it to 82 gal. That would be a known concentration of 19.997 ppm NO3- but that would waste a lot of water.

I weigh about 408 mg of ACS grade KNO3 on my analytical balance then dilute it to 250 ml with distilled water. I take 10 ml of this solution and dilute it to 0.500 L with distilled water. The final concentration of NO3 in this solution is:

408 mg KNO3 *1/250ml * 10ml * 1/.5L *62.005 (g/mole NO3-) / 101.103 (g/mole KNO3) = 20.02 mg/L of NO3- = 20.02 ppm NO3-

The test I use requires 5 ml of sample; so I use three matched cuvettes. In the first I add 5ml of distilled water, the second has 5 ml of the reference and the last has 5 ml of the sample.

I conduct the reaction. After 10 minutes I set the spec wavelength to 560 nm and set 100% transmission level using the blank.

I measure the absorbance (A) of the sample and reference at 560 nm.

The concentration of the sample is:

A(sample)/A(reference) * 20.02 ppm

In order to make an accurate reference solution you need to know a lot of chemistry, or you could use the Fertilator!

I have never found the fertilator to make an inaccurate calculation but then, what do I know. I only get good results?

You're late. ^Read one post up.

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120 Posts

http://www.fluidsensoronline.com/2010/03/mono-potassium-phosphate-kh2po4/

to my attention and I compared the info there with Fertilator and there is a nearly 3x difference in the PO4 dose rate between the two. Can someone shed a little light?

Pat

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Thank you all for the hard work.

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The difference you're seeing isn't calculation errors, but the difference between Phosphorous (P) and Phosphate (PO4). P is, by mass, about 23% of KH2PO4. PO4 is about 70% of KH2PO4. That difference of about 1:3 is what you're seeing between the link and Fertilator.

This can be confusing, of course. In the end, both measure the same thing, and as long as we keep units consistent, we can compare apples to apples. Thinking in terms of either P or PO4 isn't more correct than the other, Chemistry or not, by the way.* But Andy Ritter has started trending P as well as PO4 as he tweaks and measures PPS-Pro (with calibrated solutions) in this awesome thread, if interested: http://www.aquaticplantcentral.com/forumapc/showthread.php?p=557114

* The Fertilator and, say, your spreadsheet speaking in terms of PO4/Phosphate is good because so do us hobbyists, and that makes it easier to compare apples to apples, though.

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Thanks Wet,

That clears it up for me.

Pat

That clears it up for me.

Pat

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