A simple continuous and fail-safe water-change system

[David Cottle]

Is this concentration your weekly target or daily target? As far as I know one doses every day or every other day, in order to achieve 10-20ppm total per week. Therefore at any given time, providing your continuous water changes, you want 10-20ppm divided by 7 days? A TDS or EC measure is a snapshot in time. It is the concentration there and then and it does not get any lower than that as time goes by.....

With continuous W/C, the goal is to have water conditions constant from day to day, including the NO3 concentration (my target is 10-20 ppm), TDS and EC.

What you're calculating above is not very clear to me.

I'm not trying to calculate how much solution I need to get to 10-20 ppm - that's a different matter that isn't relevant to what I'm trying to explain. You say 65 ppm TDS in my tank is way too high. I'm explaining why it isn't, by showing what the TDS will be corresponding to 10 ppm NO3. The ratio of 85/20 is the ratio of TDS to NO3 in the PPS-Pro solution I use. For a concentration of 10 ppm NO3, the TDS that comes along with it wiil give a TDS concentration of 10 x 85/20 = 42.5 ppm. EC = 42.5/0.64 = 66.4 uS. Multiply by 2 for 20 ppm NO3.

Considering how much solution I need to get there is a different matter altogether, and trying to calculate it as you're doing just confuses the discussion. The PPS-Pro guide I follow is given here (I found the link icon ). I adjust the amount of solution added if/as needed to stay in my target range.

 

[sciencefiction]

Considering how much solution I need to get there is a different matter altogether, and trying to calculate it as you're doing just confuses the discussion.

No, it doesn't. By doing that I am trying to explain a vital point you're missing in your calculation.

the TDS that comes along with it wiil give a TDS concentration of 10 x 85/20 = 42.5 ppm

Your calculations are incorrect. Let's say you're dosing 32.5 ml from a solution with concentration "X". This does not achieve the concentration "X" in the target tank itself but way, way smaller because the tank volume is way way bigger... Your tank is diluting whatever mg/l(ppm) you're dosing into 130 litres of water.....See calculations below.

10 x 85/20 = 42.5 ppm

The 42.5 value you've calculated is not the concentration of NO3 in the solution or tank for that matter. 85/20 is the reverse ratio between NO3 in grams and total ferts in grams. Then you're multiplying that figure by 10 to achieve....??

The solution has 20G of NO3 per 500ml of distilled water. In other words that's 40g NO3 per litre which converted in ppm is 40,000 mg/l or 40,000 ppm , nof 42.5ppm!

If you're to dump the entire bottle of the solution in one go in a tank with a volume of 130 litres, you'd achieve a concentration of 40,000/130=307.7ppm NO3 but since you only want 10ppm, you should only dose 1/30th of that solution at a time. 1/30th x 500ml is 16.7ml. If you want to achieve 20ppm, then that's roughly 33.4 ml or thereabouts.

Point is, whatever the concentration X in the solution, which in this case is 40,000ppm, it gets diluted in the tank, and depending what concentration in the tank you want to achieve, you dose a certain amount of the way more concentrated solution.

However, 10ppm concentration in the tank itself always corresponds to 10ppm and, as per the above in EC, that's about 15 EC..

 

[sciencefiction]

With continuous W/C, the goal is to have water conditions constant from day to day, including the NO3 concentration

Yes, I completely understand this. What I am trying to say is, aim at lower EC/TDS as a constant than what you have right now...because you're way above the base line, i.e tap water and you need less ferts, as per your own target, than what you have at the moment, assuming all the 96 EC difference is ferts, which I highly doubt is the case.....

 

[David Cottle]

Your calculations are incorrect.

I don’t think so. They might be inaccurate, to the extent that different constituents in my fertilizer solution may be taken up by the plants or otherwise depleted at different rates, but my guess is that this would increase the added TDS/NO3 ratio in the tank over that in the fertilizer solution if anything.

The fact that the measured EC rise is in close agreement with my calculated values would seem to support my calculations and conclusions.

 

[sciencefiction]

They might be inaccurate,

I think you have not tried to read any of what I wrote. Your calculations are not just inaccurate but completely wrong. However, as long as you're happy with your results, that's what matters because in the end, one can't calculate plant health or fish health. We can only use technical data as a guide, not as ultimate truth.

 

[David Cottle]

I think you have not tried to read any of what I wrote. Your calculations are not just inaccurate but completely wrong.

Yes, I read everything you wrote. I don't know why you want to make something so simple into something so complicated. I'll try one more time.

The object is to determine how much TDS is added when I dose with enough fertilizer solution to reach 10 - 20 ppm of NO3.

The TDS will be more than 10 - 20 ppm, because the solution has other components besides NO3, and these count in the TDS measurement.

The ratio of total "solids" (electrolytes, i.e.) to NO3 in my fertilizer solution by weight is 85/20 = 4.25. This means there are 4.25 parts of total solids for 1 part of NO3.

No matter how much of the solution I add, or how much it's diluted in my tank, the ratio of total solids to NO3 will stay the same (except to the extent that different ions may be assimilated by the plants at different rates to change the ratio).

Therefore, adding enough fertilizer to raise the NO3 concentration in my tank by 1 ppm will raise TDS by 4.25 ppm.

10 ppm NO3 = 42.5 ppm TDS. 20 ppm NO3 = 85 ppm TDS.

It's too simple to calculate incorrectly.

 

[sciencefiction]

It's too simple to calculate incorrectly.

I am tired of explaining so I will try one more time.

TDS is a measure expressed as either parts per million(ppm) or mg/l. You cannot divide grams by grams

It's too simple to calculate incorrectly.

Not as simple apparently....

I'll try explaining one more time and then I am done.

TDS is a measure you can express in ppm or mg/l where 1 ppm=1 mg/l

The ratio of total "solids" (electrolytes, i.e.) to NO3 in my fertilizer solution by weight is 85/20 = 4.25. This means there are 4.25 parts of total solids for 1 part of NO3.

As explained above TDS(ppm) is a measure of total dissolved solids (in grams) divided by the volume of the solution the solids are dissolved in (in litre)

Therefore your solution of 85 grams solids to 500 ml solution has a TDS of 85 grams/ 0.5 litres. To convert in mg/l=ppm that's 170,000 mg/l .
The TDS of your solution is therefore 170,000 ppm!!!! not 4.25ppm.....

Do you understand where you are going wrong? You are dividing grams by grams and arriving at ppm!! Impossible to calculate TDS like that, which is a measure of milligrams per litre.

What you are doing is calculating that if 85g is the total grams fertiliser and 20g of that is NO3 then the concentration of NO3 in the given 500ml solution is 4.25 titmes less compared to the total grams of fertilisers. For a known TDS(ppm), which in this case is 170,000 ppm, you can then calculate using your "favourite" ratio 🙂 that the NO3 concentration in the solution is 170,000/4.25=40,000 ppm, as already previously explained several times.

Now, you DO need to take into account the volume of water of the tank and the amount dosed from the concentrated solution to see how that concentration dilutes and what ppm NO3 it will give in the tank.....as I already calculated several times in different ways....

However, once you've figured how much of the concentrated solution to dose in the tank to arrive at 20ppm, the TDS increase is simply...20ppm! Of course I am not taking into account other factors, but neither are you in your calculations....

10 ppm NO3 = 42.5 ppm TDS. 20 ppm NO3 = 85 ppm TDS.

10 ppm cannot equal 42.5 ppm and 20 ppm cannot equal 85 ppm

It's like saying that 10=42.5 and 20=85. !!!!

When you have achieved 10 ppm or 20 ppm concentration of NO3 in the tank, then that's the increase theoretically.....as simple as that 🙂

 

[David Cottle]

To resolve this tedious argument, this morning I prepared a solution containing 15 ppm NO3 + micronutrients by taking 5 ml of my PPS-Pro 'macro' solution + 0.5 ml of 'micros' and diluting to 200 ml with tap water; then diluting 15 ml of that solution to 1 L with tap water.

The EC of that solution measures at 428uS. My tap water is 344uS.

My aquarium this morning was 407uS before addition of 6 ml of ferts + micros, and 419 uS afterwards.

QED. Real science works.

The fish are fed and happy .

 

[dw1305]

Hi all,

My aquarium this morning was 407uS before addition of 6 ml of ferts + micros, and 419 uS afterwards.

That is why I like conductivity (in this case expressed as ppm TDS) as a measurement. It doesn't tell you anything specific about the ratio of ions , but it is quick and easy to measure.

The pertinent point (IMO) with regards to TDS is to observe the trend, i.e. is it rising, falling or staying steady.

Then once you know that you can choose what to do next.

If it is rising, it could be either from excess ferts, fish waste or uneaten food.

One simple way to gauge it is to simply stop adding ferts for a week.

I'm sure you knew that already, so the main point of this post is to let you know that if the plants are healthy and well fed then they can go without ferts for a week without any issues

That is it. When the plants are healthy and the tank is running well you have a datum range for conductivity.

Because I use rain-water, the duckweed index and have low nutrient tanks I use the 80 - 140 microS datum. If the tank gets below this, but the plants look healthy, I add some tap water (about 17 dKH). If the tank gets a higher, I add some more rain-water or RO. Because locally is all limestone our rain water has some dKH, presumably from dust it picks up.

If I used <"our tap water">, I'd use the same approach, but my initial datum would be much higher, because the tap water starts at about ~600 microS from all the dissolved limestone.

cheers Darrel

 

[sciencefiction]

To resolve this tedious argument, this morning I prepared a solution containing 15 ppm NO3 + micronutrients by taking 5 ml of my PPS-Pro 'macro' solution + 0.5 ml of 'micros' and diluting to 200 ml with tap water; then diluting 15 ml of that solution to 1 L with tap water.

The EC of that solution measures at 428uS. My tap water is 344uS.

My aquarium this morning was 407uS before addition of 6 ml of ferts + micros, and 419 uS afterwards

He, he. I can't help myself 😛

That's correct.

Your 500 ml solution has a concentration of total ferts of 170,000 ppm

If you add 6ml to a 130 litre that that will give you 6/130,000*170,000ppm =7,846ppm increase in the tank.

Converted to EC that's 7,846/0.64 = 12.26 EC.

Your EC moved from 407 uS to 419 uS, the difference is roughly 12 uS as per the above.

It would be interesting to measure the EC tomorrow, just before you dose the next dose, perhaps to see plant uptake between water changes....

The fish are fed and happy

Glad to hear that 🙂

 

[David Cottle]

Your 500 ml solution has a concentration of total ferts of 170,000 ppm

I find it easier to think in terms of 40 mg NO3/ml (and TDS 170 mg/ml, which I don't normally worry about - except when I'm arguing with you 😉). I already know how many ppm NO3 6 ml adds to my tank.

If you add 6ml to a 130 litre that that will give you 6/130,000*170,000ppm =7,846ppm increase in the tank.

6 ml x (170 mg TDS/ml) /130 L = 7.8 mg/L = 7.8 ppm TDS increase.

Full disclosure: I'm a chemical engineer and have been doing such calculations all my life .

Do you understand where you are going wrong? You are dividing grams by grams and arriving at ppm!! Impossible to calculate TDS like that, which is a measure of milligrams per litre.

FYI, g/g is a dimensionless ratio. Multiplying ppm NO3 by g TDS/g NO3 gives ppm TDS. Dimensional analysis helps these computations.

 

[David Cottle]

It would be interesting to measure the EC tomorrow, just before you dose the next dose, perhaps to see plant uptake between water changes....

EC dropped from 424 to 402 over the two days I stopped dosing early in the week. It all seems pretty stable, considering that my purge rate may drop a little over time and I readjust it, usually by eyeballing the drips, so the change rate fluctuates within a range. I'll also adjust fertilizer dosage if/as needed to stay in a range.

 

[sciencefiction]

FYI, g/g is a dimensionless ratio. Multiplying ppm NO3 by g TDS/g NO3 gives ppm TDS. Being proficient in dimensional analysis makes computations easier.

Yes, I agree. But you know what I meant by 85/20*10ppm not giving you the concentration of the solution...because it didn't take into account the volume of the solution or the volume of the tank later on. .

I find it easier to think in terms of 40 mg NO3/ml (and TDS 170 mg/ml, which I don't normally worry about - except when I'm arguing with you

😀😀😀 I call it a dispute, not an argument 😉

Yes, it seems easier to express in mg/ml.. My long winded explanations is a habit from 25 years back. I was in Maths High School and those days calculators of any kind were not allowed in class...Obviously we had no computers or phones either to be able to cheat.The kids today have it easy...We wrote down everything on paper as you go along with your calculation and it was compulsory to show each line of the calculation, regardless of how easy it is. I can flip sides of equations to caclulate unknowns in my head and get the answer without a calculator or paper but by habit, if I am asked to I write it down, I'll give you each simplification on a separate line....the long way.....

Those days we calculated everything manually on paper, including logarithms, sinus, cosinus, tangens and the likes. So the long way was the more accurate way 😉 Now you can just press a button on your calculator, lol I am an accountant by education by the way but I work in finance. Error prevention, error detection and impact, represents pretty much the bulk of my daily job..I release calculated "numbers"/values of complex financial securities on the market daily, which calculations can have tens to hundreds of pages of paper showing the calculations and verifications, so I've got to think twice before cutting down 3 zeros from a number

 

[Edvet]

I am an accountant by education by the way but I work in finance. Error prevention, error detection and impact, represents pretty much the bulk of my daily job..I release calculated "numbers"/values of complex financial securities on the market daily, which calculations can have tens to hundreds of pages of paper showing the calculations and verifications, so I've got to think twice before cutting down 3 zeros from a number

That's all sciencefiction to me😛

 

[zozo]

That's all sciencefiction to me😛

This whole topic is.. Very interesting but siencefiction..