Thanks for starting a new thread. It's important that we get as much feedback on our plans for dosing prior to our vacations and the sooner the better. I've got all my equipment and just now worked out some calculations using Chuck Gadd's Calculator.
First you must come to the realization that you do not have to add your nutrients on alternate days. You can add them every day, just use half the amount on each day. You should have an idea of about how much your biomass consumes nutrients per day. This is done when you have fine-tuned the EI method to suit your plant's requirements. I'm finding that with my HQI MH pendants my plants are using/storing 13 ppm nitrates and 2 ppm phosphates every two days. I assume the plants are using/storing at least 13 ppm potassium and try to dose potassium accordingly. I've also found I can go two weeks without a water change while still maintaining my dosing regimen, even after moderate pruning, with no ill effects.
So now I look at my newly purchased equipment and find that the SP3000 Aquamedic peristaltic pump pumps at a constant rate of 50 ml/min. and the digital timer has a minimmum programmable time of 1 min. so, the minimum volume I can add per dose is 50 ml. These are the minimum limits of my equipment.
How Many doses will I need? I will be gone 16 days and I want to be able to test it beforehand, so 16*50 ml = 800 ml. I'll make up 2000 ml so that I have enough to test/add for a week before I leave and some left over so my tubing stays submersed. I can use the remainder when I come back. Heck, if all goes well, I may even keep it an automated dosing system.
Using Chuck's Nutrient Calculator, I put in 2000 ml as the amount of water to mix with and entered my aquarium volume. I find that 130 g KNO3 gives a concentration of .12 ppm nitrate per ml for my 90 gallon tank. Multiplied by 50 ml I get 6 ppm per dose, which is close enough to the 6.5 ppm nitrates my biomass uses daily.
For KH2PO4 I find that 20 g in 2000L gives a concentration of 0.02 ppm phosphate per ml for my 90 gallon tank. For a 50 ml dose would give 1 ppm, which is what is used on a daily basis.
Finally Potassium. I find that I'm getting 3.5 ppm of K per dose of KNO3 (using Chuck's calculator), so I want to add another 3 ppm daily to match that of nitrate (roughly). The calculator doesn't calculate the potassium derived from KH2PO4, but I assume it is negligable as the target range for potassium is quite large. I found that 90 g of K2SO4 in 2000 ml of water gives 0.06 ppm per 1 ml for a 90 gallon tank. Multiplied by the 50 ml gives 3 ppm per dose. Add that with the potassium from KNO3 and I will get 6.5 ppm per 50 ml dose.
One thing you need to think about when figuring out your macronutrient stock solution is the solubility of each component. KNO3 dissolves quite easily as we all know. It has a solubility of 85 g per 100 ml of water. KH2PO4 and K2SO4 on the other hand have a very low solubility; only 15 g per 100 ml and 12 g per 100 ml of water. This comes into play if you were thinking about making a more concentrated stock solution.
Since I will be making up a dilute stock solution, I will only be adding 5.8 g KNO3 per 100 ml Water, 1 g KH2PO4 per 100 ml Water, and 4.5 g K2SO4 per 100 ml Water, so all components should dissolve nicely. I believe solubilities are additive, so that would give me 11.3 g per 100 ml of Water which is very close to the maximum solubility of 12 g per 100 ml Water for K2SO4. We'll see!
Since I still have not heard of a good reason for not mixing micros with macros I'm planning on doing just that. If reactions/precipitates were going to happen, they would do so in the tank itself and we'd know about it by now, IMO. I find that I'm using 20 ml per day of my stock solution of 1T CSM+B in 250 ml Water which gives me 0.2 ppm Fe in my 90 gallon tank. Again, I've been doing this with no ill effects, just the opposite, I've noticed very positive effects in plant growth and overall plant health. So in order to continue to dose 0.2 ppm dialy, my stock solution must be multiplied by a factor of 2.5 (50/20) so that I will add 1T CSM+B in 625 ml Water. But that alone still won't give me 0.2 ppm per 50 ml dose from a 2000 ml macro/micro stock solution, so I multiply it by 3, so 3T CSM+B in 1875 ml Water (rounded to 2000 ml is close enough), will give approximately 0.2 ppm, per 50 ml dose.
Actually, I may not have enough solubility in my stock solution to also add the micronutrients and expect it all to be dissolved, so I may need a seperate dosing system anyway. I have a Kent's drip system, but that would require it to be "on" 24/7, which may not be efficient. Dang, I guess I need another perstaltic pump and only 14 days to vacation! That's why we need to think about these things way in advance.
I still need to find 1/4" OD tubing for the pump and test it to see if I actually get 50 ml in one minute and adjust my calculations as necessary. There is actual tubing inner diameter and height from pump to top of tank to consider and will affect the actual pump rate. I also want to test the aquarium water after the first automatic dose to see if I'm getting what I expected and I want to keep testing regularly to see if the plants are using/storing nitrates and phosphates at expected rates.
All in all there's only so much that can be planned for. My area is notorious for power outages while I'm on vacation, but since all my equipment, including feeder is electrical, they should all be in synch when the lights back come on....unless the digital timer resets itself then the fertilization time may be off which isn't bad...I'll have to check!