Originally Posted by Newt
I find it difficult to make reference solutions for PO4 as the amounts for 1 gallon are small. My digital scale does 0.00 but I don't trust anything in the hundredths.
A digital scale gives you a lot of power! You can make serial dilutions of concentrated solutions to even microscopic amounts of substrate. Here is how to do it:
Since the Fertilator only knows liters and gallons you have to remember that 1 gram of water = 0.001L.
The next piece is the capacity of YOUR scale. Say it is 500g. That will be the biggest size solution you can make.
Now go to the Fertilater and put in aquarium volume 0.5L (500g x .001L/g = 0.5L). Then play with the amount of substrate you add until you get the right ppm of PO4. For example when I use KH2PO4 as my substrate I found that if you add 0.00072 g of KH2PO4 to 500g (0.5L) you will have a 1 ppm solution of phosphate. You cannot weigh 0.00072 grams on your scale but if you multiply this result by 1000: 1000 * 0.00072g = 0.72 g you can weigh this amount.
So place a plastic cup that can hold 1/2L on your scale and tare it off. Then add 0.72g of KH2PO4 into the cup. Then add sufficient purified water to make a final weight of 500g.
This solution is 1000x stronger than you want. It will be a 1000ppm reference.
Now take another plastic cup that holds 1/2L and tare it on you scale. Weigh in exactly 0.50g of the 1000ppm reference and then add sufficient purified water to make 500g.
This solution will be 0.5/500 = 1/1000 the strength of the 1000ppm reference solution or
1000ppm *1/1000 = 1ppm
That was a lot harder to write out than it is to do. Here is the short answer.
Make a solution that is 10x, 100x or 1000x the concentration you need. Then use your scale to make the appropriate 1/10, 1/100 or 1/1000 dilution.