Basically you make up a range of solutions of known concentration and then use these to see how accurate your test kits are. The basic reactions of test kits are very accurate, but sometimes different chemicals in the water (e.g. tannins) and different interpretations by people's eyes/brains are where some inaccuracies creep in. However, some test kits colour reactions also don't match up to the colours printed on their comparison charts very well and calibrating helps you spot these errors.
Once you've made up, for example, a solution of 25ppm Nitrate you can then test it with your test kit and see what reading you get. This then allows you to be more accurate with your tanks. If you make up a range of concentrations, e.g. 0ppm, 12.5ppm, 25ppm etc you can calibrate the kit across the range.
well in that case my suspicions were verified, because that's how I'd imagined it. So what happens when the test kits don't match up to the calibrated solutions? Do you throw them away?
Also where does one get these calibrated solutions from? I can get pH 7 and 4 sachets to calibrate my pH probe, but never heard of phosphate, nitrate, nitrite, ammonia, iron, manganese, potassium etc . . . reference solutions. And of course you are placing your faith that these reference solutions are what they say they are, right?
You make them up from fertilisers. I've never done it though as, like George intimated, you almost never need them in the ordinary planted tank. So I almost never use them. I will be using them to check on how my new tank is maturing, but the readings I get will be good enough to check Nitrite levels rising and falling which is all I'm bothered about. I have a pH meter for that and a TDS meter and GH/KH test kits which should be pretty accurate due to the way they work.
I take it from this discussion we can only apply calibration to liquid test kits and NOT the test strips that you can buy?
The reason I ask is because I'm get the impression you calibrate the test kit because of variations in the manufacturing process of the chemicals that make up the test kit, and the calibration with a made up reference solution helps you adjust for that error. With test strips you can't do that because there might be variations in the manufacture of each and every strip, am I correct in this?
Calibration solutions are made up by adding known amounts of chemicals to deionised water to give you various ppm solutions. So lets say for nitrate you could make up three calibration solutions. One at 5ppm, one at 10ppm and one at 20ppm. Making the solutions is hard as the amounts used are small and so you need accurate equipment. Then using your test kit test all these three solutions and compare against the colour chart. Now if your test kit reads 10ppm when testing the 5ppm calibration solution, you now know that when you test your tank water and it reads 10ppm on the chart it is actually 5ppm. Doesn't matter what type of test kit is used, but you do need to recalibrate at periodic intervals at the test kits do change over time.
Another way is what I like to do in that I make one calibration solution which is at the ppm that I'm aiming for. This means that I now know what the colour is I'm aiming for. When testing actual numbers aren't important to me, but if my levels are low, just right or high.
Completely agree with this. EI Freedom Fighters have sworn an oath, on pain of death, to never test for NO3 or PO4.
The first premise is simple. Algae can never be induced by NO3/PO4. Algae is only ever induced by the presence of ammonia and light. There are a plethora of ways in which ammonia can be produced in a tank, but often, a typical way is through poor plant health caused by insufficient NO3/PO4.
The second premise is ironic. Hobby grade test kits are inherently unreliable. Whether because of interactions of the reagents with unanticipated chemicals in the tank water, age of the reagents, humidity or whatever. As sods law would have it, in my experience the test results would typically show false high concentration.
Now, consider this deadly combination where you did not believe in the first premise and you were unaware of the second premise. You would underdose NO3/PO4, starve the plants, and algae would appear. You would then test and get a false high reading. You would then try to eliminate even more NO3/PO4 from the tank thus getting more algae and falling into a vicious cycle.
In my opinion, this is why we have such astounding proliferation of algae in the hobby. People simply don't believe in the first premise. Visit any LFS and tell the staff you have algae. 99% of the time the response will be "Get rid of your Phosphates", followed by "Buy this PO4 test kit" and finally, "Buy this Phosphate remover".
So now that the test kit flaws are revealed and folks are aware of the second premise the idea is to concoct a series of known concentrations of NO3/PO4 and test each concoction. If you know that a concoction is 20ppm and you test it, and you test kit results show 20 ppm then you know that your test kit can accurately test for 20 ppm. Do the same for the other standard solutions - 10ppm, 30ppm and see how the kit responds. You can even plot a graph of your test kit response to see what kind of fidelity the kit has. You can then see how much correction is necessary so that, for example you can see that the kit reads 5ppm high across a certain range of concentrations.
This is a sound scientific technique, no question, but if you are experiencing algae and are going through all this trouble and do not believe in the first premise, it's a colossal waste of time.
On the other hand, many folks who use the PPS dosing techniques must use the calibration because PPS requires the maintenance of a specific NO3/PO4 concentration profile in order to control growth, or to some extent, coloration. This is a different issue altogether and the PPS guys will chime in.
EI Freedom Fighters' response to all this is a gigantic "YAWN". We dose max nutrients, never suffer NO3/PO4 deficiency and if we want to control growth we turn the lights down. That just requires flipping a switch. We know that if we get algae then we need to track down the source of NH4. We never need to test. The 5 or 7 quid saved by not buying a test kit can be used instead to buy a cool new plant (if I can find the space for it). :idea: