Controllers have special ways to do things, but in general, here's how to calibrate a pH meter:
http://reefkeeping.com/issues/2005-02/rhf/index.htm
How to Calibrate and Use a pH Meter
The most important aspect of using a pH meter is correctly calibrating it. Each meter will have a slightly different method of calibration. A number of general rules are very useful, however:
1. Any analytical method, including pH measurement, is best calibrated with quality standards that span the range expected to be measured. Most aquarists calibrate pH meters using two solutions of known pH. A meter that allows only one calibration point is a very poor choice. Using more is fine if the meter allows more than two. When using two solutions to calibrate a pH electrode for use in a marine aquarium, one calibration point should optimally be below 8.0 (typically 7) and one should be above 8.5 (typically 10, but 9 is also sometimes used). When measuring pH in something other than aquarium water, there may be special tricks to use which are detailed below.
Using pH 4 and 7 is often done, but can be a less optimal choice because the range expected to be measured for reef aquarium water (about pH 7.8 - 8.6) is outside of this calibration range. In some cases the error is small enough that this is acceptable; while in others, it may be a problem.
The tables below show the maximum errors that are attained from various errors in the standard buffer solutions themselves (with problems with the standards being the only source of error considered; in reality, there can be additional errors in real measurements). These tables were obtained by simply looking at how much the calibration solutions might vary (first column), then seeing by how much the actual measured value can be off if both standards vary to the stated maximum error and in directions that result in the maximum measurement error (which turns out to be varying in opposite directions when using pH 4 and 7, and varying in the same direction when using pH 7 and 10 standards).
It is clear that with similar errors in the standard solutions, the errors in the measurements at pH 8-10 are smaller when calibrating at pH 7 and 10 than at pH 4 and 7. Whether these differences are important depends on the application and expectations of the aquarist.
Additionally, when measuring pH in a fluid of a lower pH (such as inside a CaCO3/CO2reactor), calibrating at pH 4 and 7 is more sensible than calibrating at pH 7 and 10.
2. Make sure the calibration standards are either new or at least adequate for the purpose. To be sure, use one of the brands recommended in this article.
I have several bottles of pH fluid that I have been using for years. Occasionally, I use a fresh bottle or packet to calibrate my meter. At that time I check the pH values of all of these older bottles, and note the pH on the bottle. I can then use that bottle for future pH calibrations BECAUSE my meter allows me to calibrate with standards at any pH (such as pH 7.03 and 8.85). If the meter does not allow the input of the pH values that precisely, then it is not possible to use this trick.
3. Rinse the pH electrode in pure fresh water before putting it into any calibration standard, and between each standard. Also, be careful to not transfer anything except a trace of purified water into the calibrations standards. Even tap water, when transferred into a calibration standard, can impact the pH.
4. It takes some time for a pH meter to get a correct reading. So let the meter equilibrate to each standard long enough that the value stabilizes (say, holding within +/- 0.02 pH unit for 30 seconds or longer). Some meters beep or otherwise indicate when they are suitably equilibrated.
5. Stirring the solution can help the pH probe equilibrate to the solution, but it also encourages CO2 to enter the fluid. This CO2 can lower the pH of high pH standards, such as pH 8 and greater. I stir mine for about 30 seconds (often with the pH probe itself, though I've also broken them this way) and then let it sit to get a reading.
6. The temperature of the standards is important for two reasons. One is that temperature changes actually change the standards' pH. The other is that the pH electrodes change their response as a function of temperature (described above). The change in standard solution pH as a function of temperature cannot be automatically adjusted for by inputting temperature into the meter, or via its ATC. It is an attribute of the exact chemistry of the buffer used. Some have pH that rises as temperature rises, and some fall as temperature rises. Others rise with temperature in some temperature ranges and fall with temperature in other temperature ranges. Aquarists should be aware of the exact pH at the temperature being used for calibration. Buffers will often have such pH values as a function of temperature printed on the bottle. For example, a standard phosphate buffer has a pH of 7.000 at 25ºC, but 7.04 at 15ºC (a small difference). At the same time, a carbonate buffer with a pH of 10.01 at 25ºC has a pH of 10.12 at 15ºC (a larger difference).
7. After calibrating the meter, go back and make sure that it reads the calibrating solutions correctly (to within whatever error is acceptable) to be sure that it was done correctly.
8. For certain kinds of pH measurements, direct comparison to a known standard may be more useful than using the absolute readings that the pH meter shows. For example, to assess the strength of limewater via pH, make a standard of known saturated limewater (from, for example, a teaspoon of calcium hydroxide in a cup of pure fresh water). That solution has a pH of about 12.45 at 25ºC, but regardless of what is measured, use the number as the standard and see how far off from actual limewater it is (if it is 0.1 pH unit lower, then the limewater is about 79% saturated; 0.2 pH units lower and it is 63% saturated; 0.3 pH units lower and it is about 50% saturated; 0.4 pH units lower and it is 40% saturated, etc.). In this case, exact temperature equivalence between the samples is important. A difference of only 3ºC means a pH difference of 0.1 pH unit for saturated limewater.
