Adding still more DIY to this test method: I have found that I can use DIY pH calibration buffers made out of Borax and Cream of Tartar to calibrate my Pinpoint pH meter!
The use of Borax as a pH calibration solution has been discussed before in aquarium hobby articles, such as this:
http://reefkeeping.com/issues/2005-02/rhf/index.htm, which refers to this:
http://web.archive.org/web/20021015005420/www.animalnetwork.com/fish2/aqfm/2000/feb/bio/default.asp. Dr. Bingman's calculations suggest using 1/2 tsp. of Borax in 1 pint of water. My measurements suggest something more like 1 1/4 tsp. in 1 quart of water (I probably didn't compress the Borax as much). As Dr. Bingman says, though, the pH of the resulting solution isn't very dependent upon exact measurements. This solution does need to be protected from atmospheric CO2, so it should be kept in a closed container, hopefully with minimal headspace. It will have a pH of 9.23 at 20C, and 9.18 at 25C.
Another easy and reliable DIY pH calibration solution can be created using potassium hydrogen tartarate, aka Cream of Tartar. Both it and Borax are discussed on page 11 of this document:
http://pac.iupac.org/publications/pac/pdf/1978/pdf/5011x1485.pdf. The Cream of Tartar calibration solution is discussed in depth in this document:
http://nvlpubs.nist.gov/nistpubs/jres/47/jresv47n6p433_A1b.pdf. A saturated solution at 25C requires about 6.4 grams per liter, which by my measurements is about 1 1/2 tsp. of the powder. The nice thing about this pH calibration solution is that you just need to make a saturated solution. In other words, you don't really need to measure at all -- just add enough to whatever amount of RO/DI water to make sure that it won't all dissolve. Once you are certain that you have a saturated solution, let the remaining crystals settle out, and then pour the clear solution off for use, leaving the undissolved crystals behind. This solution will have a pH of 3.56 at 20C, and 3.55 at 25C.
Now, I know that the instructions for the Pinpoint say that you need to use 7.0 and either 4.0 or 10.0 pH buffers to calibrate it, but in fact, the two calibration screws are just adjusting the bias (7.0 screw) and the slope (4.0/10.0 screw) of the calibration curve. I have found that if I use the Borax solution to adjust the 7.0 screw to the correct value for that buffer, e.g., 9.18 at 25C, and then use the Cream of Tartar solution to adjust the 4.0/10.0 screw to the correct value, e.g., 3.55 at 25C, then the Pinpoint pH monitor can be calibrated reliably using these solutions. I did find that adjusting the one screw tends to throw the other one out of calibration, and they tend to fight each other if you fully adjust each screw to the target value each time. I can more easily and quickly get perfect calibration of both buffers if I only adjust each screw only enough to make up about half the difference each time. For example, I start by setting the 7.0 screw to exactly 9.18 using the Borax buffer. Let's say that I then find the Cream of Tartar solution reads 3.65 instead of 3.55. Instead of turning the 4.0/10.0 screw until the meter reads 3.55, I halve the difference, and adjust it until it reads 3.60. If I switch back to the Borax buffer, I may find that it now reads something like 9.22, so, again, I halve the difference and turn the 7.0 screw until the meter reads 9.20. After just a few back-and-forth cycles like this, I can quickly get both solutions reading correctly.
I have verified the calibration using other pH calibration buffers with values of 4.0, 4.5, 4.65, and 7.0, and in all cases have found agreement to within +/- 0.03 pH units, which is certainly good enough for the DIY alk test. Obviously, these solutions can be used to calibrate other pH meters than just the Pinpoint, as long as the meter allows you to enter/set buffer values, rather than being programmed to use certain specific buffer values.
