Initially you need to remove the biofilms from the probe and any calcium deposits, then reclibrate and perform the borax test. You may just need recalibrating as per below if the probe fails the borax test. The borax test is just a means of confirming that your meter is working the way it should be working, and that it is in calibratioin and the probe is clean enough to register the correct readings of a known sample.
First, replace your battery in the meter and remove any algae or gross calcium deposits from the probe. Then make fresh Borax solution as listed blow and perform the test.
The big advantage of using the borax test is that it is idiot-proof, easily obtaiable, and allows for a wide variation in technical abilities while still giving accurate reliable results. It takes advantage of the availability of sodium metaborate as a relatively pure product known as 20 Mule Team Borax in the US (not sure about the availability of this product outside the US). The procedure is relatively simple. First, make a test soution of 20 Mule Team borax. It is NOT a soap, rather, it is a sodium metaborate powder that when mixed even anywhere close to 1/2 teaspoonful in one pint of RO/DI water (approx. 2gms/473ml RO/DI water) will result in a buffered test solution that will test out at 9.18 pH value at 80 degrees F.
Test the ability of the pH meter/controller to get this value; if it fails, then the probe is either not clean or the meter is incorrecty calibrated. Before pursuing ANY big changes in your system, first make sure the probe passes this test and has fresh batteries, if not, clean and recalibrate the probe, then recheck the pH of the borax solution.
Calibration is good, but it is always a good idea to do a separate test with the sodium metaborate (borax) every few weeks or any time you have a VOR to see if your meter is testing well during normal continuous usage. I read a lot of posts where folks say that they have calibrated their pH meters and followed the directions exactly, followed all the temp bath suggestions, corrected for every possible error, yet they consistently see pH values that are skewed either high or low in their aquaria based on even the new calibration. When questioned on their procedure after every other possibility has been explored, it turns out that many hqad used standards that had either expired, or been in their LFS’s shops long enough to have expired. I read with interest Craig Bingman’s posts in the late 90's about a similar problem with (name deleted) brand (NOT Americn Marine) of pH calibration solution where the pH 10 standard was off by almost a full pH unit. Craig had purchased a few packets of this particular brand, used a lab bench pH electrode, calibrated with fresh research-quality pH standards and tested these calibration fluids. After much board discussion, Craig proposed that folks use a freshly prepared buffering solution of sodium metaborate that will always comes to the same value under home conditions. This lead to the publishing of his article in Aquarium Frontiers on the use of 20 Mule Team Borax to prepare a home standard solution. I would suggest that you use this procedure to make sure that your pH probe and meter are in actuality measuring what they are supposed to.
Essentially, you are making a buffer standard to test your probe's accuracy in measuring the resulting solution. This solution differs from NIST standard solutions in that NIST solution standards rely on a combined solution using controlled measured amounts of an acid and its conjugate base. Usually you would need to mix precise quantities of two different compounds to make a pH standard solution in a precisely measured volume requiring a Troemner balance and volumetric flasks that most folks cannot afford at the hobby level. These buffers will have a set shelf life, and will deteriorate more rapidly if exposed to less than ideal conditions. Rather than try and make these lab standards (you could, if you had the precision lab ware and balances needed to measure end volume and the acid and conjugate base weights), we can use the properties of sodium metaborate in water to form these equimolar acid/conjugate bases for us (boric acid and hydrated borate ions). This is one of the advantages of using borax as a standard.
The safest way to do clean your pH probe is to soak it in white vinegar for an hour or two, then rinse it in RO water and soak it for 10 more minutes in clean RO water, then remove biofilms from it by soaking it in diluted chlorox. To do this, put 1 tablespoonful of Chlorox (Sodium hypochlorite solution-household bleach) in a clean glass container and dilute it with 9 more tablespoonsful of RO water (using a tall narrow CLEAN olive jar works the best). Soak for 10 minutes, then rinse in clean RO water and soak the probe again for 10 more minutes. After the two chlorox soaks, rinse the probe and soak in clean RO/DI a few times to remove the chlorine, then recalibrate the probe and meter per manufacturer instructions with fresh calibration standards.
To recalibrate the Pinpoint meter and probe, use the little packets of standards. The sachets are much more reliable when opened fresh and in date than the bottles of standards (which start to wander from the exact pH of the standard as soon as they are opened). Before opening the packets of standardizing solution, bring them to tank temp (throw the unopened packets in the sump for a bit, about 10 minutes). Get a ceramic coffee mug for each standard you intend to test, and put it in the sump as well. By doing this, both the mug and the standards will be at tank temp, and the mugs will keep the temp at close to the value for the time you need to do the calibration. Open the 7.0 pH packet at the top only, put the packet (upright) in the mug and add enough tank water at tank temp in the mug, NOT THE PACKET, so that it surrounds the packet in the mug. This forms a water bath that prevents temperature change induced variations in the pH standard. Put the probe into the standard solution and stir it gently. Set a timer for 10 minutes, stirring occasionally, then check the reading at the end of that period and adjust the reading with the meter's potentiometer to the standard's pH (7.00). Rinse the probe with fresh RO water, then shake any excess water off the probe and place it in the 10 or the 4 pH standard (use either one, yu are setting the slope for the amount of change), and set the timer for 10 minutes, stirring occasionally. At the end of that time, adjust the reading on the meter to the pH of that solution, rinse in RO water, shake off any excess water, and retest the 7.0 standard again for 2 or 3 minutes. It should read to within 0.01 pH units of 7.00. If not, repeat the calibration process. If it passes the retest, rinse the probe and put it into service.
If this doesn't fix the problem, repost and I will make other suggestions.
I do not recommend using a toothbrush to do vigorous cleaning on pH probes, as this may damage the selectivity of the probe tip to ions (it'll ruin it). Only use a toothbrush to very lightly remove any gross organic material, if at all. The tips are specially made to be very selective and allow for ionics to move across without losing all the KCl quickly. The meter works by creating a small electric gradient across the tip, and the pH is measured as a function of the electric current generated by this differential. This is why it is crutial to estabish both the center of the pH gradient (the 7.000 buffer) and the slope of the difference between the center and 3 pH units out either way (with either the 4.000 or the 10.000 buffer solution). The meter uses these values to determine what the pH values of a tested solution are once the calibration is complete and the probe is functioning correctly.
HTH
