if you really want to use the extreme accuracy of +/- 0.01 pH-values, you have to use the best possible electrode for your work. You should contact a laboratory pH-meter manufacturer or a supplier for laboratory equipment for the best recommendation for your work. Only professional laboratory pH-meters (several haundred $ or ) can use different types of dedicated electrodes. Furthermore it is important that your meter is temperature-compensated and can be calibrated by at least two, better three calibration buffer solutions. The shelf-life of these calibration standards is limited. When not in use the electrode has to be stored in a specified storage-solution in order not to get dried.