Answer: pH meters do need to be calibrated by the user every day since the membrane does change and is affected by temperature as well. If pH values are very critical then it will need to be recalibrated several times a day, or before each sample test (depending on how critical the value is). Uncontaminated calibration solution is very important as well. For each calibration, pour some solution in a separate container. Keep out of sunlight and away from heat.