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.
Answer: PPM and TDS are abbreviations that represent the concentration of elements in a solution or atmosphere. PPM is short for parts per million and TDS is short for total disolved solids. EC (electro conductivity) is another measurement for TDS. pH is a measure of acidity or alkilinity which dictates plant uptake of particular elements.
Answer: Every time you are foliar feeding & when ever the pH changes out of acceptable range on the reservoir (usually not more than a half a point, example" 6.0 - 6.5, if it went to 6.7 you would want to lower it a little) One should remember to check and adjust pH "AFTER" adding the nutrients, additives and or wetting agents as most nutrients and additives will change the pH. You are wasting your time adjusting the pH before hand. Also adjust pH "AFTER" mixing up foliar spray and before spraying plants.
Answer: At least once a month, pH meters especially. The cleaner you keep the Probe of your meter, the better it will work. Letting film or salt build up on the tips of the Probe can be just as bad as letting the tips of the pens dry out. The tip of the pH probe is porous to allow solution to be absorbed. If you look at the tip of most pH meters you will see that there is liquid in the tip, if that to dries out the meter will not work. You can use distilled water, Calibration solution or Storage solution to keep the tip moist. Put a piece of paper towel in the cap of your meter and pour a little solution in it. This will help stop the probe from drying out.