On meters without ATC the pH should be checked at the same time of day each time in order to minimize any temperature related fluctuations. The tip needs to be stored in a electrode storage solution or in a buffer solution. The tip should never be allowed to dry out. Both of these chemicals are relatively safe, although they can cause burns and should never come in contact with the eyes. Most hydroponic supply stores sell pH adjusters that are diluted to a level that is reasonably safe and easy to use.
Concentrated adjusters can cause large pH changes and can make adjusting the pH very frustrating.
Several other chemicals can be used to adjust the pH of hydroponic nutrient solutions. Nitric acid and sulfuric acid can be used to lower pH but are much more dangerous than phosphoric acid. Food grade citric acid is sometimes used in organic gardening to lower pH. Always add the nutrients to the water before checking and adjusting the pH of your nutrient solution. The fertilizer will usually lower the pH of the water due to it's chemical make up. After adding nutrient and mixing the solution, check the pH using what ever means you have.