High precision: The accuracy of the calibration powder solution of the pH tester is 0.01pH. Compatibility: Suitable for most pH meters, ideal for calibration . Good quality: Produced with top quality material. Sealed by individual moisture-proof packaging. Easy to use: Mix the buffer powder with 250ML distilled water, put the pH meter into the solution, stir gently and wait a few seconds for it to result. Pack of 15: 5* ph 4.00 calibration packets, 5* ph 6.86 calibration packets, 5* ph 9.18 calibration packets, economical and flexible to use. Specifications: Accuracy: 0.01pH at 77F(25C) Instructions for use: Step 1: Completely empty the powder in a dry glass container. Step 2: Add 250mL of deionized water (or distilled water) to the beaker. Step 3: Stir until until the powder is completely dissolved. Step 4:Put the pH meter in the solution and stir gently, then wait a few seconds for the result. Warning: non-drinkable solution!< /p> The pH meter may need to be recalibrated mainly in the following cases: -Inactivity for a long period of time, for example more than 3-4 weeks. -Very frequently used. -Can get stable pH meter reading. -After testing strong acid or alpha acid. -The requirement for test accuracy is high