HI50021P is a premium quality calibration Check solution produced in accordance with ISO 3696/BS3978 standards using high purity salts, deionized water, certified weight-Checked balances, and Class A glassware in a temperature controlled environment monitored with certified thermometers. It is specially formulated for use with the HI9813-6 portable pH/EC/TDS/Temp. meter. It allows the user to Check the meter’s calibration status at any time by simply rinsing the probe with water and immersing it in the solution. The meter will indicate the probe’s condition.