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| What’s the difference between primary and secondary calibration standards? |
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| Primary standards are required to perform an actual recalibration of a unit, whether it’s a chlorine colorimeter or a turbidimeter. Secondary standards are used as a check to see whether an instrument needs to be recalibrated with primary standards. For instance, the EPA requires that you recalibrate your turbidimeter at least once every 3 months using a primary calibration standard. After you calibrate it using the primary standard, you would place the secondary standard into the measuring chamber, take a measurement and write the measured value on the outside of the secondary standard vial. Over the course of the next few months, you would periodically place the secondary standard in the chamber and take a reading. Because secondary standards are designed to maintain a given value, the reading should match the initial reading written on the vial or at least be within the manufacturer’s specified tolerance range. If the reading is outside that range, the turbidimeter itself has drifted. You would then perform another calibration using a primary standard. |
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