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SOP for Operation and Calibration of Field Water Quality Instruments 2 <br />3.2 pH <br />Most pH meters require frequent calibration, and must be calibrated in the field before use at each sampling <br />station. pH is temperature- dependent, and while temperature compensating instruments (such as the <br />Oakton® PC 10) are preferred, non - temperature- compensating instruments are acceptable so long as the pH <br />reading is recorded then adjusted/corrected for temperature. Nearly all pH probes are sensitive to drying <br />out, and must be kept moist at all times (especially when not in use) for proper operation. A pH probe that <br />has dried out may not calibrate properly. Malfunctioning pH probes must be replaced immediately. The <br />procedures below are specific to the Oakton® PC 10 and may be generally relevant to other pH meter <br />makes and models. <br />3.2.1 Calibration <br />1. Perform a battery check prior to initiating calibration. <br />2. Calibrate the pH meter prior to use at each site in the field. The pH meter also will be re- calibrated <br />when check standards drift by greater than 0.2 pH units. <br />3. Use specific calibration techniques according to the manufacturer's instructions using the three <br />standard buffer solutions (4.01, 7 and 10 pH). Rinse the electrode with deionized or distilled water <br />between solutions. If the meter does not automatically compensate for temperature, the pH probe is <br />faulty and should be replaced. The temperature sensor included on the probe is factory calibrated and <br />should only be re- calibrated if suspected errors have occurred over a long period of time, or if the <br />probe has been replaced. <br />4. Document calibration information in the fieldbook. Information to be recorded includes all <br />calculations, readings taken during calibration, adjustments made to meter, and accuracy prior to and <br />after calibration. <br />3.2.2 Field Operation <br />1. Immerse probe end of meter into sample, allow sufficient time to equilibrate to a stable value (t 0.1 <br />s.u.). Record reading. <br />2. Rinse probe with deionized or distilled water and cover with water - filled cap or place a moist/wet <br />tissue inside the cap. <br />3.3 Specific Conductance (SC) <br />Specific conductance (SC) is also commonly referred to as "conductivity" or "electrical conductivity" (EC). <br />Units of SC are µmhos /cm (micromhos per centimeter) or µS (microSiemens), which are essentially identical <br />units. SC is even more temperature- dependent than pH, and while temperature- compensating instruments <br />(such as the Oakton® PC 10) are preferred, non - temperature - compensating instruments are acceptable so long <br />as the calibration and field readings are manually adjusted for temperature. The procedures below are specific <br />to the Oakton® PC 10 and may be generally relevant to other SC meter makes and models. <br />3.3.1 Calibration <br />1. Perform a battery check prior to use in the field. <br />2. Calibrate the meter once monthly according to the manufacturer's specifications. Check the <br />calibration, as necessary, using reference solutions between routine calibrations. <br />3. In general, calibrate the instrument to each conductivity range that will be measured. A table of <br />conductivity ranges and recommended calibration solution ranges can be found in the manufacturer's <br />manual. The minimum calibration value should be 20% of full scale for the specific range. Make <br />adjustments as directed by the manufacturer's specifications. <br />4153A.140129 Whetstone Associates <br />