Response Time of Electrodes Another parameter that is evaluated during the calibration with certain meters that have CAL Check technology is the response time of an electrode. This is evaluated based on the amount of time necessary to reach stability when the electrode is immersed in a new buer that has a dierence in pH larger than 3 pH units from the old one. Oset and Slope of pH Electrode The oset and slope are the most important parameters that can describe the quality of an electrode. With Hanna’s CAL Check technology, the oset of the electrode can be evaluated using one point calibration. Oset is generally determined using a 7.01 pH buer, however, using CAL Check allows the oset to be based on any calibration point. The acceptable range for oset is ±30 mV although a warning may be displayed. A minimum of two calibration points is necessary to determine the slope. Slope can be evaluated between two calibration points and normally should fall within a range of 92% to 110%, where 100% is 59.16 mV/pH @ 25°C. Calibration Points and pH buers The calibration of a pH electrode is normally performed using two points: 7 pH, and 4 or 10 pH. This is based on the assumption that the pH electrode is linear from 3 pH up to 10 pH. For the most accurate reading, Hanna recommends using calibration buers that bracket the sample values. For a variety of applications and measuring points, many Hanna meters oer the ability to calibrate using more than two points. Many Hanna instruments oer 2, 3, or up to 5 calibration points for enhanced accuracy. pH buers 1.68, 3.00, 4.01, 6.86, 7.01, 9.18, 10.01, and 12.45 cover the entire pH range. During calibration, the recognized pH buers are temperature compensated by the instrument in order to account for pH variation of buers due to temperature. For example, a 10.01 pH buer is 10.01 pH only @ 25°C. A table of temperature variation is printed on the label of each pH buer. Custom pH Buers Hanna has implemented the concept of custom pH buers into many of its instruments. This permits the user to add an industry specic buer for calibration. However, temperature compensation during calibration is not implemented because the temperature variation correlation is unknown. Stability During Calibration The stability of readings is important in order to avoid incorrect calibration. Based on this, the conrmation of a new calibration point is done only after stability is reached. Users are informed during all processes about the stability conditions, and any instability will restart the stability evaluation. The stability criteria during the calibration is more rigorous than during the measurement. This mode used in Hanna instrumentation avoids errors by conrmation of calibration points during unstable readings. This principle is respected in any type of calibration, manual or automatic. Out of Calibration Range A measurement is considered more accurate if the measurement readings are close to or within the calibration points. An “out of calibration range” message will be °agged if the measurement exceeds the calibration window. This feature is a Good Laboratory practice (GLP). The user can continue with the measurement and ignore the warning, but the warning suggests there is the possibility of measurement inaccuracy. Calibration Reminder The calibration reminder, like “out of calibration range," is a GLP warning message. Regularly scheduled calibrations are crucial for accurate and repeatable measurements. A warning reminder will be displayed when the sensor needs calibration. Measurements can still be used under the warning reminder. Step-by-Step Calibration In order to avoid errors during the calibration procedure, the meters display indicators that can be followed by the user for a successful calibration. If necessary, it is possible for the calibration steps to be performed in a dierent order by the user. Additional Features GLP and ISO standards require the traceability of operations. Hanna’s GLP documents the quality of calibration, plus information to identify the instrument, operator, and the time at which calibration was performed. Logging is a common feature for many instruments and can be used to record readings. Two working modes are available: log-on-demand and automatic or interval logging. With log-on-demand, measurements that are considered important can be saved with the press of the log button. With automatic or interval logging, the instrument saves all the readings according to a specied interval. Another logging mode is Auto-End logging or log on stability. Many Hanna meters include graphic LCD's with features such as tutorials, contextual help, multi-language support, and icons and messages to guide the user through operation and calibration. Introduction 2 pH 2.3 www.hannainst.com | introduction
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