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The calibration frequency for laboratory analysis and measurement instruments is influenced by various factors such as the degree of usage, accuracy requirements, operating environment, and performance. Changhui Instruments has compiled ten questions related to calibration frequencies that are of particular interest to many people, and provides answers to each one to help clear up any doubts. 1. How is the calibration cycle defined in the standard documents? Clause 7.8.4.3 of CNAS-CL01 stipulates that: “The calibration certificate (or calibration label) shall not contain recommendations regarding the calibration interval, unless an agreement has been reached with the customer.” This requirement may be replaced by regulations. It is clearly stipulated that calibration laboratories shall not provide recommendations regarding the calibration interval. The calibration cycle is determined by the laboratory itself, based on the actual usage of the measuring instrument and in accordance with the principles of science, economy, and accuracy of measurement values. After the first calibration of the instrument, the interval between calibrations is initially set at 1 year. If the instrument remains accurate after being sent to a calibration laboratory after 1 year (i.e., the error remains within acceptable limits compared to the first calibration), then the interval can be extended to 2 years, and so on. The maximum interval cannot exceed 5 years; however, periodic checks must be carried out during this period. If any instability is detected, re-calibration is necessary. 2. Why must the determination of the calibration cycle be based on sound reasoning? Let’s start with the calibration cycle, or the verification interval – it is one of the key factors in assessing the quality of measurement work, and it affects the qualification rate of the measuring instruments in use. Only by strictly adhering to the calibration schedule can the smooth progress of various activities such as scientific research and production be ensured. To ensure the accuracy and reliability of measurement values, it is necessary to scientifically determine the calibration cycle. 3. What are the consequences of an unreasonable calibration cycle? Over time, whether the calibration interval for measuring instruments is appropriate depends on the calibration success rate as well as the instrument’s historical calibration records, which can serve as the basic basis for making such decisions. However, over time, or due to changes in the operating environment, or changes in the way and conditions in which the measuring instruments are used, it is possible for the instruments to become inaccurate. Therefore, when a calibration cycle for the measuring instrument arrives, it should be calibrated immediately. Additionally, during the valid calibration period, the instrument’s deviation status should also be checked periodically. Make appropriate adjustments to the calibration cycle based on the above information, extending or shortening it as necessary. 4. What principles should be followed to determine the calibration cycle? Determining the calibration cycle must adhere to two opposing fundamental principles: one is to minimize the risk that the measuring instrument will exceed the allowable error range during this cycle ； Second, it is economically reasonable, minimizing calibration costs as much as possible. To find the optimal balance between the aforementioned risks and costs, a scientific approach must be employed, with a large amount of experimental data collected and analyzed to determine it. 5. Is calibration necessary at the intervals specified in the calibration procedure? User usage patterns vary greatly; if calibration is carried out mechanically and uniformly according to the cycles specified in the calibration procedures without any distinction, it is difficult to ensure that all measuring instruments remain qualified throughout those calibration cycles. Therefore, the calibration interval must be determined based on the actual usage of the measuring instrument. However, due to the considerable complexity of actual conditions, it is difficult to determine the calibration interval with absolute accuracy; rather, it is only possible to aim for a generally correct and reasonable approach that improves the practical situation to make it more scientific and economically sound. Note: Blindly shortening the calibration cycle will result in a waste of social resources, and it will also have adverse effects on the lifespan, accuracy of measuring instruments, as well as on production and labor costs. Extending the calibration cycle merely due to a lack of funds or insufficient personnel is extremely dangerous, as it may lead to even greater risks or serious consequences resulting from the use of inaccurate measuring instruments. 6. What are the criteria for determining the calibration cycle? Determining the calibration cycle requires various types of expertise and takes multiple factors into consideration. If it exceeds one cycle, it may lead to a deterioration in quality characteristics, due to mechanical wear, dust, performance, and the frequency of testing, among other factors. The sensitivity to changes in these factors depends on the type of measuring instrument. Those of good quality are likely to be less affected ； Those of poor quality are likely to be more affected. Therefore, each laboratory should determine the calibration cycle for each measuring instrument based on actual conditions. Basis for determining the calibration cycle: ① Frequency of use. Frequently used measuring instruments are prone to a decline in their measurement accuracy, so this issue can be addressed by shortening the calibration cycle. Of course, improving the quality of the raw materials used in measuring instruments, as well as their manufacturing processes and service life, are also important approaches. ②Requirements for measurement accuracy. For units that require high accuracy, the calibration interval can be appropriately shortened. Each unit should decide based on its own actual circumstances, choosing the accuracy level that is necessary. It should be as high as necessary and as low as necessary, without blindly pursuing high accuracy to avoid unnecessary losses; however, it is also not advisable to have too low an accuracy, as this will fail to meet the requirements for use and cause losses in operations. ③Utilize the unit’s maintenance capabilities; if the maintenance is of high quality, shorten the calibration cycle accordingly ； Conversely, it is longer. ④The performance of measuring instruments, particularly their level of long-term stability and reliability. Even for instruments of the same type, those with poor stability and reliability should have a shorter calibration interval. ⑤For measuring instruments that have a significant impact on product quality or require special specifications, their calibration cycle is relatively shorter ； Conversely, it is longer. 7. How to scientifically determine the calibration cycle? There are four common methods for determining the calibration cycle: the statistical method, the hourly time method, the comparison method, and the chart method. The following introduces each of these methods one by one: ① Statistical method: Based on the structure of the measuring instruments, as well as their expected reliability and stability, the instruments are initially grouped, after which the calibration cycle for each group is determined using general principles. For each set of measuring instruments, count the number of those that are out of tolerance or otherwise non-conforming within a specified period, and calculate the ratio of these instruments to the total number of conforming instruments in that set during the given period. When identifying unqualified measuring instruments, those that are visibly damaged or returned by the user due to suspected defects should be excluded. If the proportion of unqualified instruments is high, the calibration interval should be shortened. If it is proven that the proportion of unqualified instruments is very low, extending the calibration interval may be economically reasonable. If it is found that the instruments in a certain group (whether manufactured by a particular manufacturer or of a particular model) cannot operate in the same way as the other instruments in that group, then that group should be classified as a separate group with a different cycle. ②The hour-time method is a way to determine the calibration interval in terms of the actual number of hours of operation. The measuring instrument can be connected to a timing indicator; when the indicator reaches a specified value, the instrument is sent back for calibration. The main theoretical advantage of this method is that the number of instruments required for verification and the verification costs are proportional to the amount of time used, and in addition, the usage time of the instruments can be automatically checked. For example, Yunnan Changhui Instrument Manufacturing Co., Ltd. uses an oscilloscope from a certain company; without the need to connect a timer, it is possible to determine directly on the oscilloscope how long it has been in use, which makes management very convenient. However, the hour-time method has the following disadvantages in practice: a. This method should not be used when the measuring instrument experiences drift or damage due to storage, handling, or other reasons ； b. Provide and install appropriate timers; the initial cost is high, and supervision is required due to the possibility of interference from users, which further increases the costs. ③Comparative method: When each measuring instrument is calibrated according to the specified calibration schedule, the calibration data is compared with that from previous calibrations. If the calibration results over several consecutive cycles remain within the specified allowable range, then its calibration schedule can be extended; if the values exceed this allowable range, the calibration schedule for that instrument should be shortened. ④The graphical method involves selecting the same representative calibration point for each calibration of the measuring instrument, plotting the results of those calibrations over time to form curves. Based on these curves, the effective drift of the instrument over one or several calibration cycles can be calculated. The data from these graphs can then be used to determine the optimal calibration interval. 8. Can the calibration schedule for laboratory equipment be set by oneself? Generally, after equipment calibration, the certificate recommends calibrating it once a year; however, some people say that certain types of equipment do not need to be calibrated annually at all. Can the calibration interval for the device be set by myself? Will the review panel approve it if it is calibrated according to one’s own specified cycle? It is best to set the calibration interval yourself, as it is related to the way the device is used. The calibration cycle can be determined by oneself, but it is also necessary to take into account the requirements of domestic metrology laws (if you are applying for CNAS recognition). The equipment calibration interval can be adjusted, but only provided that you can provide a reasonable justification for the change; otherwise, it will still not be accepted during the audit. 9. Should I ask the instrument and equipment company about calibration issues? Calibration companies are not aware of factors such as the frequency of use, maintenance status, and operating environment of the equipment; therefore, the calibration schedule they recommend may not be appropriate. For example, a standard platinum resistance thermometer that is well-maintained might only need to be used two or three times a year ； Another standard platinum resistance thermometer is in use for 8 hours a day ； The calibration intervals specified by the calibration company are definitely once every 2 years. This interval is too short for the first standard platinum resistance thermometer, while it’s too long for the second one; calibration may be necessary after six or seven years. This applies only to corporate laboratories; third-party laboratories, due to the need to obtain accreditation, have different requirements, and many of their devices may need to be calibrated. 10. What is the relationship between calibration cycles and periodic inspections? **It is stipulated that during the calibration period, equipment must be recalibrated in case of maintenance, replacement of key components, or relocation of the instruments. Periodic checks on the equipment are also required during this period to ensure its stability and accuracy. If the device – referring here to devices rather than rulers, compasses, etc. – defines its own calibration cycle, that cycle must be shorter than the **specified cycle. The laboratory can customize the calibration schedule based on the characteristics of the instruments, such as their usage frequency, etc., as long as the equipment is used properly and can fulfill its intended functions. Usually, measures such as periodic inspections are required to prove that the instrument is in good condition. However, a longer calibration period is not necessarily better, as the longer the time, the lower the reliability. Measurement calibration is an important aspect for improving laboratory efficiency, and determining the calibration schedule is a key element in measurement work; it plays a vital role in ensuring product quality and service quality. When deciding on the calibration schedule for measuring instruments, it is necessary to conduct a scientific analysis of their actual usage before making a decision. Source: Changhui Instruments http://yunrun.com.cn/