The calibration period of the laboratory analysis measuring instrument is affected by many factors such as the frequency of use, accuracy requirements, use environment, and performance. It can be said that determining the calibration cycle is a complicated task. Many analysts often have questions, such as how to determine the principles and methods of the calibration cycle. What are the current standards for determining the calibration cycle? Can the laboratory's internal calibration cycle be freely changed? The answers are revealed one by one!
Provisions for calibration cycles in the standard documents
5.10.4.4 in CNAS-CL01
The calibration certificate (or calibration label) should not contain recommendations for the calibration interval unless an agreement has been reached with the customer. This requirement may be replaced by regulations.
It is clearly stated that the calibration laboratory cannot give recommendations for the calibration cycle. The calibration cycle is determined by the laboratory based on the actual use of the measuring instrument and based on the principles of scientific, economic and quantitative accuracy. However, during the period, it is necessary to check the period. If unstable conditions are found, it needs to be recalibrated.
The calibration cycle must be determined
The calibration cycle, which is the confirmation interval, is a key measure of the quality of the measurement work and is related to the pass rate of the measuring instrument in use. Only when the calibration cycle is strictly implemented can we ensure the smooth progress of scientific research and production activities. In order to ensure accurate and reliable values, the calibration cycle must be scientifically determined.
As time goes by, the calibration cycle of the measuring instrument is reasonable, depending on the calibration pass rate, and also depends on the historical calibration record of the instrument, which can be used as the most basic basis. However, changes in time or changes in the operating environment, or changes in the manner and conditions of use of the measuring instrument, may result in instrument misalignment. Therefore, when a calibration cycle of the measuring instrument has passed, it is immediately calibrated. In addition, during the effective calibration period, the state of deviation of the instrument should also be checked from time to time. According to the above information, the calibration cycle is adjusted appropriately, and the calibration cycle is extended or shortened appropriately. 
The principle of determining the calibration cycle
Determining the calibration cycle must follow two basic principles of opposition:
First, the risk of measuring the instrument exceeding the allowable error during this period is as small as possible;
Second, the economy is reasonable, so that the calibration cost is as small as possible. In order to find the best balance between the above risks and costs, a scientific method must be used to accumulate a large amount of experimental data, which is determined after analysis and research.
The user's usage is very different. If the machine is calibrated according to the cycle specified by the calibration procedure without distinction, it is difficult to ensure that all measuring instruments are qualified during the calibration cycle. Therefore, the calibration cycle must be determined according to the actual use of the measuring instrument. However, because the actual situation is quite complicated, it is difficult to determine the calibration cycle absolutely correctly. It can only be required to be generally correct and reasonable, so that the actual situation is more perfect, scientific, and more economical and reasonable.
Note that blindly shortening the calibration cycle will result in waste of social resources, which will also have an adverse impact on the life, accuracy, and production and manpower of the measuring instrument. It is very dangerous to extend the calibration cycle simply because of lack of funds or insufficient personnel, which may bring greater risks or even serious consequences due to the use of inaccurate measuring instruments.
Determine the basis for the calibration cycle
The determination of the calibration cycle requires a variety of expertise, taking into account a variety of factors. If it exceeds one cycle, it may cause deterioration of quality characteristics due to mechanical wear, dust, performance, and frequency of experiments. The sensitivity to changes in these factors depends on the type of measuring instrument. Good quality may be less affected; if the quality is not good, it may be affected more. Therefore, each laboratory should determine the calibration cycle of each measuring instrument based on actual conditions.
The basis for determining the calibration period is:
Frequent use: The use of frequent measuring instruments makes it easy to reduce the metering performance, so the calibration cycle can be shortened. Of course, improving the nature, manufacturing process and service life of the raw materials used in measuring instruments is also an important means.
Measurement accuracy requirements: For units requiring high accuracy, the calibration cycle can be appropriately shortened. Each unit should decide according to its own actual situation, and what level of accuracy is required. The high is high, the low is low, and the high accuracy is not blindly pursued, so as to avoid unnecessary loss; but the accuracy is too low, the demand cannot be met, and the work is lost, and it is also undesirable.
Maintenance capacity of the use unit: If the maintenance of the unit is better, the calibration cycle is shortened appropriately; otherwise, it is longer.
Measuring instrument performance: especially the level of long-term stability and reliability. Even for the same type of measuring instrument, the calibration cycle should be shorter if the stability and reliability are poor.
For measuring instruments with large product quality and special requirements, the calibration period is relatively short; otherwise, it is longer.
How to scientifically determine the calibration cycle
Statistical method:
According to the similarity of the structure, expected reliability and stability of the measuring instrument, the measuring instruments are initially grouped, and then the calibration cycle of each group of instruments is initially determined according to general conventional knowledge. For each set of measuring instruments, count the number of out-of-tolerances or other non-conformities within a specified period, and calculate the ratio of these instruments to the total number of instruments in the given period for a given period. Instruments that are significantly damaged or returned by the user due to suspiciousness or defects should be excluded when determining the unacceptable measuring instrument. If the proportion of unqualified instruments is high, the calibration cycle should be shortened.
If the proportion of unqualified instruments is low, extending the calibration cycle may be economically justified. If a grouped instrument (or a manufacturer or a model) is found to be incapable of working with other instruments in the group, the group should be classified as a different group with different periods.
Hour time method:
This method is to confirm that the calibration cycle is expressed in hours of actual operation. The measuring instrument can be connected to a chronograph indicator and returned to the calibration when the indicator reaches the specified value. The main advantage of this method in theory is that the number of instruments to be confirmed and the cost of confirmation are directly proportional to the time of use, and the time of use of the instrument can be automatically checked. For example, if we use a company's oscilloscope, we don't need to connect a timer, we can directly find on the oscilloscope how long it has been used continuously, which is very convenient to manage. However, this method has the following disadvantages in practice:
(1) This method should not be used when the measuring instrument is drifting or damaged during storage, handling or other conditions;
(2) Providing and installing a suitable timer, the starting point is high, and it is necessary to be supervised due to possible user interference, which increases the cost.
Comparison method:
When each measuring instrument is calibrated according to the specified calibration period, the calibration data is compared with the previous calibration data, and if the calibration results of several consecutive cycles are within the specified allowable range, the calibration period can be extended. If the range is exceeded, the calibration period of the instrument should be shortened.
Chart method:
In each calibration, the measuring instrument selects the representative identical calibration points, plots their calibration results in time, draws a curve, and calculates the effective drift of the instrument during one or several calibration cycles based on these curves. From the data in these charts, the best calibration cycle can be derived.
common problem:
Can the calibration cycle of laboratory equipment be specified by itself?
General equipment calibration is recommended on the certificate every year, some people say that some equipment is not required to be calibrated every year. Can the calibration cycle of the equipment be specified by itself? Is it approved by the review team if it is calibrated according to its own specified cycle?
It is best to specify the calibration cycle yourself, as the calibration cycle is related to the use of the device. The calibration cycle can be determined by itself, but at the same time it must refer to the domestic metrology requirements (if you are applying for CNAS accreditation). In fact, it is clearly stated in the standard (ISO/IEC 17025:2005) 5.10.4.4 that the calibration certificate should not contain recommendations for calibration intervals, unless it is agreed with the customer or is expressly provided by law. Therefore, the equipment calibration cycle can be adjusted, but only if you have to give a reasonable basis for the adjustment, otherwise, it will still not be accepted during the audit.
Should the calibration problem be asked by the equipment company?
The calibration company does not understand the frequency of use of the equipment, maintenance conditions, use environment and other factors, he gives you a relatively unreasonable calibration cycle, such as a steel ruler, well kept, two or three times a year; another steel Ruler, put it on the workbench, 8 hours a day; the calibration cycle given by the calibration company must be 1 year, so the calibration period of the first ruler is too short, and the calibration cycle of the second ruler is Too long, three or five months may be out of order. For enterprise laboratories only, third-party laboratories have to pass the qualifications and the requirements are different. Many devices may need to be verified.
What is the relationship between the calibration cycle and the period verification?
The state has regulations to recalibrate equipment maintenance, key replacement parts, and instrument migration during the calibration cycle. During the calibration cycle, equipment verification should be performed to ensure the stability and accuracy of the equipment. If the equipment, here refers to the equipment rather than the ruler, compasses, etc., the definition of the calibration cycle is less than the period specified by the state.
The laboratory can customize the calibration cycle according to the characteristics of the instrument, the frequency of use, etc., as long as the device is in the correct state of use, it can be used as expected. It is often necessary to provide measures such as period verification to prove that the instrument is in good condition. But the calibration cycle is not as long as possible, because the longer the time, the greater the uncertainty.
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