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 about the following issues, such as how to determine the principles and methods of the calibration cycle. What are the current standards for determining the calibration cycle? Is it possible to change the calibration cycle of the instrument at will in the laboratory? Don’t worry, the answer will be revealed one by one!
How is the calibration cycle explained in the standard document?
The 5.10.4.4 calibration certificate (or calibration label) in the CNAS-CL01 should not contain recommendations for calibration intervals 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.
After the first calibration of the instrument, the second calibration time is set for 1 year, and after 1 year, the calibration of the calibration laboratory is still very accurate (within the error range compared with the first calibration), it can be set for 2 years. , and so on, the maximum length can not exceed 5 years, but the period must be checked during the period, if it is found to be unstable, it needs to be recalibrated.
The calibration cycle must be determined
Let me talk about the calibration cycle, that is, the confirmation interval. It is one of the key steps to measure the quality of measurement work, and it is related to the pass rate of the measuring instrument in use. Only by strictly implementing the calibration cycle 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.
What happens if the calibration cycle is unreasonable?
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 way and conditions of use of the measuring instrument may result in misalignment of the instrument. Therefore, when one 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 should be adjusted appropriately to extend or shorten the calibration cycle.
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 costs are kept to a minimum.
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.
Must it be calibrated according to the cycle specified in the calibration procedure?
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: Blind shortening of 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 may be very dangerous to extend the calibration cycle simply because of lack of funds or insufficient personnel, which may result in 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 and considers a variety of factors. If it exceeds one cycle, it may cause deterioration of the 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:
- (1) The frequency of 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.
- (2) Requirements for measurement accuracy. For units that require 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, which is also undesirable.
- (3) The unit’s maintenance ability, if the unit’s maintenance is better, the calibration cycle is shortened appropriately; otherwise, it is longer.
- (4) The performance of the measuring instrument, especially the level of long-term stability and reliability. Even for the same type of measuring instrument, the stability and reliability are poor, and the calibration period should be shorter.
- (5) For calibration 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 period 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’s or a model) cannot be found to behave as if it were working with other instruments in the group, the group should be grouped into other groups 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, we using a company’s oscilloscope, you can directly find the continuous use on the oscilloscope without connecting a timer, which is very convenient to manage.
However, this approach 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. If the calibration result of several consecutive cycles is within the specified allowable range, it can be extended. The calibration period; if it is found to be outside the allowable range, the calibration period of the instrument should be shortened.
Chart method: The measuring instrument selects the representative same calibration points in each calibration, draws their calibration results in time, draws a curve, and calculates the effective drift of the instrument in one or several calibration cycles based on these curves. Quantity, from the data of these charts, can be derived from the best calibration cycle.
Frequently Asked Questions Q&A
1. 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 device be specified by itself? Is the review team approved 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.
2. Questions about calibration should ask the equipment company?
The calibration company does not understand the frequency of use of the equipment, the maintenance situation, the use environment and other factors. He gives you a relatively unreasonable calibration cycle, such as a steel ruler, which is 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 inaccurate. For enterprise laboratories only, third-party laboratories have to pass the qualifications and the requirements are different. Many devices may need to be verified.
3. Contact between calibration cycle and 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 inspections are also 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 use state, 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.
summary
Metering and calibration is an important part of improving the efficiency of the laboratory. Determining the calibration cycle is a key part of the measurement work. It plays an important role in product quality and service quality. When determining the calibration period of the measuring instrument, it is necessary to measure The actual use of the instrument is scientifically analyzed and evaluated.
