The internal quality control in the laboratory is primarily employed to assess the precision of laboratory testing quality, simultaneously reflecting the stability of laboratory analytical quality. The methods of internal quality control in the laboratory mainly include: the transfer measurement method, multiple set equipment comparison method, two-set equipment comparison method, standard substance method, retesting of retained samples method, inter-laboratory comparison method, and so on.
Characteristics of Internal Quality Control in the Laboratory:
(1) Convenient, Simple, and Flexible: Internal quality control in the laboratory is organized and implemented by the laboratory itself, with organizers and implementers being internal personnel. It can be adjusted according to the actual working conditions of the laboratory, making it quite flexible and adaptable. In the case of abnormal laboratory data, personnel comparisons, method comparisons, and instrument comparisons can be conducted for self-examination.
(2) Diversity: In daily experiments, there are many factors that affect data accuracy. Factors such as personnel, equipment, materials, methods, and environment can be analyzed for the cause(sources from usms.ac.ma).
Transfer Measurement Method:
When verifying measurement standards, if the laboratory possesses a higher-level measurement standard, it can easily use it to inspect the functionality and scope of the standards being verified. When the results indicate that the relevant characteristics of the verified standards meet their technical specifications, the verification is considered successful.
When checking other measuring equipment, if the laboratory has similar measuring equipment with a higher accuracy level or equipment that can measure similar parameters, and if the uncertainty of this equipment is not more than 1/3 of the uncertainty of the equipment being verified, it can be used to inspect the equipment being verified. When the results show that the relevant characteristics of the verified equipment meet their technical specifications, the verification is considered successful.
This method can also be applied when the measuring equipment belongs to a standard signal source.
Multiple Set Equipment Comparison Method:
When the laboratory does not have higher-level measurement standards or other measuring equipment but has multiple sets of similar measurement standards or equipment with the same accuracy level, this method can be employed.
Two-Set Equipment Comparison Method:
When the laboratory has only two sets of similar measuring equipment, they can be used to measure the verification standards. The obtained measurement values are denoted as y1 and y2. If their uncertainties are U1 and U2, respectively, verification is considered successful when the following condition is met:
Believe that the verification results are satisfactory. If these two sets of equipment are traced back to the same measurement standard and have correlation between them, consideration should be given to this correlation when assessing uncertainty.
Standard Substance Method:
When the laboratory possesses standard substances for the verified equipment, standard substances can be used as the verification standard. If standard substances are used to check the parameters of the verified equipment, and the obtained measurement value is denoted as y, the judgment criterion is:
In the formulas:
y represents the measured value,
Y represents the value represented by the standard substance,
△ corresponds to the tolerance limit corresponding to the accuracy level of the verified equipment.
For periodic verification, standard substances should be traceable to the SI or certified standard substances within their validity period. When standard substances are not available, calibrated standard solutions can be used to verify the measuring equipment, such as pH meters, ion meters, conductivity meters, etc.
Retesting of Retained Samples Method:
After measuring equipment has been calibrated to obtain its performance data, it is immediately used to measure the verification standard, and the obtained measurement value y1 is taken as the reference value. The verification standard can be either the measuring equipment or a physical sample. Then, under specified conditions, store the verification standard and avoid using it for other purposes as much as possible. At the specified or planned verification frequency, measure the verification standard using the measuring equipment, obtaining measurement values y2, y3, …, yn. The judgment criterion is given by the formula:
where U represents the expanded uncertainty after deducting the uncertainty component caused by system effects.
Inspection of Physical Samples Method:
For certain measuring equipment used to measure limit values, automatic alarms are triggered when the measured values exceed the set limits. This method can be used for periodic verification of such equipment(quotes from usms).
First, based on the working principle of the verified equipment and the nature of the verified parameters, design, manufacture, or purchase corresponding physical samples. Then set the limit value for the parameter and apply the physical sample to the measuring equipment, operate the equipment, and adjust it to the specified output level, observing whether the measuring equipment exhibits the corresponding response.
For example, when conducting periodic verification of an insulation tester with an accuracy level of 5, an output voltage of 1500V, and a set leakage current of 5mA, a 300kΩ resistor can be used as the verification standard. Connect it to the two test rods of the insulation tester, adjust the output voltage to (1500±5%) V, and an alarm should be triggered. At this point, the performance of the insulation tester is considered normal.
Direct Measurement Method:
When the measuring equipment belongs to a standard signal source and the laboratory has measurement standards, the transfer measurement method can be used directly. If there are no measurement standards, this method can be employed.
First, determine the function and measuring points that need verification, then select measuring equipment with the corresponding function as the verification standard. Calibrate the performance of the verification standard at the corresponding measuring points, obtain the corresponding correction values, then use the verification standard to measure the performance of the verified equipment. After correcting the verification results, observe whether they meet the respective technical requirements.
For example, when verifying a standard voltage source, first determine the measuring point to be verified (e.g., 5V), and choose a digital multimeter as the verification standard. Calibrate the indication value of the 5V measuring point on the DC voltage range of the digital multimeter, obtaining the correction value e. Then, use the digital multimeter to measure the actual value of the 5V output of the standard voltage source, obtaining the result V. The corrected verification result is (V+e). Based on the technical requirements of the standard signal source, it can be determined whether the result is satisfactory.
When the laboratory where the verified equipment is located is identified as the lead laboratory for the comparison, the judgment principle follows the two-set equipment comparison method. When there is no identified lead laboratory, the judgment principle follows the multiple-set equipment comparison method.
When the measuring equipment in the participating comparison laboratories is traceable to the same calibration laboratory and the same measurement standard, the influence of correlation should be considered when assessing uncertainty.
Different methods can be used to verify measuring equipment. When using the same verified measuring equipment to measure the verification standard, the judgment principle for the verification result can follow the retesting of retained samples method. When two methods involve measurements on different measuring equipment, the judgment can follow the method described in the two-set equipment comparison method.