Regarding the calibration of ferrite detectors

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Recently, our company was assigned the task of producing a stainless steel pressure vessel (DN300). In accordance with the requirements of GB/T150-2024, it is necessary to conduct a retest for ferrite content in the cold-pressed head. Our company has newly purchased a ferrite detector; however, the personnel from the local special inspection agency have expressed their opinion on the calibration of this detector, stating that without calibration, the accuracy of the test results cannot be guaranteed. Therefore, an order was issued to halt production of this pressure vessel manufactured by our company. After consulting by phone with the local provincial metrology bureau, the China National Institute of Testing Technology, and the National Institute of Metrology, it was confirmed that none of these institutions offer such calibration/services at the moment. When consulting other companies in the same industry, it was found that they had also purchased ferrite detectors, but they all used the calibration blocks included with the equipment for self-calibration. I would like to ask the experts: where in China can a ferrite detector be calibrated? Thank you all