Many measuring devices need to be recalibrated at regular intervals. These devices might be petrol pumps. They might be weighing scales. They might be complex medical equipment like MRI scanners. A certificate is issued when the device is recalibrated. The device that is used to do the calibration must itself be calibrated. These devices are often recalibrated in a national metrology laboratory. Legal problems can arise if a calibration done in one country is not recognised in another country.