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Does the device have to remain within the specifications for size throughout the entire testing process? Or solely for impound, assuming it is in ready position to test at impound?

The device must only be within specification in the ready to use position. This means that they must not have to make adjustments to the device that will put it out of spec before using the device. If the device incidentally falls out of specification during use, that will be acceptable as the measurement only occurs before operation (but when it is ready to use.)