X-Rite battery-operated sensitometers (Model 396 and others) are designed to provide repeatable, consistent exposures for many years of reliable process control. It is also X-Rite's recommendation that the instrument is returned for recertification every 12 months. In the event that a customer experiences inconsistent exposures with their sensitometer (assuming that their densitometer is verified to be reading consistently with its Calibration Reference), the 9-volt alkaline battery might need replacing. Typically, a new 9-volt alkaline battery will be good for thousands of exposures.
Keep in mind that these instruments should be inspected, tested, and re-certified every 12 months.
For battery replacement details, please see the the "Related Support" link below entitled: "Battery Replacement with X-Rite Sensitometers"