"I found a company that made an electrochemical equivalent. It was
housed in a glass cylinder, similar to a common 3AG glass fuse. Inside
was some chemical solution. When a few volts of DC was applied,
electrolytic action caused one end to slowly turn dark, thus
indicating the amount of time that the DC was applied. Sorry, but I
couldn't find the vendor or an equivalent online. When the required
maintenance was performed, the indicator would be replaced as it could
not be reset. "

They used something similar in time lapse video recorders. When those links or whatever went, you are required to replace the video head.