Not all pain is visible. There are many different kinds of intense internal pain. The way that pain was always explained to me is as swelling of some sort, some organ or vessel is engorged enough to be too big for its container. There are other types of pain, (strains, psychological pain, etc.), but it seems to me that this type of pain would be good to measure. If you told a doctor you were in pain and then you gave them a measure of it, they’d be more likely to believe you and treat you. If, you weren’t able to measure it using that pain measuring device you would know that it was a different kind of pain and they would be able to treat you better because of it. win-win

Why can’t we measure pain? We understand a lot about pain these days, how it works in the brain, different mechanisms for its transmission, yet we rely completely on patient assessment for pain reporting. It is important (probably the most important thing) to take that into account, but it should not be our sole source of information.

Also this article is amazing:
No Evidence of Disease