Here is a pretty good article (2018) on pain measurement and calibration.
While the early research in pain concentrated on developing a calibrated scale by managing all the variables (environment, mood, stress, language, etc.) and working with external stimuli that could be most precisely varied (heat, electric), it seems that more recent work is trying to measure our physiological reaction to pain stimuli to see if a common scale can be derived from the measurements.
The article talks about brain imaging as a measurement of pain (and, to its credit, describes the drawbacks). Another paper I ran across measured the biochemical reaction of pain receptors (they produce a chemical signal in response to pain stimuli).
The article also points out the recent efforts to address chronic pain calibration, which is different than pain from a traumatic event. Most of the early studies (and recent studies) try to calibrate response to immediate pain stimuli. But does this correspond to calibration of ongoing chronic pain from disease or debilitating conditions? Calibration of chronic pain is arguably much more important than calibrating pain from a single trauma to the body.