dynamic tension measurement?

That's pretty much how a strain gauge works. A calibrated 'spring' deforms a known amount under a given tension. The sensor is actually measuring the deformation of the spring.

Most strain gauges don't have a broad range of travel, so the spring must be pretty stiff. If this isn't possible, then some sort of cam or lever arrangement can translate a small amount of beam travel into the required distance. Or some other position sensing can be used with a more flexible spring.

One thing you may need to consider is the dynamic characteristics of the system. If the tension being measured changes rapidly, the added mass of a position sensing system, or beam, strain gauge and cam system may be too high to follow the thread accurately. The readings will lag the actual thread tension. Worse yet, the sensor mass may adversely affect the thread tension and break it.

Or, use the existing spring and just put an instrument on the fixed end where it's mounted.

That way the mass will not affect the dynamic movement, the spring will be the original so no changes to that, and the force the spring exerts on the mount will be very close to the force the spring exerts on the thread.

daestrom

done with a tool called a Tentelometer. I don't know if they are still in business, but you can google for them. Their gauges are pricey , but they do measure dynamic tension. I still have one and it'll cost you at least an arm if you are interested. This gauge is not the kind of thing that you would want to leave connected all day and it doesn't provide feedback to control tension, it just measures it. bg