@AA7 another way is an app that takes a frequency reading when you hit the stringbed with something, I use a screwdriver handle. you can preset the head size, type of string, string pattern etc, and when I take the racquet off the stringing machine I ping it to have a starting point. then it is home for an hour or two, and then I ping it before play, it'll lose a kg/2lb maybe, and then after play when I'm back, another half a kg/1lb lost. overnight it'll lose a bit too etc.
RPM Blast is a (cheaply made and) expensive string that has a simple very short lived silicone coating, so it's very slippery when you string it, it leaves a trace on the fingers. when fresh, it gives the string a lot of elastic snap back, so although the poly material itself is stiff, mains will glide on the crosses when you hit the ball, it provides spin and softness. after a short while, 20ish minutes, silicone coating is gone, and strings stop snapping back as much, and it can seem dead, boardy and stiff if strung higher or if you don't hit hard enough to force the mains to displace. so it's the matter of coating, not the tension itself, as any poly string surely loses tension over time, just by being strung in a frame, and by hitting the ball. if you take other, newer strings that are also engineered for nice elastic snap back, like Head Lynx Tour, Hawk Touch, Yonex PT Rev and others, you'll see that they keep it over the string lifespan almost uniformely, and even after the initial period where there is more elastic snap back as the new string is slick, it'll mostly stay like that.
when you pinch two main strings in a racquet freshly strung with RPM Blast it might seem softer than the same stringbed after an hour of hitting, so a device measuring that, could maybe measure more resistance to pinching.