Thanks Kyrpa for helping me out :)
Today I tweaked my approach and got some promising results from the session.
Here’s what I did (in detail!):
When putting on the vacuum cup and the silicone sleeve, I sealed all gaps where the air might potentially leak in. This includes the valve and both ends of the silicone sleeve. This is really important! The vacuum cup can slip and blisters can appear only if there is air inside the system. Seal it extremely tight!
Phase #1:
The initial force in the stress relaxation phase was 1.5kg (using an extender). Every 10 minutes when the traction had decreased a little, I tightened the screws more until I had reached 2kg. When the shaft didn’t give any more slack for 10 minutes, I moved to the heating phase. The first phase took 40 minutes.
By the way, when you think stress relaxation happens, make sure it’s not caused by a loose noose in the extender or any other system you use to produce the traction force.
Phase #2:
With ultrasound I used _plenty_ of gel and pressed the probe _firmly_ against the shaft. A cold(!) rice sock was supporting the shaft on the other side. I didn’t use aluminum foil or any other reflector. Only a small area of the shaft was being heated (2 times the radius of the probe).
8 minutes in, the shaft started to feel the burn. When that happened, I briefly paused the ultrasound and increased the extender traction to roughly 2.2kg and then continued with the ultrasound.
The burning sensation came back quickly and I paused whenever it became intolerable. Each pause was 10 to 20 seconds long. You can spend them increasing the traction and applying more gel. I continued this process until the 20 minute mark.
You shouldn’t hurry with this phase. Just like in phase #1, take your time.
If you are feeling intolerable burning, you are surely above 40c degrees. Then pausing the ultrasound is fine.
But keep your pauses relatively short (less than 30 seconds). The temperature in the shaft decreases rapidly.
Phase #3:
When the cooldown began, I simply stayed in the extender, made sure it stays firmly in place and won’t slip, and then increased the traction force. The traction was more than the lines in the extender rods could show, so I had to approximate. I’d say it was a little more than 3kg.
The noose of the extender actually helps to keep the vacuum sealed in high traction forces. The vacuum cup didn’t slip at all; therefore no glans pain or blisters. This last phase was super easy! 10 minutes of chilling. No manual stretching needed.
If you are going to use multi-kilogram traction forces with a vacuum cup on, I recommend putting a tight shoelace or something similar on top of the silicone sleeve to make absolutely sure it won’t leak. Same thing with the other end of the sleeve (the one wrapped over the cup). Also, find a way to block the vacuum valve.
Results:
Minimum of 2.7% strain. Possibly 3.2%, but I believe the glans had some temporary expansion because of the vacuum and the traction. I take the conservative estimate because I’m always skeptical.
The feeling in the shaft during the heating phase was purely the heat and the burning. No soreness.
After the session when I measured the strain, the feeling in the shaft was this unique mild tingling. It felt like I don’t want to stretch it any more, lol. It just felt sore in a good way. This feeling faded gradually during the first 15 minutes after the session.