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The Skylotec Sirius has the following curious label:
Does this mean that it has a working life of 28 descents? No, that would be silly.
One of the standards mentioned on the Sirius is EN 341:2011 "Personal fall protection equipment — Descender devices for rescue." This standard may or may not apply in your application, but Skylotec tested to this standard so let's look at EN 341:2011.
The standard defines descent energy as the energy measured in Joules and expressed as W, which results from the product of the descent load, the gravity, the descent height and the number of descents. More simply:
W = m × g × h × n
where
W := descent energy, expressed in joules (J)
m := descent load, expressed in kilograms (kg)
g := gravity 9.81 m/s2
h := descent height, expressed in meters (m)
n := number of descents.
This formula that relates five numbers. Knowing or choosing four, we can calculate the fifth.
The first is the descent energy. The manufacturer classifies their device as Class A, defined in EN 341:2011 as "descent energy W up to 7.5 × 10^6 J." Does this make any sense? This is a lot of energy, roughly corresponding to me doing a free rappel from the top of Mt. Everest to sea level. I do not know why the standard writers picked this number. It seems absurdly large to me.
The second is the descent load. The manufacturer picked m=141 kg, or about 310 pounds. Does this make any sense? This would be the mass of the test mass used in their test rig. Many cavers and climbers weigh less than this, even when loaded with gear (most of the time). The manufacturer also had to consider industrial workers carrying tools. This number is reasonable but some people would exceed this mass.
The third is the acceleration of gravity, g=9.81 m/s2 on Earth. Does this make any sense? This is accurate enough for the purposes of the test.
The fourth is the descent height used in their testing. This is printed on the device: h=190m. Does this make any sense? This is a characteristic of their test setup. Why 190 m. and not some other number? I don't know, but a reasonable guess is that they had a 200 m. rope, and needed 10 m. for the line on the pull-through system and leading from there to the descender, leaving 190 m. able to be used for the simulated descent.
Now we can calculate the number of descents needed to reach the Class A descent energy limit:
n = (W/mgh)×1
= [(7.5 × 10^6 J)/(141 kg)(9.81 m/s2)(190 m)]×(1 (kg m2)/(s2))
≈ 28.
This number tells the testing crew that they have to run the descender through the test setup 28 times to reach the desired descent energy. The length of the rope used in the test rig is what determines the number of descents.
Let me emphasize:
"28" is a parameter determined solely by the test rig characteristics and has nothing to do with the device being tested.
After running the descender through the test rig 28 times, the testing crew needs to measure how hot the descender got. The acceptance criteria is "none of the parts of the descender device handled by the user to control the descents" can exceed 48°C (118°F).
Does this make any sense? The control handle is the only part of the device in question that the user handles to control the descent, so as I read this, the only thing that matters is how hot the handle gets. If another part gets red-hot and melts the rope in two, you're fine as far as the standard cares since your hand will not get warm!
You can draw your own conclusions about how meaningful and important the "Max. 28 descents" marking is to the end user.