Mark Wright said:I do have quite a bit of experience with all the devices you mention.
Firstly the Sala Stop Descender and the SRTE Noworries descenders. These were designed primarily for an industrial application and whilst they both do everything it says on the box they are quite cumbersome and the SRTE device is very very very heavy and some of the moving parts are very likely to get clogged up with mud.
The DSD device is one of the most complicated to load a rope into and isn't that smooth a descent when compared to other devices.
The I-Block Evo is the same as the DSD but with a different name on the front.
The ISC A-B device has been around for years and has had a number of different names. Its first incarnation was the SAR Scarab, later changed to the SAR Sarab. ISC manufacture them for a number of different companies.
I was involved to a small extent in the development of this device, carrying out long descent trials as part of the requirement for determining the maximum descent distances before they get too hot. If a device claims it can be used on, e.g. 200m decent with a 140kg load then this test would have been carried out with a 140kg test weight with a non-stop 1m / second descent rate for 200m.
If you are looking for a descender device that is good for using as a locking rescue pulley then I'm not aware of anything better. Unfortunately it isn't a very good descender.
During one of the recent International Berger expeditions we had some Bulgarians join the trip and they used a toothed ascender above their Stop / Simple descenders. We had to replace most of the ropes in the entrance series due to the sheaths being seriously damaged due to this technique being used.
If you could be sure you would let go of the Shunt if you lost control of the descent then this could be a useful method, however, it is most unlikely you would let go. You would more likely squeeze the Shunt or its towing cord even tighter and never let go.
Using a Shunt as a back-up device as is being shown on the videos is generally considered to be very dangerous. Most of the fatalities in the rope access industry have been down to not letting go of the device.
I've never heard of a Rack twisting the rope but it has been over 30 years since i've had a go on one.
The ASAP and ASAP Lock devices were again designed for industrial use. I wouldn't recommend using either devices in a muddy cave environment due to the many moving parts. They are also very expensive. ASAPs are the only devices that can't be overridden by the user, i.e. they will still work if you lose control of the descent and grab them or grab the rope above them.
If a Petzl Simple is a bit quick on new and/or thin rope then simply use a breaking carabiner.
Personally I would go for a Petzl Stop every time. If you aren't in a rush there will soon be a new version of the Petzl Stop on the market, I'm currently trialling a prototype.
Mark
How is that different to the same reaction with a stop then ?