Ben van Millingen
On expeditions to Austria in the last two years several types of ascenders in different prusik systems were used. Pitches of varying lengths were climbed in muddy, icy, wet, and dry conditions. The most popular prusiking system used was the Frog, as its ease of handling was suitable on the pitches encountered in Austria which tend to be long and split by rebelays. This system in turn favoured the "Jumar" type ascenders rather than Ropewalkers.
In Austria, caves under exploration were left rigged during the whole expedition, being used by at least two people each day for two weeks. Some ropes rigged in muddy areas of the cave became mud-coated straight away, but after two weeks mud had spread onto nearly all the ropes, even in clean areas of the cave. Mud was often transfered from dirty racks, but also just by clothing rubbing against the ropes. The Fiesta Run in Eislufthöhle was particularly bad - a sloping pitch with the rope being lost in the mud lying on the wall. Similarly one arrived at the top of the big pitch in Stellerweghöhle as if one had just walked through a ploughed field. Often the seemingly dry, dusty mud was the worst for producing a fine coating of lubricant on the ropes. Under these conditions the difference between ascenders showed most strongly.
Jumars performed very well with almost no slippage, as did Petzls; by far the worst were CMI's. Though they were little used, the most certain ascender for use on muddy pitches were Ropewalkers, where the cam bites the rope by direct application of the climber's weight. On muddy ropes CMI's frequently slipped, providing an agonising ascent only made possible by fingering the cams into position at each step. Usually only one ascender slipped at a time, but occasionally the second slipped when shock loaded by the fall from the first.
Jumar type ascenders rely on spring loading to close the cam and thus grip the rope. The spring may not have the force required to grip on a very muddy rope, particularly if the cam teeth are also muddied. CMI's seemed especially vulnerable to clogging with mud. Even if ascenders were cleaned at the bottom of a pitch the cam was clogged with mud by the top.
The spring in CMI's is much weaker than that of a Jumar, and so does not provide sufficient force on a muddy rope to clamp it. Three springs distorted so badly that the cam was able to flap loosely in the frame rendering them useless. The cause of this might have been mud getting in behind the cam where it is pressed by the thumb each time the cam is opened. Furthermore the pin on which the cam pivots is held in place by a small circlip which does not look up to the task. This should never be used again once removed, as it is very easy to deform permanently.
Though no serious accidents were experienced because of the failure of CMI's; several people had falls which must have badly shock loaded both ropes and bolts at the head of pitches. From these experiences I do not feel that the design of CMI's is up to the conditions found underground.