vince said:
am I right in thinking that the tensioner springs take up the slack in the primary chain and when running the oil doesn't add to the tension it simply holds the plungers in place by virtue of the non-return ball??
Well you are on the right track but it is slightly more complicated than that.
The oil acts as a damping medium for the plungers (like a shock absorber as Jason mentioned) but imagine that the two tensioner plunger rods are pistons, one at each end of an oil filled tube, if you push one piston inwards this will have an effect of moving the other piston outwards (like a hydraulic brake system).
As the upper and lower chain runs constantly alternate between slack and tight when the engine is running as the crankshaft doesn't really rotate at a constant speed (which is more pronounced at lower revs). The tight run of the chain will try and push the tensioner plunger back into its housing (against the spring pressure), this 'action' will then cause a 'reaction' pushing the opposite plunger outwards increasing the amount of tension on the run of chain that is loose.
The one way ball valve closes as this happens otherwise the pressured oil between the plungers would simply escape back to the feed trough.