Thanks so much everyone who responded.

One design aspect that I was not aware of until today is that on the Pelagian Diluent can only be added to the loop through the ADV. This means (I think) that to do a diluent flush or indeed any addition of diluent it is necessary to vent air from the loop by breathing into the mask and your next breath will draw diluent into the loop through the ADV. It is the ADV which blows diluent across the sensors giving you an almost instant validation.

I must admit I am not sure about this. I have heard that ADV's sometimes malfunction and that it is common to have a valve which enables the diver to stop the ADV working should it start putting air into the loop unecessarily.

Trouble is, I simply do not have enough knowledge at this stage to know whether this is a neat and minimalist design feature or something which could cause problems. I am probably misunderstanding something here.

What do you all think? Should I be concerned?