Air entry with barrel change

A frequent and very delicate problem with the product supply from barrels! If pasty or very viscous products are taken out of barrels and supplied to a filling up or dosing equipment, it is to be avoided compellingly that air comes here into the system. Finally no user wants a gap in its adhesive crawler-type vehicle, or a filler air in the sales product. Almost all systems present on the market work with a subsequent plate, which takes the entire surface off of the barrel. This is pressed downward or pulled according to the level. With a barrel change the large risk exists that air arrives under the plate and thus into the production stream.
Here there are different solutions, in order to avoid an air entry with barrel change: The most usual method is to discharge the product after the change by hand or automatically until you are sure that air escapes together with the product. In this case a not insignificant quantity of product is like in most cases rejected, which also costs much money with expensive adhesives. Another method consists of using a disposable plate into which by vacuum product is sucked into this throw-away part. Product goes lost too, but less than with the aforementioned method. Whether a hundred percent air liberty is reached here, depends reliably on the individual case, I personally have here however doubts.
A further method consists of degassing the product after the barrel emptying unit over a Inline degas station. We already built such stations, which work with products to 1.000.000 mPas well. The advantage is that no product must be rejected at all, and beside the air, which comes with a barrel change into the system also the air, which is possibly in the product itself, is taken from the system. However, the plant costs of this Inline degas station stands opposite. And still the possibility remains last, if one cannot exclude air entries, of detecting this air at least before incorrect construction units cause large damage. For this there are different sensor systems over ultrasonic, flow meter or optical systems. Which experiences have you as users? I would be pleased much about comments for this, since I judge this topic for very important, and only by a good co-operation between users and equipment manufacturers improvements here are possible.