Yes the clear silicon tubing is different from the opaque tubing. I recently had an issue with mine on the small bit of tubing between the metal sample needle in bottle A and the 90deg fitting. Alk measurements were all over the place. I took the cover off and watched what was happening and there were air bubbles in the line as it was adding A to the vial.. sometimes... I bought some 1.5mm ID silicon tube and replaced that small bit of tubing inside the bottle and no more air bubbles. I suspect the tubing that comes with the unit is getting old and the seal around those little barb fittings is not as good as it used to be.