Everyone knows how the mercury Carbtune works.
There is a column of mercury in a tube. The tube is connected to the engine. The vacuum in the engine pulls the mercury up the tube. You take readings off the scale.
It's a good idea until you drop it, turn it upside down or try to transport it.
The mercury free Carbtune Pro is used vertically like a mercury manometer and works in a similar way - except you have a column of stainless steel rising and falling. No chance of that leaking!
The ground and polished stainless steel columns float smoothly through guides that have been machined to fit them. The guide acts like a seal and the column moves through it vertically. This is contained inside a clear plastic tube which gets connected to the engine. The vacuum from the engine draws the stainless steel column up through the guide.
As with the mercury manometer the stronger the vacuum, the higher the column rises.
Just like the mercury manometer except it's smaller, there are no liquids to spill and as the Carbtune Pro is made from high impact plastics and metal it is much more robust.
Photos on this page are of the old Carbtune II. The Carbtune Pro is the new upgraded model which can be seen on the homepage.