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Reference Level for Ruska piston gauges (L1 and D values)

The true reference plane of the piston gauge is the effective bottom of the measurement piston. This location can be correlated to the mass loading location by using the L1 and D dimensions that are provided on the calibration certificates of the piston-cylinder and the mass set. The L1 dimension is from the effective bottom of the piston to the mass loading location, typically the top of the piston or mass table. The D dimension is from the bottom of the sleeve weight (also called hanger mass or mass bell) to the mass loading location. 

The effective bottom of the piston is different from the actual bottom of the piston when the piston is an irregular shape. The effective bottom is calculated as being the bottom of the piston as though it was made from a solid piece with the same area throughout. For example, the 2465 low range TL piston (2465-725) is a hollow piston. The effective bottom is actually above the physical bottom of the piston. The total length of travel for a 2465 piston is 0.5 inches (12.7 mm).

More diagrams from the 2450 manual related to head height (h) and zeroing while vented...

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