The RAHN REPEAT-O-METER is on sale at Gaging.com!  You will need this instrument if you want to be compliant with Federal Spec. GGG-P-463C to test repeatability (repeat measurement) on your Surface Plates. 

Use the Repeat-O-Meter to scan the work surface of the surface plate. The indicator readings (delta) shall not exceed the values shown in Federal Specification GGG-P-463c. The Repeat-O-Meter substitutes the traditional height gage and gage block method, but is far more dependable and easier to use.

Rapidly check the repeat measurement accuracy of a surface plate by moving the Repeat-O-Meter over the work surface with the specified method, noting the total indicator range (T.I.R.). In just a few more minutes, by zeroing the R-O-M at the center along a diagonal or centerline and then recording readings at 5” intervals, you can chart the readings from the plate.  Maximum error of the plate cannot exceed the grade indicated in the Federal Specification.  If it does, the plate will have to be refinished or downgraded.

Repeat-O-Meter