METHODS OF SAMPLING AND TESTING HIGHWAY MATERIALS 107 
the standard weights is the amount of standard weights multiplied hy the lever 
ratio m n. Figure 44. 
6. The knife-edges, as well as their supports, shall be of hardened tool steel. 
The knife-edges shall be ground sharp with an angle of 90°. The load on any 
knife-edge shall not exceed 7,000 pounds per linear inch. The three knife- 
edges in each lever shall be parallel, and their edges shall lie in a plane. Each 
lever shall have machined surfaces in this or in some parallel plane, upon 
which a spirit level can be placed. 
7. The lever ratio of a proving lever shall be determined by the use of at 
least three test loads ; the amount of weights used shall not be less than the 
maximum load applied upon one of the arms of a lever in using the levers to 
verify testing machines. The proving lever shall be balanced over its center 
knife-edge with suitable weight trays suspended from the end knife-edges. 
Standard weights shall then be applied to the trays in three steps, correspond- 
ing approximately to 50, 75, and 100 per cent of the weights available, and the 
proving lever shall then be brought to a balance by the use of small weights and 
by observations of the freely swinging proving lever. From the weights in the 
two weight trays the lever ratio shall be computed. 
8. The proving levers shall be placed symmetrically in the testing machine 
to be verified and both levers brought as near to a horizontal position as is 
U I II 
Cross- head of testing Machine 
Weighing Table of Testing Machine 
Standard i » j Standard 
Weights Weights 
~>nk-- 
I r 
->|nj«- 
n l n 
m 
Fig. 44. — Proving levers 
feasible, after applying each increment of load, by means of the movable head 
of the testing machine. 50 The testing machine shall be balanced with the levers 
in place and the weight trays empty. Standard weights shall be applied in 
increments, half an increment in each tray, and they shall be removed in the 
reverse order. The weights shall be placed symmetrically on the weight trays, 
with the center of gravity of the weights over the center of the tray. The 
applied load and the indicated load shall be recorded for each test load applied 
and the error computed from these data. 
(c) METHOD OF VERIFICATION BY MEANS OF AN ELASTIC CALIBRATION DEVICE ri 
9. In this method the testing machine to be verified is tested by comparing 
its load indications with the amount of elastic deformation of a calibration 
device (2, 5), the deformation being measured with a delicate strainometer. 
10. The calibration device may be a bar tested in tension or in compression 
or an elastic loop or other shape whose elastic deformation can be measured, 
and if a tension or compression bar is used it shall be fitted with spherical- 
50 The method of verification by standardized proving levers is used at the present 
time only on vertical testing machines. It is often used up to loads of 10,000 pounds. 
51 The term " calibration device " shall be interpreted to include solid bars, hollow bars, 
elastic loops, and other members whose elastic deformation can be measured. 
