106 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
The word " accurate " applied to a testing machine shall be used without 
numerical values. The word " error " shall be used with numerical values. 
For example : "An accurate testing machine was used for the investigation " ; 
but " The error of the testing machine at a load of 30,000 pounds was 15 pounds." 
A testing machine is said to be accurate if the indicated load is within the 
specified tolerance of the actual load. 
Note. — The accuracy of a testing machine should not be confused with sensitiveness. 
For example, a testing machine might be very sensitive — that is, it might indicate 
quickly and definitely small changes in the load — but, nevertheless, be very inaccurate. 
On the other hand, the accuracy of the results is, in general, limited by the sensibility. 
(f) Loading range. — In the case of a testing machine, the range of indicated 
loads for which the testing machine gives results within the specified tolerances. 
The loading range shall not include loads either greater than the largest 
load or less than the smallest load applied in verifying the testing machine. 
METHODS FOR VERIFYING TESTING MACHINES THAT MEASURE 
LOAD 
2. Wherever practicable, testing machines shall be verified by applying 
standard weights directly to the weighing mechanism of the testing machine, 
or by the use of standardized proving levers in connection with standard 
weights. The test loads selected shall be applied in both ascending and descend- 
ing order, and the indications for both the ascending series and the descending 
series of loads shall be considered in the application of tolerances. 
Note. — For machines in which the load-indicating device is actuated by a Bourdon 
pressure tube, a hydraulic (or steam-engine i indicator spring, or other device depending 
on the elastic properties of a material, the tolerances for the descending load shall be 
applied to differences in the indication for the corresponding ascending and descending 
readings. In such testing machines the test for ascending load shall be applied as 
rapidly as practicable after the testing machine has remained unloaded for at least six 
hours, and the time of applying the ascending series of loads shall not exceed 30 
minutes. The descending load test shall be made after the maximum test load has 
remained on the machine for one hour, and the readings for descending loads shall be 
taken as rapidly as possible, and in any event the descending load test shall be completed 
within a period of 20 minutes. 
3. For any loading range, the testing machine shall be verified by at least 
four test loads. It is recommended that the test loads used be 10, 20, 50, and 
100 per cent of the maximum capacity at which the testing machine is to be 
used. 
Note. — Where a large amount of testing of small specimens is to be done on a large 
capacity machine, it is recommended that the verification outlined above be supplemented 
by a series of smaller test loads selected in reference to the light work to be done on the 
testing machine. 
(a) METHOD OF VERIFICATION BY STANDARD WEIGHTS 
4. Standard metal weights of suitable design, finish, and adjustment shall be 
placed on the weighing platform of the testing machine or upon trays or other 
supports suspended from the load-measuring mechanism in place of the speci- 
men. The weights shall be applied in increments and removed in the reverse 
order. They shall be arranged symmetrically with respect to the weighing plat- 
form, so that the center of gravity of the load lies in the vertical line through 
the center of the platform. The applied load and the indicated load shall be 
recorded for each te^st load applied, and the error computed from these data. 
Note. — The method of verification by standard weights can be used only on vertical 
testing machines. The total load is limited by the size of the platform and the number 
of weights available. Often 20 weights of 50 pounds each are used, a total of 1,000 
pounds. This method of verification is seldom used for any but small testing machines, 
as the loading range does no include the loads for which large machines are used. 
(b) METHOD OF VERIFICATION BY STANDARDIZED PROVING LEVERS 
5. The arrangement of proving levers for verifying testing machines is shown 
in Figure 44. The two levers rest on supports on the weighing platform of the 
testing machine. These supports shall move easily in a horizontal direction, 
which insures that the forces at each of the knife-edges shall be very nearly 
vertical. The inner knife-edges in each lever bear against a suitable block in 
the movable head of the testing machine. Weight trays or hangers are sus- 
pended from each of the outer knife-edges, and these trays or hangers are loaded 
with standard weights. The increment of load put on the testing machine by 
