METHODS OF SAMPLING AXD TESTING HIGHWAY MATERIALS 109 
12. The loading range of a calibration device is the range of indicated loads 
for which the device gives results within specified tolerances. Obviously, the 
loading range may not include loads either greater than the largest load or less 
than the smallest load applied in verifying the calibration device. 
13. (a) The loading range of a calibration device shall be determined by 
verifying its indications by the use of standard weights or by standard weights 
and proving levers. In verifying the indications of the device, an initial load 
of 1 per cent of its full load shall first be applied. All changes of strainometer 
reading and of applied load shall be reckoned from the readings at this initial 
load. Successive loads shall be applied, a rec-ommended series of loads being 
10. 20. 50. and 100 per cent of the full load of the device. Such a series shall 
be applied to the bar at least five times to determine the relation of applied load 
to strainometer reading, the shackles or bearing blocks being readjusted before 
each application of the series. 
(&) The average of the five (or more) strainometer readings obtained in this 
way shall be taken for each load applied. The average of the differences 
(taken without regard to algebraic sign) between the individual strainometer 
readings for any load and the mean reading for that load constitutes the aver- 
age deviation for that load. The lowest value of load for which the average 
deviation is not greater than 0.5 per cent of the strain for that load is the 
lower limit of the loading range of the calibration device. 
Note. — If in taking five or more strainometer readings for any load the deviation of 
any one reading is more than 1 per cent from the mean reading, 'then this series of five 
or more readings shall be repeated. 
14. The strainometer for elastic calibration devices which employ elastic bars 
in axial tension or axial compression shall be so designed as to indicate the 
average elongation or compression of the bar. It shall be sufficiently sensitive 
to indicate a change of 0.25 per cent of the minimum load in the loading range 
of the device. If the strainometer is not permanently attached to the calibra- 
tion device, the strainometer shall be removed and replaced for each successive 
reading taken during the calibration of the device itself. This, of course, does 
not mean that the strainometer must be removed for each reading in the regular 
use of the instrument. 
15. In using the calibration device it is important that just previous to taking 
any reading, including the initial reading of the series, there shall be no reversal 
in the direction of motion of the indicating mechanism of the strainometer, 
because sueh reversal may introduce error due to backlash in the instrument. 
That is. when taking readings for a series of increasing loads the load should be 
brought up to each predetermined test load, and when taking readings for a 
series of decreasing loads the load should be brought down to each predeter- 
mined test load. 
16. In using a calibration device made of steel, correction for variation in 
mean temperature during any calibrations made on different occasions should 
be made by allowing a diminution of 0.015 per cent (ttV^o of 1 per cent) in 
the modulus of elasticity of steel for each 1° F. increase in temperature. Dur- 
ing any calibration with an elastic calibration device, temperature conditions 
should be kept as uniform as is possible. 
(d) VERIFICATION BY COMPARISON METHOD 
17. When the direct method of verification of a testing machine can not be 
carried out (as is the case at present with horizontal testing machines, for 
example), or when an elastic calibration device is not available, a comparison 
method of verification may be used. In this method the machine to be verified 
is tested by comparing its indications with the corresponding readings obtained 
under the same conditions by the use of another testing machine, especially 
tested and verified, as specified in section 18. The method of verification by 
comparison shall be carried out by the use of a series of companion specimens, 
half of which are to be tested in tension in the machine to be verified and half 
of which are to be tested in tension in the especially verified testing machine 
which serves as a standard machine. The general provisions respecting the 
application of the test loads and the loading range which are outlined in the 
