
12
Increasing tensions cause the rod to elongate so that the required correction would be positive,
when Dtcorrected is positive.
Values for S are as follows:
Model Number
Range
Spring Rate
Inches
mm
Lb./in.
Newtons/mm
4425-12
½
12
34
5.95
4425-25
1
25
34
5.95
4425-50
2
50
17
2.98
4425-100
4
100
17
2.98
4425-150
6
150
8.5
1.49
Table 5 - Spring Rates
4.4 Correction for Sag
Correction for sag = - LW
2
/24P
2
or -L
3
ω
2
/24P
2
Equation 8 - Sag Correction
Where;
L = length of rods (The distance over which the measurement is made).
W = weight of rods
P = tension. The springs used have an initial tension of 13 lb. (57.8N), and this must be added to
the force caused by the displacement = SD
ω
= weight of rods per unit length (see Table 6)
For convergence the correction is negative, for extensions the correction is positive.
Rod Material
Weight per unit length
Lb./inches
Kgm/mm
Stainless Steel
0.0139
248 x 10
-6
Graphite
0.00275
49.1 x 10
-6
Invar
0.0139
248 x 10
-6
Fiberglass
0.0035
62.4 x 10
-6
Table 6 - Densities
Summary of Contents for 4425
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Page 27: ...21 APPENDIX D EXAMPLE CALIBRATION REPORT Figure 13 Typical 4425 Calibration Report ...