Geokon 4425, Instruction Manual, Page: 18 / 27

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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