Geokon 4425, Instruction Manual, Page: 17 / 27

11
All of the above thermal corrections describe an expansion of components with an increase in
temperature. Hence, the calculated thermal corrections must be added to the deformation
calculated using Equation 2.
Dtcorrected = Duncor Dtemperature
Equation 5 - Thermally Corrected Deformation Calculation
Experience has shown that the most stable readings are obtained when the system is at a stable
temperature. Taking readings late at night or early in the morning will eliminate the transient
effects of sunlight and rapid warming of sensor components. If a datalogger is used, readings
will show the trends associated with thermal effects during the day and through the seasons, and
allow corrections for these effects to be accurately made.
4.3 Rod Stretch Correction
Rod Stretch=
PL
aE
Equation 6 - Rod Stretch
Where;
P is the rod tension (lb. or Newtons).
L is the rod length (inches or mm).
a is the rod area of cross section (sq. inches or square mm).
E is the Young’s modulus (lb./sq. inch or MPa).
P depends on the spring rate,
S
, of the large exterior tension spring and the amount of
deformation (D
tcorrected
), i.e. P = S D
corrected
so that:
D
rodstretch
=
SDL
aE
Equation 7 - Rod Stretch correction
Values for K
R
and E are as follows:
Rod Material
E, Young’s Modulus
K
R
Thermal
Coefficient
Lb./sq. in. MPa Per ºC
Stainless Steel 28.5 x 10
6
0.196 x 10
6
17.3 x 10
-6
Graphite 17 x 10
6
0.117 x 10
6
0.2 x 10
-6
Invar 21 x 10
6
0.145 x 10
6
1.1 x 10
-6
Fiberglass 6 x 10
6
0.041 x 10
6
6.0 x 10
-6
Table 4 - Young’s Modulus and Thermal Coefficients