9
4.2. Temperature Correction
The Model 4400 Vibrating Wire Jointmeter has a very small coefficient of thermal expansion
so in most cases correction is not necessary. However, if maximum accuracy is desired or
the temperature changes are extreme (>10° C) corrections may be applied. The temperature
coefficient of the mass in which the Jointmeter is embedded should also be taken into
account. By correcting the transducer for temperature changes the deformation of the mass
may be distinguished. The following equation applies;
D
corrected
= ((R
1
- R
0
)
G) + ((T
1
- T
0
)
K) + L
C
Equation 3 - Thermally Corrected Deformation Calculation
Where; R
1
is the Current Reading.
R
0
is the Initial Reading.
G is the Calibration Factor.
T
1
is the Current Temperature.
T
0
is the Initial Temperature.
K is the Thermal Coefficient.
L
C
is the correction for the expansion/contraction of the universal joints and
flanges, (see figure1).
.
Tests have determined that the Thermal Ceofficient, K, changes with the position of the
transducer shaft. Hence, the first step in the temperature correction process is determination
of the proper Thermal Coefficient based on the following equation;
Thermal Coefficient = ((Reading in Digits
Multiplier)
Constant)
Calibration Factor
K = ((R
1
M)
B)
G
Equation 4 - Thermal Coefficient Calculation
See Table 2 for the Multiplier and Constant values used in Equation 4. The Multiplier (M)
and Constant (B) values vary for the stroke of the transducer used in the Jointmeter.
Model: 4400-12
mm
4400-0.5"
4400-25 mm
4400-1"
4400-50 mm
4400-2"
4400-100mm
4400-4”
Multiplier (M):
0.000375
0.000369
0.000376
0.000398
Constant (B):
1.08
0.572
0.328
0.0864
Length of joints, stand-
offs and flanges, (L):
267 mm
10.5”
259 mm
10.2”
162 mm
6.38”
162mm
6.38”
Table 2 - Thermal Coefficient Calculation Constants
The correction for expansion/contraction of the universal joints,stand-offs and flanges, (L
C
),
is calculated using Equation 5.
L
C
= 17.3
10
-6
L
(T
1
- T
0
)
Equation 5 - Gage Length Correction
Where L is from Table 2 in millimeters or inches, to match the Calibration Factor units.
