Checkline TI-MINIMAX - Manual (Page 105)

MINI-MAX Ultrasonic Bolt Tension Monitor
X L E
D C L
+ =
Effective Length
C
L
= Clamp length
D
X
= Additional diameter added for head,
nut(s), and or blind hole.
Note: A convenient way to perform the above Load Factor calculations is to use
the bolt calculator included in the MINI-MAXview PC software.
It’s important to note that performing the calculations above is only a good
approximation at best. In all cases, if possible, a field calibration should be
performed if the user intends to perform measurements in terms of load.
11.2 Calibrating a load factor (Field Calibration)
Clearly, the most effective way of calibrating the load factor is by performing a
field calibration. Often times, material constants are unknown or cannot be
calibrated in a laboratory environment. In cases such as these, a field calibration
is the most accurate means of calibration. A field calibration is accomplished
using a tensile tester, load cell, or other calibrated load device to compare known
loads against ultrasonic elongations. The MINI-MAX is equipped with a self-
calibrating feature that uses a linear regression or vector equation to produce a
best-fit line through the known loads, minimizing error.
The result is an added load offset (intercept), in the case of regression, and a
load factor (slope) to correct the ultrasonic measurements to the known loads.
The regression produces a load offset and load factor, while the vector sets the
offset to zero and produces only a load factor. Why are both options needed? In
applications where a small amount of load is applied to the fastener, thus
producing very little elongation, the regression option can potentially produce
non-linear results. In cases such as these, a vector may sometimes produce
better results as the offset is set at zero. It is recommended to use and compare
both results, following a field calibration, to determine which method is most
suitable for the application and producing the best results.