Operating Manual

Chapter 4: Temperature Compensation
Guide to Ultrasonic Inspection of Fasteners Page 17
Chapter 4: Temperature Compensation
The temperature of a fastener affects its physical length.
As the temperature of a fastener increases, its physical
length increases. In addition, as a fastener’s tempera-
ture increases the amount of time it takes for sound to
travel through the fastener also increases. In other words,
when a fastener is subjected to increased temperature,
its acoustic velocity decreases and, therefore, its ultra-
sonic length increases. In fact, temperature’s effect on
ultrasonic length is even greater than its effect on physi-
cal length. The thermal expansion of the fastener and
the ultrasonic velocity change with changing tempera-
ture are two separate effects. However, in the BoltMike’s
logic they are compensated for with a single combined
factor known as the
Temperature Coefficient (Cp) .
The BoltMike relies on its temperature compensation
system to normalize the time of flight of a fastener and
thus correct for temperature-caused changes in its physi-
cal and ultrasonic length. The compensation system
normalizes the TOF to the value expected at 22.22 de-
grees C (72 degrees F) before attempting to calculate
the change in the fastener’s ultrasonic length. This
compensation greatly improves accuracy when the tem-
perature has changed during the time period between
recording a reference length and a tensioned length.
4.1 Measuring Fastener Temperature
In some applications, significant differences in tempera-
ture exist from one portion of the fastener to another.
Compensating for these temperature gradients is
extremely difficult. Instead, the fastener’s average tem-
perature is used for temperature compensation. While
the BoltMike allows manual input of temperature, it is
preferable to input fastener temperature using the tem-
perature probe.
The BoltMike’s temperature sensor provides a conve-
nient way to input fastener temperature. Because it
magnetically couples to the metal of the fastener joint, it
provides a very accurate temperature reading.
Typically, the temperature sensor is attached to the
superstructure or frame that is being fastened, not each
individual bolt. The probe is then left in place while the
lengths of all fasteners in the area are ultrasonically
measured.
NOTE: In most cases, air temperature has very little
effect on fastener temperature and should not be
entered as the temperature of the fastener. For opti-
mum accuracy, use the temperature sensor and au-
tomatic temperature compensation.
NOTE: The range of the BoltMike temperature sen-
sor is -55 degrees to 150 degrees C (-67 to 302 de-
grees F). Use of the sensor outside of these ranges
will damage the sensor.
NOTE: Large accuracy problems can occur from han-
dling the temperature sensor. Body heat conducted
into the housing of the sensor will greatly increase
the temperature reading. After holding the sensor in
a bare hand, allow approximately ten to fifteen min-
utes for the temperature probe to stabilize. If while
fastener measurement is underway a temperature
sensor must be moved, handle it only while wearing a
thick glove. Alternatively, you may carefully remove
the temperature sensor by pulling on and handling
only its cable.
4.2 Limits of Accurate Temperature
Measurement
Errors in temperature compensation can have several
causes including:
• Manual input of air (rather than) fasten tempera-
ture
• Contact between the operator’s hand and the
temperature sensor
• Variation of the material’s temperature coefficient
• Materials’ non-linear response to changes in
temperature
The last two of these sources of error should be further
explained. If a sample of physically identical bolts is tested
for temperature coefficient, some bolt-to-bolt variation
will be found. The amount of variation will depend on the
type of material, and the uniformity with which the fas-
teners were manufactured. One way to compensate for
this variation is to determine the range of actual tem-
perature coefficients in the sample then decide of the
difference between the actual and average values is too
significant. Alternatively, a temperature calibration can
be preformed for each fastener.
A materials actual response to changes in temperature
(as represented in the BoltMike by the temperature co-
efficient) is not necessarily linear over a large range of
temperatures. Although the thermal expansion of a fas-
tener, when plotted against change in temperature, is
very nearly linear, non-linearity is present in all materi-
als. When trying to compensate for a large variation in
temperature (in the range of fifty degrees Centigrade or