Dakota Ultrasonics MINI--MAX - Manual (Page 46)

Dakota Ultrasonics
40
Chapter 8 Measuring System Zero
(calibration)
8.1 Introduction
System zeroing is the method of calculating the time required for each MINI-MAX
and transducer combination to detect the echo. When an echo travels back
through the bolt to the transducer face, there is an electronic delay before the
MINI-MAX detects the echo. Because of differences in the electronic parts, or
the transducer and cable, the delay is slightly different for each combination of
MINI-MAX and transducer. The time delay is called the delay factor or zero time
offset.
The MINI-MAX is designed for use with two calibration bars having a known
ultrasonic velocity and known length. Comparing their known physical length and
measured ultrasonic length determines the system delay factor, or zero time
offset. The system delay factor makes the Dakota Ultrasonics MINI-MAX
interchangeable with all other calibrated MINI-MAX systems.
Once the delay factor is determined, the MINI-MAX software automatically
subtracts it from the apparent measurement of the time required for the sonic
signal to travel through the bolt. Thus, the actual time of signal travel is
determined, and compensation is made for the slight difference in each system.
Since every zeroed MINI-MAX makes the same time measurement, the systems
give the same result and are interchangeable.
System Zeroing is only required if:
• More
than one MINI-MAX or transducer will be used while measuring a set of
bolts. If the same MINI-MAX and transducer are used, the amount of the
delay is constant. If unloaded length measurements were made with one
MINI-MAX, and the load measurements were made with another, the
difference in the delay factor could cause an error, though in most cases the
error would be very small. If it is uncertain that the same unit and transducer
will always be used, it is strongly recommended that System Zeroing be
checked whenever the MINI-MAX used.
• The application requires that the bolts be monitored over periodic intervals
through their service life. In this case, the MINI-MAX must be zeroed in order
to maintain reliable and accurate measurements. Once again, If a
transducer, cable, or unit is replaced, the delay factor will change, resulting in
erroneous measurements.