ZX-1
Ultrasonic Thickness Gauge
9
3.5 Temperature
Temperature has an effect on sound velocity. The higher the temperature, the slower
sound travels in a material. High temperatures can also damage transducers and
present a problem for various liquid couplants.
Since the sound velocity varies with temperature it is important to calibrate at the
same temperature as the material being measured.
Normal temperature range
Most standard transducers will operate from 0
F to 250
F.
High temperature measurements
Special transducers and couplants are available for temperatures above 250
F up to
1000
F with intermittent contact. It is necessary to cool the transducer by
submerging it in water between measurements.
Modes and temperature errors
In addition to errors caused by velocity changing with temperature, some modes
(measurement techniques) are affected more than others. For example, dual
element pulse-echo mode has larger errors due to changes in the temperature of the
transducer. However, multi-echo techniques offer temperature compensation help to
minimize these errors.
3.6
Measurement Modes
This section will cover the different measurements modes of the
ZX-1
, the
transducers required, and the reasons for using specific modes:
Pulse-Echo (P-E) Mode:
Pulse-echo mode measures from the initial pulse (sometimes referred to as an
artificial zero) to the first echo (reflection). A manual zero must be performed by
placing the transducer on the reference disk located on top of the
ZX-1
, and the
PRB
0
key pressed to establish a zero point for the transducer connected.
In pulse-echo mode, errors can result from surface coatings and temperature
variations. Since pulse-echo only requires one reflection, it is the most sensitive
mode for measuring flaw/defects when measuring heavily corroded metals.
V-Path Correction
Dual element delay line transducers have two piezoelectric elements focused
towards one another at a slight angle, mounted on a delay line. One element is used
for transmitting sound, while the other element receives the sound reflection. The
two elements and their delay lines are packaged in a single housing but acoustically
isolated from each other with an insulated sound barrier. This allows the transducer
the ability to achieve very high sensitivity for detecting small defects. Also, the
