Multiple Echo Measuring Mode
All Cygnus Thickness Gauges have the facility to use the Multiple Echo Measuring Technique with a single crystal probe. This provides, accurate, error-checked thickness measurements, ignoring coatings.
How it works
A Thickness Gauge with a probe uses the time taken for echoes of sound to travel through a material and converts this into a thickness measurement. Multiple sound pulses are used hence the name “Multiple Echo Technique”.
A short pulse of ultrasound is generated and coupled into the material from the probe. The instrument then listens for back-wall echoes to return to the probe which is now in receiving mode. When the gauge detects three consecutive echo signals of equal spacing it proceeds to measure the time between these echoes and calculates the material thickness. As three consecutive echoes are used any coating will be automatically ignored. Also because the spacing between the three consecutive echoes must be equal means measurements are checked and far more reliable than single-echo or echo-echo modes of ultrasonic thickness measurement.
The measurement displayed is an accurate thickness measurement. If the timings do not match, no measurement is displayed.
The ‘multiple echo’ beam travel is depicted in the diagram, spread out for time, to illustrate the timing method. The beam path is in fact straight, at 90° to the surface, and the ultrasonic energy reverberates up and down within the metal. Each time the echo is reflected back down, a small portion of the energy comes up through the coating, striking the probe which now acts as a receiver.
The delay between echoes at the probe face is exactly equal to the time taken to pass through the material (excluding the coating) twice, therefore coatings such as paint are ignored and the measurement displayed is of the metal thickness only.
Advantages of Multiple Echo
The most obvious advantage is that coating and paint do not have to be removed and will not be included in the measurement displayed. Using Multiple Echo, Cygnus gauges can read through coatings up to approximately 6mm (0.24”) thick. With the deep coat function this can be increased to 20mm (¾”) of coating. A single crystal probe having a straight sound beam can be used to measure on curved surfaces. Changing the orientation of the probe doesn’t affect the measurement. As the timings between the second and third echoes have to match this ensures accurate, verified measurements. Ships surveys to classification standards usually specify Multiple Echo/Through Coating mode is required.