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How Ultrasonic Thickness Gauges Work?

Sep. 26, 2021

Sound energy can be generated over a wide range of the frequency spectrum. Audible sound occurs in a relatively low-frequency range, with an upper limit of about 20,000 cycles per second (20 kilohertz). The higher the frequency, the higher the pitch we perceive. Ultrasound is sound energy at much higher frequencies, beyond the limits of human hearing. Most ultrasonic inspections are carried out in the frequency range between 500 kHz and 20 MHz, although some specialized instruments will go down to 50 kHz or lower, or even as high as 100 MHz. regardless of the frequency, acoustic energy contains organized patterns of mechanical vibration that propagate through a medium such as air or steel according to the fundamental laws of wave physics.

 

Ultrasonic thickness gauges work by measuring very precisely the time it takes for an acoustic pulse generated by a small probe called an ultrasonic transducer to pass through the specimen and reflect back from the inner surface or far wall. The measurement is usually carried out in a "pulse/echo" mode from one side, as the sound waves are reflected from the boundary between different materials.


HS F91 Electromagnetic Ultrasonic Thickness Gauge

 HS F91 Electromagnetic Ultrasonic Thickness Gauge     

The transducer contains a piezoelectric element that is excited by a short electrical pulse to produce an ultrasonic pulse train. The sound waves are coupled into the test material and pass through it until they encounter a back wall or another boundary. The reflection is then transmitted back to the transducer, which converts the acoustic energy back into electrical energy. Essentially, the gauge listens for the echoes from the other side. Usually, this time interval is only a few millionths of a second. The gauge is programmed with the speed of sound in the test material and the thickness can then be calculated using a simple mathematical relationship

 

T = (V) x (t/2)

where

T = thickness of the part

V = velocity of sound in the test material

t = measured round trip transmission time

 

It is important to note that the speed of sound in the test material is an important part of this calculation. Different materials propagate sound waves at different speeds, generally faster in hard materials and slower in soft materials, and the speed of sound varies significantly with temperature. Therefore, it is always necessary to calibrate the ultrasonic thickness gauge to the speed of sound in the material being measured and the accuracy can only be as good as this calibration.

 

Sound waves in the megahertz range do not travel effectively through the air, so a drop of coupling fluid is used between the transducer and the specimen to achieve good sound transmission. Common coupling fluids are glycerine, propylene glycol, water, oil, and gel. Only a small amount is required and this is sufficient to fill the very thin air gap that would otherwise exist between the transducer and the target.


HS F91 electromagnetic ultrasonic thickness gauge is a new type of portable electromagnetic ultrasonic thickness gauge that uses an EMAT probe to detect any metal or magnetic material. The instrument does not need couplant when working, has fast detection speed and good repeatability, and can be used for high-temperature workpieces (up to 600°). The probe is not in direct contact with the measured object and has the function of automatic high-temperature sound velocity compensation. It is suitable for the detection of materials with rough surfaces and coating (paint, rust, etc.) on the surface.

 

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