March, 2020
www.us-tech.com
Page 67
Acoustically Examining A Ceramic Pressure Sensor
By Tom Adams, Consultant, Nordson SONOSCAN
service life. Typically, these diaphragms are made of a thin, flexible metal. Metal sur- faces, however, may be altered chemically by contact with some liquids or gases. Where this problem exists, a ceramic diaphragm, typically made of alumina, can be used because it is chemically inert. The ceramic diaphragm is metallized
S
on its inside surface, which is not in contact with the liquid or gas. The ceramic pres- sure-measuring sensor described here con- sists of two circular ceramic disks, both having the same diameter. One of the disks is rigid and the other is thin and slightly flexible. The inner face of each disk is plated
with a thin metal layer. A thin air gap sep- arates the metallized faces of the two disks and forms an electrical capacitor. The ceramic diaphragm is more linear than a metal one would be. Its flexing under pres- sure is more proportional to the pressure applied. This allows it to provide a more precise measurement of pressure. Flexing of the diaphragm by a pres-
surized liquid or gas alters the thickness of the air gap and changes the capacitance. The change in capacitance reports the change in pressure.
Acoustic Micro-Imaging The acoustic images that illustrate
this article were produced in Nordson
Figure 1: Acoustic image gated on the surface of a ceramic sensor element. The small dark line near the center is a crack.
ensors that measure the pressure of liquids and gases use a diaphragm capable of responding accurately to variations in pressure over a long
SONOSCAN’s laboratory by a C-SAM® acoustic micro imaging tool. The return echoes from a pulse of focused ultrasound are very effective at locating
and imaging internal structural defects in materials. The tool’s transducer scans over the surface of
the part from a distance of a few millimeters above and sends pulses of ultrasound into the part at thousands or millions of x/y locations, depending upon the image resolution desired.
Echoes are returned only from materi-
al interfaces and not from homogeneous material. An echo from an x/y location where a pulse has been launched becomes one pixel in the acoustic image. In the images shown here, the trans-
ducer launched pulses into the exterior sur- face of the ceramic diaphragm. Diagnostic echoes were received from the vertical zone beneath the transducer at each x/y location. If there were no defects within the diaphragm material itself, the ultrasound would reach its back surface and a large echo would be returned to the transducer, creating a bright pixel in the image. If there were defects within the ceramic, additional echoes would be received and the pixel brightness altered accordingly. Interfaces between two solids will gen-
erate a pixel that is some shade of gray in the acoustic image. “Bright” means that the pulse was nearly 100 percent reflected from a solid-to-air interface, such as the interface between the back surface of the diaphragm and an air gap. Note that a pulse inserted into one disk will not cross the air gap and thus the rigid disk cannot be imaged from the diaphragm side.
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