Solid materials whose atoms or molecules are regularly and repeatedly called crystals (the structure of these cells has a unit cell). Piezoelectric materials are electrically neutral and although the structures of the atoms of these materials are not symmetric, they neutralize each other in terms of electric charge. When a mechanical force is drawn into the piezoelectric material, the distance between the atoms is changed, and they move away from each other or together. As a result, the balance of electrical charges is mixed up and a net positive or negative charge is generated. On the other hand, when an electric current is given to the crystal it starts to oscillate. The frequency of the oscillation is depending on the shape, mass, thickness, and applied flow to the crystal.
Deposition of the different materials atom on the crystal during the coating process causes the crystal mass changes and leads to a change in the crystal frequency.
The crystal used in the thickness gauges is usually a quartz crystal with a frequency of 6 MHz, and when the crystal surface is covered by atoms, its initial frequency decreases. The frequency counter circuit calculates the frequency variation and converts it to a thickness value.
HOW DOES QUARTZ CRYSTAL WORK?