Physical Process
Movement of a thin metallic or ceramic membrane under pressure transfers force to a sensing element while protecting it from the process medium. Isolation diaphragm deflection must be sufficient to displace the fill fluid but stops providing protection if the pressure exceeds the mechanical yield point of the disk.
Geometric Influence
Sensitivity of the pressure transmitter is determined by the diameter, thickness, material modulus and corrugation pattern of the barrier. In a typical isolation diaphragm deflection, the center of the disk moves in proportion to the applied force according to thin plate theory. Small changes in the material modulus or the tension of the weld can alter the response.
Error Source
Residual stresses from the forming process or the installation of the seal introduce nonlinearity into the measurement. When isolation diaphragm deflection occurs, the volume of fluid moved must match the requirements of the internal sensor without causing excessive back pressure. Temperature fluctuations change the stiffness of the metal and the volume of the internal oil.
Qualification Process
Verification involves cycling the part through its full range of motion to check for hysteresis or permanent set. Fatigue life for isolation diaphragm deflection is a function of the stress amplitude and the corrosive nature of the external environment.
