A diaphragm seal is a thin and flexible membrane that seals and isolates. The flexible nature of this seal allows pressure to transmit but not the process material.
Commonly used to protect pressure sensors from the fluid whose pressure is being measured.
There are mainly recommended for following conditions:-
In viscous (>200cST) application
In waxy, sticky, coke forming, plugging service.
Highly Toxic, corrosive fluids or fluids with solids
Where impulse line require process tracing to prevent condensation/clogging.
Plugging concerns requiring rod out facilities and urge
Process Temperature are at abnormal operating conditions exceeding measuring elemnet conditions.
For low pressure applications it is also recommended
Diaphragm Sealed pressure sensing instrument along with filled fluid capillary are being used in the industry for several year to protect the instrument from direct exposure of heat or process.
Fluid is filled between the diaphragm and the sensing element so that only pressure gets transfered to the sensing instrument not the heat or damaging process fluid
Generally Silicon is preferred fluid material because of it characteristic.
What I have seen is that for the grade of Silicon filling changes with temperature for e.g
DC200 till 200 deg C
DC704 till 338 deg C
KN32 till 400 deg C