In many industries it is required to have a ‘plastic’ or polymer coating over the diaphragm seals or wetted parts of the diaphragm seals in order to protect your instruments. These coatings can be PTFE, FEP, PFA or ECTFE, each with their own characteristics.
Typically all of these coatings give excellent chemical resistance in heavy-duty corrosion applications. However, chemical resistance is only secured when the coatings have a thickness of approx. 2 mm. Due to the nature of the flexible and thin diaphragm (75μm) of the diaphragm seal, it does not allow a thick layer of this polymer coating to be applied, as it would make the diaphragm less flexible and thus no longer transferring the pressure accurately. To maintain the flexible and sensible function of the diaphragm, only a 35-50μm thick layer can be applied, which is not sufficient to warrant chemical resistance. It may provide additional protection for chemical resistance and it may make the instrument more durable, but this cannot be warranted. It is also often used for non-stick purposes. It ensures that the process medium does not stick to the metal diaphragm and therefore the flexibility of the diaphragm is not deteriorated.
A PTFE lining (approx. 2mm thick) cannot be used for protecting the diaphragm, but it can be used to protect the inner part flange (the lower part). Due to its thickness, this does provide excellent chemical resistance. Often customers use a combination of options: a tantalum diaphragm (for the wetted parts) and a PTFE lined lower part of the seal (together with an inert filling fluid that is often used for chlorine applications) or a coated wetted part (incl. diaphragm) with a PTFE lined lower part.
Please note that these coatings and lining have temperature limitations. Badotherm can always support you, or help you in choosing the right option.