FEP is short for Fluoroethylenepropylene and is used to protect the sight or gauge glass from aggressive mediums.
Applying glass protection extends the life of the glass and reduces the need for glass changes.
The material is milky-white transparent, has good resistance to chemicals and high pH values. A sight or gauge glass wears out quickly when exposed to high pH values, for example at a pH of 12 and a temperature of 200°C, the glass wears out about 8 mm per month!
By default, mica is applied as a glass protection to extend the life / stand time of the sight or gauge glass. In some situations mica is affected by the medium, in which case the use of the thicker FEP (or Kel-F®/PFTE) is necessary.
The material properties of FEP and PFTE are very similar; FEP is less milky in color and more transparent than PFTE. Despite using the FEP or PFTE protection disc, with milky color, you can see well through the sight or gauge glass to monitor your process.