A possible alternative to glass sight glasses for hydrofluoric acid (HF) service are plastics. Acrylic sight glasses are readily available and resistant to dilute concentrations of HF. One potential problem with acrylics is that they are not resistant to most organics at mid to high concentrations. The reader does not indicate the purpose of the sight glasses. If they are being used to detect interfaces for separation of organic/water layers there are instruments available that can detect phase changes which could eliminate the need for sight glasses. HF is a difficult chemical to deal with, good luck!