HF Resistant probes incorporate a special glass which is resistant to Hydroflouric Acid. As a general rule of thumb, this glass, which has a blue tint, will last up to four times longer than the general purpose glass, when exposed to Hydroflouric Acid. HF probes are only required when Hydroflouric Acid is present. If there are other acids present, such as HCL, Sulphuric Acid or Nitric Acid, general purpose glass is fine. HF attack is usually seen as chemical etching or erosion of the glass membrane, resulting in a frosted appearance.