The PR‑21 refractometer series can be used in locations with potentially explosive atmosphere with following modifications, made by Vaisala.
The PR‑21‑…‑FM refractometer is certified by Factory Mutual Research Corporation, Certificate No. 3050975. Equipment Ratings: Nonincendive for use in Class I, Division 2, Groups A, B, C & D, Hazardous (Classified) Locations. Temperature identification rating for PR‑21‑…‑FM is T6 (Tamb = +45 °C).