The optical lens for the Wedeco TMO IV. It sits in the measurement path, so its condition is part of the measurement: a lens that has fouled, etched or solarised attenuates the signal and the instrument reads low without reporting a fault. The matching 21 × 2 mm lens O-ring is stocked under item 13223-0415. Wedeco was founded in Herford, Germany in 1976 and is now a Xylem brand.