The Bacharach 0024-1027 is a NO2 sensor for the Bacharach ECA 450 series, Bacharach PCA 3, and the Bacharach PCA 400 series. The 0024-1027 allows the Bacharach ECA 450, PCA 3, and PCA 400 analyzers to analyze NO2. This NO2 sensor replacement is not calibrated and may need to be calibrated manually or sent in to the factory to be calibrated.