Stablcal Turbidity Standard for all Turbidimeters. USEPA accepted for calibration purposes. Stablcal Stabilized Formazin Turbidity Standards are true Formazin dilutions developed for use in any turbidimeter. NTU Value: 200. Bottle: 500 mL. or TL23, TU52, 2100Q, 2100N, 2100AN series.