Thermco's Petroleum Gold Brand Hydrometers are a superior brand of high precision hydrometers calibrated to the highest standard that exceeds the exacting ASTM standards. Made from the finest materials available; the glass is free from imperfections, has the suitable thermal properties for the entire temperature range that would be subjected and resists any reactions from chemical agents. Each hydrometer has been calibrated against NIST standards and furnished with a NIST traceable certificate.