Abstract: A valve stem gas leak sensor or detector configured to surround a possibly leaking petrochemical or like flow line valve stem and provide for external monitoring equipment an electrical resistance related to the amount of hydrocarbon gas leakage occurring around the valve stem. Such a gas leak detector, termable a ring sensor, is configured as a stem surrounding enclosure, with interconnecting segments for ease of installation around and removability from a valve stem, with one or both of each such segments having an interiorly arranged layer of gas sensor particulate material characterized by change in electrical resistance in the presence of a changing concentration of hydrocarbon gas leakage around a valve stem, and with interengaging connector/conductor pins holding the segments together around the valve stem and across which the electrical resistance of one or both gas sensor material layers can be measured with the ring sensor in place on the valve stem.