A gas injector apparatus and method for use with a tracer gas airflow measurement system that allows for large, continuous tracer gas flow rates while avoiding operation at very cold temperatures that will damage equipment. The controlled and variable heating of high pressure tracer gas flow before a gas regulator allows all points in the system to remain above freezing, and within a safe temperature range. The system has a tracer gas source and a heater that heats the gas isobarically at high pressure (>100 psi) and the hot gas is expanded across a pressure regulator to a lower working pressure to ˜30 psi. The gas flow is modulated using a mass flow controller with a high turndown ratio and injected into a bulk fluid flow with an injector.