A lightweight, compact gas standards generator for permeation tubes is described. This system provides reliable temperature control during periods of intermittent power and is ideal for field measurements. A eutectic alloy with a high heat of fusion is used as a phase change material. This thermal source allows the system to maintain a constant permeation tube temperature of 46.6 °C for over 5 h in the absence of external power. This permeation system is currently being used in an aircraft chemical ionization mass spectrometer to provide HNO3 calibration.