Abstract: An internally isothermal control volume of a flow verification system for measuring gas pressure drop relative to time to calculate flow rate has a thermal reservoir provided therein. The thermal reservoir is de-coupled from ambient effects so that the temperature of the thermal reservoir will be driven to the gas temperature during steady state flow of gas through the known volume and during gas expansion, will transfer heat to maintain a contrast gas temperature for accurate gas flow rate calculation with minimal effect from external influence during the verification process.