Abstract: An improved instrument for preparing quality control solutions of liquid and gas for use in calibrating blood gas analyzers includes a gas inlet providing gas to a humidifier, a diffusion tube, flexible tubing connecting the gas from the humidifier to the diffusion tube and a temperature controlled environment for the diffusion tube. These standard components are arranged in a unique combination permitting the tubing connecting the outlet of the humidifier to the diffusion tube and the diffusion tube itself to both be replaced or individually replaced without disturbing the other components of the instrument. For this purpose, a metal heat block is used as the temperature controlled environment and is provided with an open top for receiving the diffusion tube. The humidifier and connecting tubing are exterior of the heat block so that the referred to replacement of the tubing can readily be carried out.