Ordinarily, the plumbing connections between the sample manifold, analyzer, and calibrator must be changed for each calibration. The Model 450’s unique “through-port” feature eliminates this chore. The mode control switch selects among standby, zero, span 1 (low concentration), span 2 (high concentration), and remote modes. In the remote mode, the Model 450 can be externally programmed to operate in all of its functional modes.