The respirometer under-reads at low flow rates and over-reads at high flow rates, but during anaesthesia, the respiratory waveform and the nature of the respired gas combine to minimize the low error which would otherwise result from hypoventilation. Over-reading is to be expected during hyperventilation of the anaesthetized patient. The error will always exaggerate a departure from normality. The response of the ventigrator is proportional to the square of the gas flow rate. Consequently the sensitivity is poor at low flow rates and excessive at high flow rates and, furthermore, the signal is markedly influenced by the peak flow/minute volume ratio. The response of the ventigrator is directly proportional to the vapour density of the respired gases. Large corrections are, therefore, required during anaesthesia.