Measured-Flow Vaporizers

Measured-flow vaporizers are obsolete, but may still be encountered in the form of the Copper Kettle, Verni-trol and Metomatic vaporizers.

The principle of operation is similar to that of precision vaporizer, in that gas saturated with anesthetic is mixed with anesthetic-free gas. However, the flow through the vaporizing chamber and the flow bypassing the chamber are controlled by flowmeters, rather than a dial on the vaporizer. The delivered concentration must be calculated from the SVP of the anesthetic vapor at the current temperature and the proportions of the total flow going through and bypassing the vaporizing chamber. This calculation is facilitated by a circular slide rule provided with the machine, but this has often been lost by the time it is needed.

Note that if the bypass flowmeter is turned off, it is possible to supply gas fully saturated with anesthetic to the patient.

Metomatic Vaporizer
The Metomatic vaporizer, found on Pitman-Moore 980 machines, is similar except that the gas supply to the vaporizer flowmeter is taken from the output of the main flowmeter, the excess passing to the gas outlet via a bypass valve. Consequently, any change in the total gas flow rate will also change the gas flow through the vaporizer, and the vaporizer output will remain unchanged.

Uses
Although these vaporizers are accurate and can in theory be used for any agent, the calculation is cumbersome and errors are liable to occur. This is particularly so since every change in concentration or temperature requires a new calculation to be made.

Breathing circuits