vaporizers are obsolete, but may still be encountered in the
form of the Copper Kettle, Verni-trol and Metomatic vaporizers.
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
that if the bypass flowmeter is turned off, it is possible to
supply gas fully saturated with anesthetic to the patient.
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.
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.