Back Pressure (Steady); Low And Moderate Pressures; High Pressures; Back Pressure (Fluctuating) - Datex-Ohmeda Tec 5 Operation And Maintenance Manual

7.2.4

Back Pressure (Steady)

7.2.4.1

Low And Moderate Pressures

The vaporizer cannot distinguish between pressures at the outlet due to barometric pressure and
pressures in excess of barometric due to steady back pressures applied by downstream components.
Equation 1 therefore applies with the term P now being the absolute pressure at the outlet, that is,
barometric pressure plus back pressure. Steady back pressure reduces the % v/v.
Currently it is unlikely that the steady back pressure imposed by commonly used downstream
components, other than some ventilators, exceeds 30 mm Hg at commonly used flowrates. Back
pressures as high as 30 mm Hg would reduce the delivered % v/v, at 760 mm Hg barometric pressure,
to the following:
760
Equation 2:
790
Under normal clinical circumstances effects of this magnitude can be ignored.
7.2.4.2

High Pressures

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Warning: Pressures in excess of 400 mm Hg must not be imposed on the vaporizer since pressures
of this magnitude may overcome the internal pressure balance and cause a variation in output.
7.2.5

Back Pressure (Fluctuating)

Fluctuating back pressures may be imposed on the vaporizer by downstream components and/or
assisted or controlled ventilation to the patient. These fluctuating back pressures can affect the
vaporizer and increase the concentration by intermittently altering the pressures, and hence the flow
distribution, within the vaporizer.
The greatest effects are observed at combinations of very low flowrates and low dial setting with large
and rapid pressure fluctuations. The effects become progressively less important as the dial setting and
flowrate increase and the magnitude and rate of cycling of the pressure fluctuations decrease.
7.2.6

Carrier Gas Composition

Small effects on the output can occur when the carrier gas composition is changed. These effects may
either be transient, arising from the varying solubility of gases in anaesthetic agent, or steady, due to
the variations in gas density and viscosity affecting the flow split within the vaporizer.
When either air or nitrous oxide at low flows is employed as the carrier gas, the output is depressed
compared to the output when oxygen is the carrier gas. This effect is greatest when nitrous oxide is
employed, but using nitrous oxide reduces the required inspired concentrations of volatile agent
thereby mitigating the small depression in output from the vaporizer.
At high flows and low dial settings a small increase in output may be observed.
If the proportion of nitrous oxide passing into the vaporizer is increased, a temporary decrease in
vapour output is obtained. Conversely, if the proportion of nitrous oxide is decreased, a temporary
increase in output is obtained.
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Warning: Do not operate the vaporizer other than with dry medical gases.
7.2.7

Time Out Of Service

If the anaesthetic machine on which the vaporizer is fitted is left for a period of time with no gases
flowing, small concentrations may be detected at the machine outlet immediately after the gas flow is
turned ON. This is a normal machine characteristic and is caused by residual vapour left in the machine
from previous use.
Tec 5 Vaporizer
O & M Manual Part No. 1105-0100-000
= 0.96 of what would otherwise be expected.
Page 27
August 1999