Page 144 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF RESPIRATORY DEVICES 123

Reference
Reference

Pump
Signal
output

–

+
Sample

FIGURE 4.8 Schematic drawing of a thermal conductivity meter. The elec-
trical elements are configured in a typical Wheatstone bridge circuit. The vari-
able resistors are usually thermistors.

be removed by passing the gas stream through a canister of calcium sulfate (CaSO ). Alternatively,
4
water vapor may be removed, or equilibrated to a known level, by passing the gas mixture through
a length of nafion, tubing with walls that are able to remove water because they are impregnated with
a material with a high affinity for water. Placing the nafion within the lumen of larger tubing through
which is run dry nitrogen (running in the direction opposite to the gas mixture to be analyzed) will
remove all the water vapor from the mixture, whereas leaving the nafion tubing exposed to room air
will equilibrate the water vapor concentration with ambient levels. Carbon dioxide is most often
removed by passing the gas stream through a canister containing either barium hydroxide [Ba(OH) ]
2
or sodium hydroxide (NaOH). In both cases, the chemical combines with the gaseous CO remov-
2
ing it from the gas mixture. Also, both chemical CO scrubbers produce water as a by-product of the
2
reaction and therefore should be placed upstream to a water removal system, if needed. Usually, any
chemical scrubber will be impregnated with an indicator that changes color as the chemical becomes
consumed, alerting the user to the need for replacement.
Thermal conductivity meters measure the ability of a gas mixture to conduct heat away from a
heat source (Fig. 4.8). They usually employ two thermistors, one exposed to sample gas and the other
to a reference sample containing none of the gases to be measured, arranged in a conventional
Wheatstone bridge. Thermal conductivity is most commonly used to measure helium concentration,
although it can be used to measure carbon dioxide concentrations if used carefully. Both carbon
dioxide and water vapor interfere with helium analysis and must be removed from the gas mixture
prior to helium analysis. Water vapor, like helium, has a higher thermal conductivity than other gases
found in air, and thus will cause false elevation in the helium reading. Carbon dioxide, on the other
hand, has a lower thermal conductivity and will cause a lowered helium reading. Thermal conduc-
tivity meters tend to be quite linear, making them easy to use, but they have relatively slow response
times on the order of 20 to 30 seconds.
Infrared (IR) absorption is a common method of gas analysis and can be used for measurements of
carbon dioxide, carbon monoxide, and methane, among others (Fig. 4.9). IR analyzers run the gas mix-
ture through a sample chamber illuminated at one end with an IR light source. At the other end of the
chamber is an IR detector. Parallel to the sample chamber is a reference chamber, which is usually
filled with room air and uses the same IR source and detector as the sample chamber. The gases to be
analyzed absorb some of the IR radiation, decreasing the amount reaching the detector. A motor
rotates a “chopper blade” which alternately blocks one of the two chambers from the source. By syn-
chronizing the chopper rotation with the signal at the detector, the analyzer can determine the relative