Gas Mixing Applications for Low Flow with Precision Measurement

Any laboratory or biomedical research setup that requires gas flow under controlled conditions will need a gas flow controller. This is all the more true when two or more gases are to be mixed within a chamber to reach specific partial pressures for each component gas. In order to obtain reliable data from such experiments, a failsafe gas flow controller is essential and is the object of search for all such scientific researchers.

Suspended Animation: an Illustration from Real-Life

In one such experiment on suspended animation, using a mixture of gases which required to be present in precise proportions within a mixture at very low flow rates, Dr. David Kraus, Ph.D. experienced this concern. A sulfide biologist working at the University of Alabama, he needed to find the right mass flow controller that could ensure that accurately measured and reproducible amounts of a specific gas, namely, hydrogen sulfide (H₂S) was delivered to the laboratory mice. This blend actually had 5% H₂S and 95% nitrogen gas, and was mixed with an incoming stream which itself comprised air and nitrogen gas. The H₂S-N₂ mixture was flowing into the air-N₂ stream at 1-10 sccm, a very low rate of flow, which was supposed to reduce the metabolic rate, as well as decrease the temperature and heartbeat, inducing suspended animation.

In effect, Kraus was attempting to achieve a time-freeze in living animals. To bring them back to normal life, the H₂S would be removed.

Accurate control of the flow rate of each gas was absolutely critical as excessive H₂S would be lethal. On the other hand, too little would not achieve the state of suspended animation.

Highly Precise Gas Flow Rate Assessment

The key to a successful experiment was a gas flow controller that would be capable of fulfilling the biologist’s expectations of precise gas measurement in a mixture at low rates of gas flow.

The most frequent complaint with regard to conventional mass flow controllers is their inability to measure mixed gases accurately. One cause is the nature of the calibration curve, which is often adjusted by fitting the readings to the curve because of non-linearity. Again, one gas is used for calibration, but the instrument is used to measure gases in a mixture, often with properties quite dissimilar to the original gas. This means that the original curve would not fit the mixture.

Again, some types of flow controllers, in particular those which measure differential pressure (ΔP), may not provide accurate data in situations where the pressure changes during the process. For instance, when the chamber is filled with a gas mixture, the pressure will go up leading to back pressure on the pressure meters. As this reading changes, the accuracy of measurement will go down in a differential pressure device.

In low flow situations too, differential pressure flow controllers do not give precise readings because of the absence of sufficient difference in the pressure at such low degrees of flow, which falls below the sensitivity of the instrument.

Example of a typical gas mixing application

Which Type – Volumetric or Mass Flow?

In the experiment described here, Dr. Kraus had to choose between measuring mass or volumetric flow. His decision was made because of the higher reliability of capillary thermal technology. This meant that he used a controller using mass flow which offered dial-a-gas facilities besides having state-of-the-art laminar flow technology which relies on a linear relationship between the preset K-factors in the mass flow controller program and the gas flow, thus enabling it to compensate for all types of gas blends over a wide range of flow rates. Linearity has the advantage of avoiding frequent recalibrations, which are required only at zero and full scale.

The dial-a-gas capability is one which makes use of multiple flow controllers of the same type to regulate each pure gas in the mixture and the final blend itself without the need for recalibration. This was used in this experiment to set a custom listing for the 5% H₂S mixture, thus allowing the researcher to use the same controller to either simply select this mixture or select and measure any other gas from the menu of nine gases which was already set.

This mass flow controller also has a high-efficacy solenoid valve to ensure accuracy of control even when the flow is at a very low rate, within the range of 1-10 sccm. This is in contrast to the differential pressure controllers which are not accurate at such low flows. This would have been disastrous in this experiment as it would lead to the delivery of excessive H₂S and lethality among the rodents. The successful outcome due to the use of the mass flow controller meant strict regulation of the H₂S gas levels even at very low flow rates, which allowed all the mice to enter suspended animation.

The Big Three Requirements

The crucial parameters which decided which gas flow meter was used in this setup were:

• Precision of measurement of gas flow, which precluded more than 1% error in H₂S levels – an excess would be lethal, while too low levels would lead to failure to achieve suspended animation.
• Easy to use because the experiments demanded extreme care, requiring a user-friendly and simply operated gas mass flow controller
• Capability to function with customized mixtures, since H₂S is not a standard gas included in the measurement menu of most gas flow controllers

Conclusion

This unique and successful experiment is likely to create a ripple effect both in the medical and astronomical research areas. Dr. Kraus commented that NASA was looking into replicating suspended animation through the use of cryosleep chambers in space capsules which were bound to very distant space targets, enabling the astronauts to hibernate during extremely long trips.

This information has been sourced, reviewed and adapted from materials provided by Sierra Instruments.

For more information on this source, please visit Sierra Instruments.