>Technical Bulletin

Monitoring Oxygen-in-Helium Applications

In the past couple months there has been a flurry of discussions on the "Web" regarding the response of Oxygen sensors in an environment that contains some Helium. The discussion started with a government laboratory that uses liquid Helium to draw heat away from a process.

During a test of their emergency systems they released a quantity of Helium that they calculated would create an Oxygen deficiency, but the Oxygen alarm did not sound. Following their standard procedure using an Oxygen-in-Nitrogen mixture they were able to verify that the monitoring equipment was "calibrated."

Further study eliminated potential variations due to flow rate, "nonlinear" response, operator technique, and various equipment manufacturers and models. Only after carefully blending mixtures of Oxygen-in-Helium and Oxygen-in-Nitrogen were they able to determine that there was a significant variation in response - when calibrated using a Nitrogen blend and then challenged with a Helium blend the Oxygen reading was 18% Vol. when it should have read 15% Vol.!

There are generally two types of Oxygen sensors in the market today - capillary and galvanic - (though you may not have a choice when you select a particular brand or model of instrument). There are several good reasons to select one or the other, but they are not necessarily interchangeable in a given application.

As you may know, the output of an Oxygen sensor is determined by how many Oxygen molecules get through the sensor membrane and react. For a galvanic type sensor, the diffusion rate of Oxygen through the membrane is determined by the pressure and partial pressure of the Oxygen surrounding the membrane. The other gases that are mixed in with the Oxygen, whether it is air (Nitrogen) or Helium, for example, do not make much (or any) difference in the diffusion rate of the Oxygen molecules through the membrane.

For a capillary type sensor, the surrounding sample must first travel through a small capillary (tube) to get to the sensor membrane. Therefore, the rate that the gas can travel through the capillary determines how many Oxygen molecules reach the membrane. Since Helium is such a small atom, Oxygen/Helium mixtures will travel through the capillary faster than Oxygen/Nitrogen mixtures (air). So, 10% Oxygen-in-Helium will read higher than 10% Oxygen-in-Nitrogen - as much as 20% higher, indicating a safe exposure that may in reality be unsafe!

When should one use the capillary type sensor?

When...

• The application involves frequent changes in altitude (i.e., working in Colorado)
• The application involves rapid or significant changes in pressure or partial-pressure (frequent changes in barometric pressure - entering the Silverdome, for instance, sampling from a pressurized duct, etc.)

When should one use the galvanic type sensor?

When...

• The application includes actual or potential Helium concentrations
• The application involves moisture or particulates that could block the capillary

Where one does not have a choice of different types of Oxygen sensors or must deal with an existing situation, the choices might be:

• Use a calibration mixture that duplicates the expected atmospheric conditions (in this case use an Oxygen/Helium calibration gas)
• Apply a correction factor (in this case set the calibration at 15% when calibrating with an 18& Oxygen/Nitrogen mixture)
• Adjust the alarm threshold to provide an earlier warning (this may not be possible in the case of an Oxygen deficiency alarm where the setting the alarm threshold at 20.5% instead of 19.5% might not allow it to clear due to relay hysteresis)

The source of this "problem" can be traced to the current situation that allows "electronic display" manufacturers (both fixed and portable) easy entry to the market place when they use sensors developed and sold by sensor subcontractors. Virtually all of these instrument manufacturers use the City Technology Oxygen sensors, which are the capillary type. Not having a choice in this case means having to try to live with a less than ideal situation.

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