#164 - PPM O2 Analysis – Part 1

We have had quite a few inquiries lately about oxygen analysis in the parts-per-million (PPM) scale.

The ambient air we breathe has 20.95% oxygen (O2) in it. This proportion of O2 is suitable for sustaining many forms of life and biological processes on this planet. However, some processes require atmospheres of low O2 or even trace O2. When verifying the purity of a produced gas, instead of measuring the target gas, it is sometimes best to measure the impurities. If the impurity is known to be O2, measuring the O2 will make good sense. Instead of being stated as a percent (%) O2, some very low O2 atmospheres are stated in parts-per-million (PPM).

Measuring ppmO2 can be a challenge for several reasons:

• PPM measurements require a suitable degree of accuracy and
  precision to be meaningful.
• Not all O2 measurement technologies are sensitive enough to
  function at low ppmO2 levels.
• Ambient O2 is ubiquitous and is difficult to exclude.

In our experience, the electrochemical sensor is most commonly requested by our customers for ppmO2 measurement. In Nova’s product line, this methodology can be used for any range from 0-100 PPM to 0-19,999 PPM O2. The electrochemical sensor provides good sensitivity, accuracy, and speed of response to O2 levels in these ranges.

One disadvantage of electrochemical ppmO2 sensors is their sensitivity to high levels of O2. If a sensor is set up for a range of 0-1,000ppm and it is exposed to ambient air, it will be ‘blinded’ temporarily. The effect is similar to stepping out into bright sunlight after spending time in a low-light environment. An electrochemical sensor may have long recovery time after the exposure for a couple of reasons:

• Chemical recovery of the sensor and removal of high O2
  from the cell’s diffusion layer.
• Purging the excess O2 from the internal sample tubing
  and related components.

Therefore, it is important to avoid exposing the sensor to ambient air. This may not be too difficult with a continuous analyzer that is permanently connected to a low O2 environment. However, a portable analyzer may not have this advantage.

In these analyzers, we internally protect the O2 sensors by purging out the ambient air out before exposing the sensors to the sample. When finished analyzing, we trap the last ppmO2 sample in the sensor before decoupling the analyzer from the process. We do both of these operations by using PURGE / SAMPLE switch on the front panel.

Before the analyzer is exposed to the sample gas,
the switch on analyzer is moved to the PURGE position.
Running the analyzer in this mode allows some time
for the ambient air to purge out of the sample tubing.
After the purge time, the switch can be moved over to SAMPLE.

The Model 325 is designed for ppmO2 analysis. If a system like this is of interest to you, contact Nova for details.

1-800-295-3771

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