Chemical Oxygen Sensor Overview

By far the most popular way to measure Oxygen has been to use a chemical sensor that produces a small current by a chemical process when exposed to oxygen gas.  These sensors are relatively fast (10 to 15 seconds to 95%  of final value) and very specific to oxygen, so they can make accurate oxygen measurements in complex gas mixtures.  The downside is that the chemical reaction that is produced is sensitive to temperature, and the sensitivity of the sensor gradually reduces with time as the chemically active elements of the sensor are consumed.  Both of these issues can be easily rectified by periodic recalibration using ambient air, which contains a known and stable concentration of oxygen.  Periodic monitoring of sensor response to ambient air also allows the sensor end of life to be determined.  Chemical sensors have become clearly the most popular way to measure oxygen – and there are literally millions of installations worldwide that use these sensors to measure oxygen.  They have been shown to provide reliable service between 12 to 18 months in typical applications – and are relatively low cost and field changeable – as the instruments they are used in are generally self-calibrating.

The Effect of High Concentrations of CO2 on Chemical Oxygen Sensors

The principle use of the sensors is in analysis of oxygen in the exhaust gas from combustion processes.  In these applications, the  chemical sensor may be exposed to other gases in the exhaust gas mix – with the CO2 level never reaching much above 15% due to the chemistry of the fuels used.  The oxygen sensors used in this application are generally designed to withstand CO2 levels of up to 20%.  Above that level, Carbon Dioxide in the gas mix is at a high enough level to react with the chemical sensor internal components, causing a precipitate to coat the active elements in the sensor and causing early sensor failure.  This effect is strictly a matter of sensor chemistry, and is directly a function of the level of CO2 to which the sensor is exposed during use.  CO2 concentrations above 20% will cause shortened life – with typical values of 6 months life for 35% CO2, and 3 months life for 50% CO2.  (The actual sensor life depends on the details of sensor design, exposure time duty cycle, and the characteristics of the individual sensor.)

Suffice it to say that the chemical sensors designed to measure Oxygen in relatively low (less than 20%) concentrations of CO2 have drastically shortened life when exposed to high CO2 concentrations.

The Solution – A CO2 Resistant Chemical Oxygen Sensor

There are chemical oxygen sensors that are designed specifically for operation in high concentrations of CO2.  They are more expensive initially – but they also produce longer typical operating life, so the actual cost per month remains the same between the two designs.  Bridge Analyzers, Inc. has elected to use the standard (strong and fast) oxygen sensor for low CO2 (below 20% – Model 900107, 900109) applications – and use the more expensive and exotic CO2-resistant sensor for high CO2 concentrations (above 20% – Model 900108, 900110).  This necessitates that the consumer be sensitive to the range of CO2 that the analyzer will be exposed to and order the correct range analyzer.