Oxygen analyzers measure oxygen concentrations in a given environment for two main purposes: 1). industrial process monitoring or 2). safety monitoring purposes. Analyzers measure oxygen either as a percentage (0-100%) or trace amount (parts per million) for greater precision. 

Accurate measurements are important for industrial processes that require precise amounts of oxygen. These industries include aerospace, biotechnology, chemical, food and beverage, medical manufacturing, oil and gas, pharmaceuticals, pulp and paper, water and wastewater treatment, and more. Specific applications include:

• Food packaging
• Glove boxes
• Laboratories
• Manufacturing
• Oven applications (curing/heat treating)
• Welding

For process safety purposes, analyzers provide crucial measurement to ensure worker safety by making sure process levels of oxygen stay within defined limits.

Oxygen deficiency is considered among the most common and dangerous atmospheric hazards for personnel safety within confined spaces. This is especially true for laboratories, production complexes, gas storage areas, medical facilities, food processing plants, grain elevators, welding facilities, and other installations where significant amounts of inert gases ─ nitrogen, argon, helium, or carbon dioxide ─ are used. 

A leak of any of these gases can pose a significant danger to personnel due to oxygen depletion. Fresh air contains 20.9% oxygen by volume; OSHA considers breathing air to be oxygen deficient at 19.5%. Permanently installed oxygen deficiency monitors should be used as a first line of protection.

Oxygen analyzers function differently depending on their intended use application. There are two main application types–open diffuser applications and extractive applications. In an open diffuser application, the analyzer’s sensor is directly exposed to the ambient air surrounding it at close to atmospheric pressure. Conversely, in an extractive application, the sensor is installed in a sensor housing (small chamber) as part of a sampling system. Below, we explore how an analyzer works in each of these scenarios. 

Open Diffuser Applications

Oxygen deficiency monitors are the primary example of analyzer technology used for an open diffuser application. In this scenario, no sampling system is needed and the only requirements are continuous measurement and notification (i.e. alarms) of gas levels outside the desired range. 

For example, Alpha Omega Instruments’ Series 1300 Oxygen Deficiency Monitor features an ambient temperature electrochemical sensor with an Enhanced Electrolyte System (EES) with a weak acid electrolyte system. The weak acid electrolyte system retards passivation of the sensor anode by allowing the products of oxidation to dissolve in the acid electrolyte. In effect, the sensor is renewed continuously as the weak acid electrolyte tolerates over 20 times the lead oxide (PbO) than potassium hydroxide (KOH) based sensors. 

The result is a sensor with a greatly extended useful life as well as providing exceptional measurement stability. By measuring the voltage that is a result of the electrochemical process, the oxygen concentration can be accurately determined.

Another open diffuser scenario involves percent oxygen measurements within processes using environmental control chambers. Some examples are glove boxes, anaerobic chambers, and food packaging machines where an open diffuser sensor can be utilized as long as the chamber pressure is at atmosphere or ≤1.5 psig. For this need, Alpha Omega Instrument’s OXY-SEN™ Oxygen Monitor and Series 2000 Percent Oxygen Analyzer (with our OXY-SEN™ sensor) are both suitable. 

Another option for an environmental control chamber application is to use analyzers that are equipped with an extractive sample system, including a pump directly inside the environmental chamber, conditions permitting. Battery-operated portables Series 2520 (percent) and Series 3520 (trace) are great options for this application. If, however, conditions do not permit locating the instrument in the environment, then we recommend using one of our instruments with an extractive sample system outside the chamber.

Extractive Applications

Most process applications measure gases from a process rather than an ambient environment. In the process, there are conditions (e.g. pressure, temperature, relative humidity, etc.) that may vary widely, so “extracting” and conditioning a sample may be required. In extractive applications, the sensor is installed in a housing (small chamber) as part of a sampling system. As the sample gas passes through the sensor housing, the voltage produced by the electrochemical process can be measured and the oxygen concentration can be determined accurately. 

For extractive applications requiring percent level measurements, Alpha Omega Instruments’ Series 2000, 2500, 2510 and 2520 analyzers feature an extended life oxygen sensor with a weak acid electrolyte system. The weak acid electrolyte system retards passivation of the sensor anode by allowing the products of oxidation to dissolve in the acid electrolyte. In effect, the sensor is renewed continuously as the weak acid electrolyte tolerates over 20 times the lead oxide (PbO) than potassium hydroxide (KOH) based sensors. The result is a sensor with a greatly extended useful life. Different options are available to meet your sample condition requirements.

For extractive applications requiring trace (ppm) level measurements, Alpha Omega Instruments’ Series 3000, 3500, 3510 and 3520 trace oxygen analyzers feature an advanced trace oxygen sensor. This sensor is a lead-oxygen battery comprised of a lead anode, a gold plated cathode, and an electrolyte consisting of potassium hydroxide.

For applications with CO2, we also have a sensor with an electrolyte that is a weak acid, so that it can handle up to 100% CO2. All of our trace level instruments come with manual isolation valves so the sensor can be isolated from ambient levels of oxygen when they are not being used to measure trace level process gases. Different options are available to meet your sample condition requirements.

Choosing the right oxygen analyzer depends on your intended application(s) and the type of oxygen sensor used. There is no “universal” oxygen sensor type that fulfills every use case. Knowing the strengths and weaknesses of each oxygen sensor type is vital for choosing the most suitable oxygen analyzer given your needs.

The four main oxygen sensors include:

• Ambient Temperature Electrochemical
• Paramagnetic
• Polarographic
• Zirconium Oxide

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Below are additional details for various gas phase oxygen sensors. Use this information to help you select the right oxygen sensor type for your application:

AMBIENT TEMPERATURE ELECTROCHEMICAL SENSOR

• Often referred to as a galvanic sensor, is typically a small, partially sealed, cylindrical device (1-1/4” diameter by 0.75” height) that contains two dissimilar electrodes immersed in an aqueous electrolyte, commonly potassium hydroxide.
• Refinements in electrode materials, and enhanced electrolyte formulations, the galvanic oxygen sensor provides extended life over earlier versions and are recognized for their accuracy in both the percent and traces oxygen ranges.
• They are easy to damage when used with samples containing acid gas species such as hydrogen sulfide, hydrogen chloride, sulfur dioxide, etc.

PARAMAGNETIC OXYGEN SENSORS

• This is the magnetodynamic or `dumbbell’ type of design and is the predominate sensor type.
• The paramagnetic oxygen sensor consists of a cylindrical shaped container inside of which is placed a small glass dumbbell. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field.
• A precision optical system consisting of a light source, photodiode, and amplifier circuit is used to measure the degree of rotation of the dumbbell.
• Some paramagnetic oxygen sensor designs have an opposing current applied to restore the dumbbell to its normal position.
• In general, paramagnetic oxygen sensors offer very good response time characteristics and use no consumable parts, making sensor life, under normal conditions, quite good.
• Offers excellent precision over a range of 1% to 100% oxygen.
• They are quite delicate and sensitive to vibration and/or position.
• Due to the loss in measurement sensitivity, in general, the paramagnetic oxygen sensor is not recommended for trace oxygen measurements.

POLAROGRAPHIC OXYGEN SENSORS

• Often referred to as a Clark Cell [J. L. Clark (1822- 1898)].
• This sensor, both the anode (typically silver) and cathode (typically gold) are immersed in an aqueous electrolyte of potassium chloride.
• The electrodes are separated from the sample by a semi-permeable membrane that provides the mechanism to diffuse oxygen into the sensor.
• The current output generated from the sensor is measured and amplified electronically to provide a percent oxygen measurement.
• An advantage of the polarographic oxygen sensor is that while inoperative, there is no consumption of the electrode (anode).
• Storage times are almost indefinite. Similar to the galvanic oxygen sensor, they are not position sensitive.
• One major advantage of this sensor type is its ability to measure parts per billion levels of oxygen. 
• The sensors are position sensitive and replacement costs are quite expensive, in some cases, paralleling that of an entire analyzer of another sensor type.
• Not recommended for applications where oxygen concentrations exceed 25%.

ZIRCONIUM OXIDE OXYGEN SENSORS

• This sensor is referred to as the “high temperature” electrochemical sensor and is based on the Nernst principle [W. H. Nernst (1864-1941)].
• Zirconium oxide sensors use a solid-state electrolyte typically fabricated from zirconium oxide stabilized with yttrium oxide. The zirconium oxide probe is plated on opposing sides with platinum which serves as the sensor electrodes.
• The zirconium oxide oxygen sensor has excellent response time characteristics.
• The same sensor can be used to measure 100% oxygen, as well as parts per billion concentrations.
• Due to the high temperatures of operation, the life of the sensor can be shortened by on/off operation.
• A major limitation is their unsuitability for trace oxygen measurements when reducing gases (hydrocarbons of any species, hydrogen, and carbon monoxide) are present in the sample gas. At operating temperatures of 650 degrees Centigrade, the reducing gases will react with the oxygen, consuming it prior to measurement thus producing a lower than actual oxygen reading.
• The magnitude of the error is proportional to the concentration of reducing gas.  
• Zirconium oxide oxygen sensors are the “defacto standard” for in-situ combustion control applications.

The cost of an analyzer depends on the use application and the type of oxygen sensor required. Oxygen analyzers used for industrial process monitoring applications start at $1,625 with Alpha Omega Instruments (AOI), which produces the highest quality analyzers manufactured in the United States. AOI provides a minimum, two-year electronics warranty and one-year oxygen sensor warranty on analyzers. 

Unlike other manufacturers, we offer several, semi-custom configurations, which eliminates wasteful spending on unneeded “bells and whistles”. We also offer a base analyzer option, which allows you to provide your own sample conditioning.

The cost of oxygen deficiency monitors, used for continuous safety monitoring applications, start at $2,243. AOI’s oxygen deficiency monitors are manufactured in the United States and offer three-year electronics and oxygen sensor warranties.