In this series, we've compared the pros and cons of the various combustible gas sensors and toxic sensors available in today's market. This week, I'll wrap up the series by sharing my findings on oxygen sensors.

The 5 Types of Oxygen Sensors

There are five common types of oxygen sensors. Let's look closely at the merits of each.

1. Ambient Temperature Electrochemical Oxygen Sensors

Ambient temperature electrochemical oxygen sensors, often referred to as galvanic sensors, operate much like a battery. Oxygen gas flows past an electrode and becomes a negatively charged hydroxyl ion. The ions move through electrolytes in the oxygen sensors to positively charged electrodes, typically made of lead, react with the lead and releases electrons. The electron flow is measured and the measurement can be mathematically converted to an oxygen concentration.

Pros

• The only true chemically specific sensor (similar to the electrochemical toxic sensor described last week).

Cons

• Susceptible to freezing
• Affected by altitude
• Nominal operational life of one year.
• Susceptible to damage when used with samples containing acid gas species such as hydrogen sulfide, hydrogen chloride, sulfur dioxide, etc. Unless the offending gas constituent is scrubbed prior to analysis, their presence will greatly shorten the life of the sensor.

2. Paramagnetic Oxygen Sensors

Oxygen has a relatively high magnetic susceptibility as compared to other gases such as nitrogen, helium and argon; and it displays a paramagnetic behavior. The paramagnetic oxygen sensor is based on these qualities. It typically consists of a cylindrical shaped container in which a small glass dumbbell is placed. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field. The dumbbell is designed to move freely as it is suspended from the wire. When oxygen is processed through the sensor, the oxygen molecules are attracted to the stronger of the two magnetic fields created by each side of the dumbbell. This causes a displacement of the dumbbell and causes it to rotate. A precision optical system consisting of a light source, photodiode and amplifier circuit is used to measure the degree of rotation of the dumbbell.

In some sensor designs, an opposing current is applied to restore the dumbbell to its normal position. The current required to maintain the dumbbell in its normal state is directly proportional to the partial pressure of oxygen and is represented electronically in percent oxygen.

The magnetodynamic or dumbbell type of design is the predominate sensor type of paramagnetic oxygen sensors. But design variations are available, depending on the manufacturer. Also, other types of sensors have been developed that use the susceptibility of oxygen to a magnetic field, which include the thermomagnetic or "magnetic wind" type and the magnetopneumatic sensor.

Pros

• Offer very good response time characteristics
• Use no consumable parts, making sensor life (under normal conditions) quite good
• Offer excellent precision over a range of 1% to 100% oxygen

Cons

• Quite delicate and sensitive to vibration and position
• Not recommended for trace oxygen measurements in general, due to the loss in measurement sensitivity
• Other gases that exhibit a magnetic susceptibility can produce sizeable measurement errors. Manufactures of paramagnetic oxygen sensors and analyzers should provide details on these interfering gases.

3. Polarographic Oxygen Sensors

Polarographic oxygen sensors are often referred to as a Clark Cell. In this type of 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 diffuses oxygen into the sensor. The silver anode is typically held at a potential of 0.8V (polarizing voltage) with respect to the gold cathode. Molecular oxygen is consumed electrochemically with an accompanying flow of electrical current directly proportional to the oxygen concentration (based on Faraday's law). The current output generated from the sensor is measured and amplified electronically to provide a percent oxygen measurement.

Pros

• While inoperative, there is no consumption of the electrode (anode)
• Almost indefinite storage times
• Not position sensitive
• The sensor of choice for dissolved oxygen measurements in liquids

Cons

• Relatively high frequency of sensor replacement
• Sensor membrane and electrolyte require maintenance
• For gas phase oxygen measurements, the sensor is suitable for percent level oxygen measurements only

4. Non-Depleting Coulometric Oxygen Sensors

Non-depleting coulometric sensors are a variant of the polarographic oxygen sensor in which two similar electrodes are immersed in an electrolyte consisting of potassium hydroxide. Typically, an external EMF of 1.3 VDC is applied across both electrodes which act as the driving mechanism for reduction/oxidation reaction. The electrical current resulting from this reaction is directly proportional to the oxygen concentration in the sample gas. As with other sensor types, the signal derived from the sensor is amplified and conditioned prior to displaying.

Pros

• Can be used for both percent and trace oxygen measurements
• Can measure parts per billion levels of oxygen

Cons

• One sensor cannot be used to measure both high percentage levels as well as trace concentrations of oxygen
• 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%.

5. Zirconium Oxide Oxygen Sensors

Zirconium oxide oxygen sensors are occasionally referred to as the "high temperature" electrochemical sensor and are based on the Nernst principle.

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.

For a zirconium oxide sensor to operate properly, it must be heated to approximately 650° C. At this temperature, on a molecular basis, the zirconium lattice becomes porous, allowing the movement of oxygen ions from a higher concentration of oxygen to a lower one, based on the partial pressure of oxygen. To create this partial pressure differential, one electrode is usually exposed to air (20.9% oxygen) while the other electrode is exposed to the sample gas. The movement of oxygen ions across the zirconium oxide produces a voltage between the two electrodes, the magnitude of which is based on the oxygen partial pressure differential created by the reference gas and sample gas.

Pros

• Can be utilized in high temperature environments
• Excellent response time characteristics
• Can be used to measure 100% oxygen, as well as parts per billion concentrations
• The "de facto standard" for in-situ combustion control applications

Cons

• Due to the high temperatures (typically) of operation, the life of the sensor can be shortened by on/off operation
• Constant heating and cooling often causes "sensor fatigue"
• Unsuitable 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°C, 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.

Conclusion

Selecting the right gas detector and attendant sensor is a crucial safety decision. I hope this and the previous installments help you make the right decision for your workplace and workforce.

EDITOR'S NOTE
Happy Makeover Week

To celebrate SafetyXChange's new look, all this week we will publish an interesting - at least, we think so - safety-related fact about cosmetics and cosmetic surgery. Thank goodness it's a short week!

Catherine Jones
Editor
SafetyXChange

BY THE NUMBERS
Cosmetic Surgery

By Catherine Jones

Okay, we admit it. Part of the reason for the SafetyXChange facelift is vanity. We want to look good for you. But we're not alone. In fact, last year the total number of surgical and non-surgical cosmetic procedures performed in the US was 11.5 million.

Here are some other cosmetic surgery statistics:

• 444% increase in cosmetic procedures since 1997
• 455,489 liposuction procedures were performed last year (down 5% from the previous year)
• 91.4% of total cosmetic procedures were performed on women
• 35- to 50-year-olds had the majority of cosmetic procedures
• $12.4 billion was spent on cosmetic procedures by Americans in 2005

(Source: American Society for Aesthetic Plastic Surgery)

THE FACE OF SAFETY
8 Questions to Ask Before You Get a Facelift

By Catherine Jones

We didn't take our make-over lightly. And neither should anyone considering cosmetic surgery. The Mayo Clinic says there are several questions you should ask before having a facelift or other form of cosmetic surgery. Here are eight of those questions and how we at SafetyXChange answered them:

1. Is there any other treatment that might work just as well?
   SafetyXChange Answer: No. Diet and exercise wouldn't have worked for us. We needed to call in the surgeon, that is, the web designers.

1. Am I a good candidate for this procedure?
   SafetyXChange Answer: Yes. SafetyXChange was in good overall health and had benefited from a balanced diet, in our case of nutritional content.

1. How many times has the surgeon performed similar procedures and what were the results?
   SafetyXChange Answer: Our web design team was made up of seasoned professionals. Their stuff rocks and we felt safe in their hands.

1. Can the desired effect be accomplished in one procedure, or are multiple procedures anticipated?
   SafetyXChange Answer: SafetyXChange will never shy away from making changes necessary not only to look good but to serve our members' needs better. Over time, we will rely on you to tell us what changes you desire.

1. What results can we expect?
   SafetyXChange Answer: SafetyXChange members can expect more interaction with colleagues, easier navigation, deeper insights, fresher ideas and greater value.

1. What are the possible complications?
   SafetyXChange Answer: We don't want the new look to cause us to become vain. But rest assured that we are not about to give up safety for a career in fashion and glamour.

1. How will our progress be monitored after the procedure? Will there be follow-up care?
   SafetyXChange Answer: We're going to survey our members and help them where needed.

1. Will touch-up surgery be needed?
   SafetyXChange Answer: We expect SafetyXChange members will answer that one, too.

ON THIS DATE IN HISTORY
September 5, 1997

"Performed miracles and waited for no one"

By Catherine Jones

I think it's safe to say that the 20th century will not be remembered for its saints. But, thankfully, there were a few. One of them died nine years ago on this date: Mother Teresa (born Agnes Gonxhe Bojaxhiu) in Calcutta on September 5, 1997.

A quote attributed to Mother Teresa, while likely in reference to her charitable work, is also appropriate for SafetyXChange members:

"Do not wait for leaders. Do it alone, person to person."

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