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Oxygen Deficiency Monitor for Your Application

Written by Terri Melle-Johnson on . Posted in Alpha Omega Instruments, Process Insights. No Comments on Oxygen Deficiency Monitor for Your Application

When selecting an oxygen analyzer for your critical process application, consider factors like sensor type, maintenance, and calibration needs. Different oxygen sensors are suited for specific applications, so choose one based on the requirements of your process.

Oxygen Sensor Types:

  • Ambient Temperature Electrochemical Sensors: Known for accuracy in both trace and percent oxygen measurements, these sensors have extended life but are vulnerable to damage from acid gases and over-pressurization.
  • Paramagnetic Sensors: Offering precise measurements across 1%-100% oxygen, these sensors work by detecting the magnetic properties of oxygen. However, they are delicate, sensitive to vibration, and not suitable for trace oxygen measurements.
  • Polarographic Sensors: Ideal for dissolved oxygen in liquids, these sensors are suited for percent oxygen measurements in gases. They have low maintenance requirements but are prone to frequent sensor replacements.
  • Zirconium Oxide Sensors: Operating at high temperatures (650°C), these sensors are ideal for combustion control applications. They can measure a wide range of oxygen concentrations but are unsuitable for trace oxygen measurements when reducing gases are present.

Each sensor type has its strengths and limitations, so assess your process needs carefully before choosing the right oxygen analyzer.

Compare Oxygen Deficiency Monitors

Written by Terri Melle-Johnson on . Posted in Alpha Omega Instruments, Process Insights. No Comments on Compare Oxygen Deficiency Monitors

Not all oxygen deficiency monitors are the same. Like automobiles or cell phones, they vary in quality and performance.

Many monitors use “fuel cell” oxygen sensors, which typically need replacement every 10-14 months. However, as these sensors age, their electrical output decreases, mimicking a low oxygen signal. This can lead to false alarms, which, when ignored, can create dangerous situations. Personnel may dismiss alarms as false, potentially missing real low-oxygen events. This frustration may even lead to disabling alarms, increasing risk.

Zirconium oxide-based monitors often claim to be calibration-free for over 10 years, but these claims are misleading. One major supplier of zirconium oxide sensors makes no such claims. Closer inspection of the user manuals reveals that these sensors require periodic adjustments and calibration. Experts agree that gas monitors need occasional checks to ensure accurate readings and protect personnel. The stakes are high, and cutting corners can be dangerous.

CAPABILITIES
Series 1300 Oxygen
Deficiency Monitor
Fuel Cell Oxygen Monitor
High Temperature (450 °C) Zirconium Oxide Oxygen Monitor
Three-year warranty on both the electronics and      sensor

YES

NO – Typically one year

NO – Typically two years
Accepts up to 3 oxygen sensors with one set of electronics drastically reducing the per point monitoring costs

 

YES

NO

NO
Built-In data logger standard

YES

NO

Limited Availability
Easy field replacement of the oxygen sensor

YES

YES

NO – Both sensor and mating electronics need replacement – an expensive repair
Built in alarm relay contacts

YES (4 Standard)

Some at extra charge

Often an extra charge
Can be affected by changes in ambient air now caused by HVAC / air handling systems

NO

NO

YES – Changes in airflow may sufficiently cool the high temperature sensor producing erroneous oxygen readings.
Can be used in the presence of combustible gases, refrigerant gases, other reducing gases

YES

YES

NO
Long-life oxygen sensor

YES

NO

Can fail prematurely from heat fatigue

Know the Difference Between Oxygen Monitor Sensors

Written by Terri Melle-Johnson on . Posted in Alpha Omega Instruments, Process Insights. No Comments on Know the Difference Between Oxygen Monitor Sensors

KNOW THE DIFFERENCE BETWEEN OXYGEN MONITOR SENSORS

Do you know the difference between oxygen monitor sensors?  Oxygen analyzers use one of a several types of oxygen sensors.   As industrial process applications call for improved measurement accuracy and repeatability, users are demanding analyzers that require a minimum of maintenance and calibration.   There is no one universal oxygen sensor type.

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.
  • Response times have also been improved.
  • 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 is 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.

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