Oxygen is one of the very few common gases that is strongly attracted into a magnetic field. Paramagnetic analyzers turn that unusual property into a fast, selective and non consumptive oxygen measurement for combustion, purity and safety duties.
A paramagnetic oxygen analyzer measures oxygen by the force it feels in a magnetic field, without consuming the gas. This guide explains the three sensor types, sample handling, calibration and how it compares with zirconia.
What Is a Paramagnetic Oxygen Analyzer?
A paramagnetic oxygen analyzer is an extractive gas analyzer that measures oxygen concentration by detecting how strongly the sample gas is pulled into a non uniform magnetic field. Because oxygen is strongly paramagnetic and most other gases are not, the reading is highly selective for oxygen.
It is one of several oxygen technologies used in plants, alongside the in situ zirconia oxygen analyzer and electrochemical cells. For an overview of where it fits among other methods, see gas analyzers explained.
Oxygen molecules have two unpaired electrons, which give each molecule a small permanent magnetic moment. In a magnetic field these moments align and the gas is drawn toward the strongest part of the field.
Yokogawa lists the relative magnetic susceptibility of oxygen as 100, while most common flue gas components are close to zero. That huge contrast is why the measurement needs no chemical reaction at all.
How Magnetic Force Reveals Oxygen
HORIBA explains the dumbbell principle with a simple force equation. The force on the test body depends on the difference between the susceptibility of the sample gas and that of the body, its volume and the magnetic field.
F = force on the dumbbell
X₁ = magnetic susceptibility of the sample gas
X₂ = magnetic susceptibility of the dumbbell
V = dumbbell volume
H = magnetic field strength (HORIBA notation)
Since X₁ rises with oxygen partial pressure, F is proportional to O₂ %
Susceptibility of oxygen falls as temperature rises, following the Curie law, so the measuring cell is held at a constant temperature. Yokogawa, for example, keeps its MG8 sensor housing at a constant 55 °C.
Susceptibility also rises with gas pressure, because more oxygen molecules sit in the same volume. For this reason every paramagnetic oxygen analyzer reads partial pressure and needs pressure compensation for accurate percent readings.
3 Types of Paramagnetic Oxygen Analyzer
Nitrogen filled glass spheres on a torsion suspension rotate as oxygen pushes them out of the field.
Heated oxygen loses susceptibility, creating a gas flow that cools a hot wire bridge.
A pulsed field creates a pressure difference sensed by a microflow sensor on a reference gas line.
All three types rely on the same physics but detect it in different ways. The choice depends on accuracy, sample cleanliness, background gas and budget.
Dumbbell Type Paramagnetic Oxygen Analyzer
In the dumbbell cell, two small glass spheres filled with nitrogen hang on a thin fibre between shaped magnet poles. HORIBA describes a cell volume of about 2 cm³ with a mirror on the dumbbell and a light source aimed at two photocells.
When oxygen enters, it crowds into the strong field and pushes the spheres aside, rotating the dumbbell. The photocells sense the rotation and a feedback coil drives a current that pulls the dumbbell back to its null position.
The feedback current needed to hold the null is proportional to oxygen concentration. This null balance design gives excellent linearity and a stable zero, which suits both trace ranges and high purity oxygen measurement.
Never apply compressed air or a sudden pressure pulse to a dumbbell cell. The suspension is delicate, and a shock can shift the zero or break the fibre.
Thermomagnetic and Magneto Pneumatic Types
In the thermomagnetic cell, a heated filament sits in the magnetic field. Oxygen is drawn in, warms up, loses susceptibility and is displaced by cooler oxygen, creating a steady magnetic wind that cools the filament.
The cooling changes filament resistance and unbalances a bridge. Because the effect also depends on the thermal conductivity of the background gas, this type needs correction when hydrogen, carbon dioxide or hydrocarbons vary.
The magneto pneumatic type keeps the sample away from the detector. Yokogawa describes an auxiliary nitrogen flow of 35 to 55 ml per minute, where the pressure difference created by oxygen in a pulsed magnetic field is sensed on the nitrogen side.
Yokogawa states that the MG8 reaches 90 percent response within 3 seconds at the sensor and within 6 seconds for the system. Fast response makes the analyzer useful for continuous emissions monitoring and combustion trim.
Comparing the Three Detector Styles
| Feature | Dumbbell | Thermomagnetic | Magneto Pneumatic |
|---|---|---|---|
| Detection | Torque on glass body | Hot wire cooling | Pressure on reference gas |
| Sample touches detector | Yes | Yes | No |
| Background gas effect | Very small | Thermal conductivity effect | Very small |
| Shock sensitivity | High | Low | Low |
| Typical use | Purity, trace, lab | Boilers, simple combustion | Corrosive flue, process gas |
Excess Air Calculator From Dry Oxygen
In combustion control, the oxygen reading is used to estimate excess air. On a dry basis, excess air equals the oxygen percentage divided by 20.9 minus that percentage, multiplied by 100.
Air ratio λ = 20.9 ÷ (20.9 minus O₂)
Example:
Dry O₂ = 3.0 %
20.9 minus 3.0 = 17.9
Excess air = 3.0 ÷ 17.9 × 100 = 16.76 %
λ = 20.9 ÷ 17.9 = 1.168
This formula assumes complete combustion and a dry sample, which is exactly what an extractive paramagnetic oxygen analyzer delivers after the sample cooler. A zirconia probe reads on a wet basis, so its value is slightly lower for the same flue gas.
Second Worked Example: Barometric Pressure Correction
Suppose an analyzer was calibrated at 1013 hPa and now vents to a room at 990 hPa during monsoon weather, while it shows 4.20 percent. Since the reading follows partial pressure, the corrected value is 4.20 × 1013 ÷ 990 = 4.30 percent.
Yokogawa reports a measured atmospheric pressure effect of about 0.12 percent of span per 10 hPa on the MG8 with compensation. Without compensation, the error in the example above would be a full 0.1 percent oxygen, which matters for tight measurement uncertainty budgets.
Yokogawa notes that nitric oxide is also paramagnetic. In typical exhaust gas it is present at only about 100 ppm, so its effect on the oxygen reading can be ignored.
Paramagnetic Oxygen Analyzer vs Zirconia Probe
A zirconia probe works in situ at high temperature and responds to the net oxygen after any combustibles burn on its hot electrodes. A paramagnetic oxygen analyzer works cool and extractive, so it reads the true oxygen even in flammable gas mixtures.
Yokogawa highlights this ability to measure oxygen in flammable gas, which a zirconia analyzer cannot do. For flue gas trim, zirconia is usually cheaper and faster to install, while paramagnetic wins for process gas, purity and inerting duties that need dry basis readings.
- Highly selective for oxygen.
- Non consumptive, no cell to replace.
- Linear from trace to 100 percent oxygen.
- Works in flammable or inert process gas.
- Fast response of a few seconds.
- Needs an extractive sample system.
- Reads partial pressure, so needs compensation.
- Dumbbell cells dislike shock and vibration.
- Condensate and dust can damage the cell.
- Higher cost than a simple zirconia probe.
Sample System and Installation
The sample system is where most problems begin, so design it as carefully as the analyzer. Where the gas is flammable, choose a suitable certified housing, as explained in explosion proof vs intrinsically safe instruments, and follow the hazardous area zone drawing.
Yokogawa offers its MG8E version with Ex d IIB T4X certification, so check the gas group before specifying any unit for hydrogen service. Analyzer houses are often pressurised with instrument air, following the ideas in nitrogen purging.
Keep the analyzer vent line short, open and free of back pressure. A long or blocked vent raises cell pressure and makes the paramagnetic oxygen analyzer read high.
Calibration and Commissioning Checklist
- Confirm sample flow is within the maker range and steady.
- Check the cell temperature has stabilised before calibrating.
- Zero with pure nitrogen of known low oxygen content.
- Span with instrument air at 20.9 percent or a certified gas.
- Verify barometric compensation is active and correct.
- Leak test the sample line, since air ingress reads high.
- Record as found and as left values in the calibration sheet.
General procedures for zero and span gases are covered in oxygen analyzer calibration. Always calibrate at the same flow and vent condition used in normal operation, or the pressure effect will offset the result.
Where the Paramagnetic Oxygen Analyzer Is Used
In emissions work, the oxygen value is used to correct other pollutant readings to a reference oxygen level, often alongside an NDIR gas analyzer for CO and CO2. Complex hydrocarbon streams may need a gas chromatograph as well.
Do not confuse gas phase oxygen analysis with dissolved oxygen in water, which uses different sensors described in optical dissolved oxygen sensors. Oxygen deficiency alarms for personnel are part of the fire and gas system, not the process analyzer.
Yokogawa MG8 Technical Information
Dumbbell Analyzer Working Video
The video explains the dumbbell cell step by step in Hindi. Watch it alongside the HORIBA force equation above to connect the physics with the hardware.
Paramagnetic Oxygen Analyzer FAQ
It is an extractive analyzer that measures oxygen from the force oxygen feels in a magnetic field. Oxygen is strongly paramagnetic while most other gases are not.
The measurement is selective, fast and does not consume the sample. It is widely used for combustion control, oxygen purity checks and safe inerting in refineries, steel plants and chemical plants.
Each oxygen molecule has two unpaired electrons that give it a permanent magnetic moment. In a magnetic field these moments align, and the gas is drawn toward the strongest region.
Most gases such as nitrogen and carbon dioxide are weakly diamagnetic instead. Yokogawa rates oxygen at 100 on a relative scale where these gases are near zero.
Two nitrogen filled glass spheres hang on a fine fibre between magnet poles. Oxygen pushes them out of the strong field, and the dumbbell rotates slightly.
A light, mirror and photocells detect the rotation, and a feedback coil pulls it back to null. The feedback current is proportional to the oxygen concentration in the sample.
A zirconia probe works hot and in situ on a wet basis and burns any combustibles at its electrodes. It therefore reports net oxygen after that burning.
A paramagnetic unit is extractive and reads dry basis oxygen at a cool cell. It can also measure oxygen in flammable gas mixtures where zirconia cannot be used.
The analyzer responds to oxygen partial pressure, which rises when total pressure rises. A change in barometric pressure or vent back pressure therefore changes the displayed oxygen value.
Most modern units include an internal barometric pressure sensor and compensation to correct this effect. Keep the vent short and free, and calibrate at the normal operating flow and vent condition.
Zero it with high purity nitrogen and span it with dry instrument air or a certified gas. Allow the cell temperature and sample flow to stabilise fully before any adjustment.
Check that barometric compensation is working and that the sample line has no air leaks. Record the as found and as left values for every calibration in the maintenance system.
Nitric oxide and nitrogen dioxide have measurable magnetic susceptibility. In normal boiler and furnace flue gas their concentration is so low that the effect is usually negligible.
Thermomagnetic types are also affected by the thermal conductivity of the background gas. Dumbbell and magneto pneumatic designs of paramagnetic oxygen analyzer are much less sensitive to that effect.
Related Articles
- Zirconia Oxygen Analyzer Working Principle
- Oxygen Analyzer Calibration
- Gas Analyzers Explained
- CEMS Continuous Emissions Monitoring System
- NDIR Gas Analyzer Working Principle
External References
- MG8G and MG8E Paramagnetic Oxygen Analyzers, Yokogawa
- Paramagnetic Method, Dumb Bell Type, HORIBA
- Paramagnetism, Wikipedia
What We Learn Today
- A paramagnetic oxygen analyzer measures the magnetic force on oxygen molecules, giving a selective, fast and non consumptive reading from trace levels up to pure oxygen.
- The three detector styles are dumbbell, thermomagnetic and magneto pneumatic, and each suits a different sample cleanliness and accuracy need in the plant.
- Readings follow partial pressure on a dry basis, so barometric compensation, a clean vent and a good sample cooler are essential for accurate results.