Paramagnetic Oxygen Gas Analyzer

Features and typical applications

Paramagnetic oxygen measurement - 19“ & field housing O2 process gas analyzer – CONTHOS 3 - PMD

Typical Applications

  • Fast response process gas measurement
  • Flue gas control analyzer
  • Inertization plants
  • Biogas O2 measurement
  • Air separation, O2 gas purity
  • Power plants, metallurgical, chemistry, petrochemistry

Key Features

  • Oxygen specific analysis utilizing paramagnetic sensor
  • Magnetomechanical measuring principle (dumbbell principle)
  • Temperature controlled for increased stability and performance
  • Up to 3 measuring ranges
  • Optional paramagnetic cells for corrosive gases and solvents
  • Optional intrinsically safe measuring cell for flammable gases



The CONTHOS 3 PMD state-of-the-art process gas analyzer is an analytical instrument developed for use in process industry.

Some of the outstanding technical features of LFE's 3rd generation, microprocessor-controlled gas analyzer for oxygen analysis are:

  • Temperature controlled paramagnetic sensor
  • Magnetomechanical measuring principle ("dumbbell" type)
  • High selectivity to O2
  • Fast response time: time constant < 5 sec
  • Excellent precision and outstanding performance for ranges from 0 - 1 vol.% O2 up to 0 - 100 vol.% O2
  • Optional automatic pressure compensation
  • Intuitive user-interface based on NAMUR recommendations
  • Automatic self-diagnosis
  • Optional paramagnetic cells for corrosive gases and solvents
  • Optional intrinsically safe measuring cell for flammable gases


Oxygen sensor

The basic measuring principle of the CONTHOS 3 PMD makes use of the fact that oxygen has a paramagnetic susceptibility that is significantly greater than other gases. This property causes oxygen molecules to be attracted much more strongly into an inhomogeneous magnetic field than other gases.

The paramagnetic sensor employed in the CONTHOS 3 PMD is of the so-called "dumbbell" type utilizing the magnetomechanical measuring principle. Two miniaturized, nitrogen filled gas spheres configured in a dumbbell shape are symmetrically suspended in a strong, inhomogeneous magnetic field. Any oxygen contained in the surrounding (sample) gas is drawn into the magnetic field thereby displacing the glass spheres and forcing the dumbbell to rotate outward. The resulting torque is proportional to the oxygen concentration.

A mirror mounted on the rotational axis of the dumbbell reflects a beam of light onto a pair of photocells which detect any rotational displacement. The photocells are part of a control loop which subsequently drives current through windings arranged around the dumbbell. The current through the windings generates an electromagnetic counter moment which moves the dumbbell back to its null position. The required current level is proportional to the oxygen concentration and as such is passed on to the CONTHOS' signal processing unit.

Paramagnetic O2 sensor using magnetomechanical measuring principle – "dumbbell" type

Model variations

Model variations

19“ rack housing for cabinet mounting – absolute and suppressed oxygen ranges - CONTHOS 3 - PMD
19"-rack housing
IP65 field housing - absolute and suppressed paramagnetic oxygen ranges - CONTHOS 3 - PMD
Field housing
(protective class IP65)


  • Maximum 3 switchable ranges: independently configurable; suppressed ranges as special solution on request
  • Automatic pressure compensation (from 800 to 1200 mbar absolute; extended pressure range on request)
  • Digital I/O board for remote range switching, threshold contacts, etc.
  • RS-485 interface with Modbus RTU protocol
  • Interference correction in conjunction with external, selective gas analyzers for multiple gas constituents

Technical data

Technical data

19” rack housing
Field housing

Technical specifications subject to change without notice

  19“ rack housing paramagnetic oxygen detector to analyze 0 – 1% O2 up to 0 - 100% oxygen Purgeable wall mounting housing paramagnetic oxygen detector to analyze 0 – 1% O2 up to 0 – 100% oxygen
Enclosure & electical data
Housing 3HE/ 84TE housing
for mounting in 19" cabinet
purgeable steel housing for wall mounting;
with separate compartments for the electronic components
and the analytical components
(H x W x D)
3HU / 84TE
133 x 483 x 427 mm
434 x 460 x 266 mm
Weight approx. 10 kg approx. 25 kg
Power requirements 100-240 VAC (48-62Hz; nominal voltage range: 88-253 VAC; 100 VA max. during warm-up period)
Sample gas connectors Standard: Swagelok® (SS 316) for tubing o.d 6 mm
Option: Swagelok® (SS 316) for tubing o.d. ¼"
Measuring characteristics
Measuring principle Paramagnetic sensor ("dumbbell" type)

Measured quantity
Gas interference

Oxygen concentration in gas mixtures
Note: Although the paramagnetic susceptibility of O2 is much greater than other gases, these can also exhibit lower degrees of paramagnetic susceptibility and therefore as interfering components possibly influence the accuracy of the analysis. For this reason the gas matrix should be evaluated.

Measuring ranges

Up to 3 independently configurable, switchable ranges.
Suppressed ranges as special solution on request.
Range switching is accomplished manually, automatically and/or remotely (via optional digital inputs).

  • lowest range: 0 - 1% O2
  • largest range: 0 - 100% O2
Response time T90 < 5 sec (dependent upon gas flow and analyzer configuration; integration time configurable)
Influence of gas flow between 30 – 60 l/h: < 1% of range span for a gas flow change of ±10 l/h
Detection limit 1 < 1% of span
Reproducibility 1 < 1% of span
Linearity 1 < 1% of span

Response drift 1

Zero: < 2% of span per week
Span: < 1% of span per week

Ambient temperature influence

Zero: < 1% of span per 10 K
Span: < 1% of span per 10 K

Calibration Manual: 2-point (offset/span) calibration (The optimal span gas concentrations should be chosen between 75 and 100 % of the corresponding range).
Option: automatic or remote calibration in conjunction with the optional digital I/O board or RS-485
Pressure compensation optional: from 800 to 1200 mbar absolute; extended pressure range on request
Interference correction for static and/or dynamic interference correction (dynamic correction only in conjunction with the optional analog inputs or RS-485).
One of the prerequisites for dynamic interference correction is the availability of a selective signal, proportional to the particular gas component to be corrected for. The processing of analyzer ranges with a suppressed zero range is not possible.
Materials in contact with sample gas
Paramagnetic sensor platinum, epoxy, glass, FPM, stainless steel 1.4571
Sample gas connectors standard: stainless steel (SS 316)
Sample gas lines standard: PTFE
optional: stainless steel tubing (SS 321 - similar to 1.4541) and 1.4571
Data display, inputs & outputs

User interface

LC-display (40 characters x 16 lines) + bar graph
Plain text description of instrument status as well as digital status output
Language: switchable between English & German

Analog signal output

2 independently configurable, galvanically isolated analog outputs (with common ground; RLoad = 600 Ohm max)
Available output levels: 0 - 20 mA, 4 - 20 mA, 4 - 20 mA with superimposed instrument status (NAMUR NE 43 compliant) as well as test signal levels (0, 4, 10, 12 & 20 mA)

Digital outputs
1 to 3
(instrument status)

Instrument status (NAMUR NE 107 compliant) via floating contacts (28 V max.; 350 mA max.)

Analog inputs
3 galvanically isolated, configurable analog inputs for interference correction and pressure compensation
0 – 20 mA or 4 – 20 mA (Ri = 50 Ohm)

Digital I/O

Digital inputs: 8 configurable, optically isolated inputs (6 - 24 VDC; 10 mA max.)

  • remote range selection
  • remote triggering of zero and span calibration
  • remote triggering and cancelling of automatic calibration
  • switching of interference correction analog inputs to a secondary input range
  • mapping of user defined input to a digital output

Digital outputs: 7 configurable, floating relay contacts (28 V max.; 350 mA max.)

  • threshold monitoring (1 threshold per measuring range)
  • feedback as to the current range
  • calibration gas selection
  • mapping of user defined input to a digital output

Modbus interface

  • Modbus RTU - RS485
  • Modbus TCP
Service interface non-isolated serial interface for accessing the instrument's configuration

1 at constant temperature and pressure


  • The stability data is valid for analyzer operation with pure bottled gases. Instrument accuracy is based on binary or quasi-binary gas mixtures. Deviations from the above data can occur in conjunction with process gases depending upon the gas quality and the degree of gas handling.
  • Unless otherwise specified the CONTHOS gas analyzer is neither ex-proof nor intrinsically safe in terms of explosion protection.
  • The CONTHOS may not be employed for the analysis of ignitable gas-mixtures. The customer must ensure compliance with applicable regulations when using the analyzer with inflammable or toxic gases or when installing within explosion endangered environments.
  • The customer must ensure that the sample gas is dry and free of particulates.

Go to top