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Miniature Probes

General Information

Potentiometric oxygen probes developed for the exhaust gas measurements in motor vehicles (λ-probes) are frequently used in industrial equipment. Due to their construction, these probes have some disadvantages: because of the uncontrolled heating (steady working voltage) and ceramic coating of the electrodes (which increase the diffusion of special gas components) significant measuring errors can result.

The ZIROX miniature probes MS27 and SS27 with controlled heater and integrated primary electronics features high accuracy at comparatively lower costs. The main part is the approved ZIROX potentiometric zirconia-based solid electrolyte cell (drift-, calibration- and maintenance-free). The use of high-quality components and materials guarantees high long-term stability.

The SS27 was developed for trace measurements under oxidizing conditions. The MS27 was constructed for reducing conditions or oxygen concentration of more than 100 ppm. Both probe types only differ in the type of sensor element.  


  • Oxygen measurements in protective gases
  • Exhaust gas measurements in large-scale engines
  • Control optimization in small combustion plants
  • Furnace gas measurements in heat treatment equipment
  • Optimisation of incineration plants
  • Monitozing of industrial processes under protective or inert gas atmosphere

Technical Data Miniature Probe SS27/MS27

Installation length65 mm (incl. M18x1.5), 75 mm (DN40KF)
Diameter10 mm
Weight0.5 kg
Dimension clamp head64 x 58 x 36 mm
Protection degreeIP42, others on request
Power supply24 V DC
Power consumption17 W
Warm-upapprox. 5 min
Sensor voltage (raw signal)0...200 mV (0...300 mV on request)
Range (SS27)15 ppm...20.6 Vol.-% O2 (0...200 mV)
0.13ppm...20.6 Vol.-% (0...300 mV)
Accuracy< 5 % rel. error
Measuring gas temperaturemax. 300 °C
Probe working temperature700 °C
Surrounding conditions probe headmax. 50 °C, rel. humidity < 80 %
Offsetapprox. -2...-8 mV (compensable)
Output signal4-20 mA (0-5 V, 0-10 V on request)
Plug assignment: 
(1) Analog output+ IA
(2) Analog outputGND IA
(3) Power supplyGND VB
(4) Power supply+ 24 V VB
(5) RelayRDY

Calculation of oxygen concentration:
The oxygen partial pressure is calculated by the NERNST equation.
For normal pressure applies:

ϕ(O2) = 20.64 * e (-46.42 U/T)

  • ϕ(O2): oxygen concentration in Vol.-%
  • U: cell voltage in mV
  • T: cell temperature in K (700 °C = 973.15 K)
  • 20.64: oxygen concentration in ambient air at 50 % rel. humidity

Translation of the output signal (range 0...200mV):

Current output 4-20 mA1 mA ≙ 12,5 m cell voltage
Voltage output 0-5 V1 V ≙ 40 mV cell voltage
Voltage output 0-10 V1 V ≙ 20 mV cell voltage

Example for 0...200 mV range:
At a current of 5.5 mA ((5.5-4)mA x 12.5 mV = 18.75 mV) an oxygen concentration
ϕ(O2) = 8.43 Vol.-% results.


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