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Coulometric Sensors: FAQ and Expert Insights

Friday, July 16, 2021

What is a Coulometric Sensor?

A Coulometric Sensor is a fuel cell that performs in accordance with Faraday’s Law. When oxygen enters the coulometric sensor, the oxygen molecules release four free electrons. This reaction is measured by the sensor, the magnitude of which is proportional to the amount of oxygen flowing into the sensor per unit of time.

What are Coulometric Sensors used for?

Coulometric Sensors are commonly used in Oxygen Permeation Testing (OTR) analyzers in order to meet the requirements for ASTM D985 and ASTM F2622.

What is ASTM D3985 and how does it relate to Coulometric Sensors?

ASTM D3985 is described by ASTM International as the “Standard Test Method for Oxygen Gas Transmission Through Plastic Film and Sheeting Using a Coulometric Sensor”. Per ASTM’s description, “[this] test method covers a procedure for determination of the steady-state rate of transmission of oxygen gas through plastics in the form of film, sheeting, laminates, coextrusions, or plastic-coated papers or fabrics. It provides for the determination of:

  1. Oxygen gas transmission rate (OTR)
  2. The permeance of the film to oxygen gas (PO2)
  3. Oxygen permeability coefficient (P′O2) in the case of homogeneous materials

Is calibration required for Coulometric Sensors?

Most Coulometric Sensors require calibration, but MOCON’s patented COULOX® Sensor does not. Every oxygen molecule that enters a COULOX® Sensor is analyzed, allowing for OTR measurement that is 95%-98% efficient. Absolute Coulometric Sensors such as the COULOX® used in most MOCON oxygen permeation systems are not affected by flow, pressure or temperature changes.

How is the MOCON COULOX® Sensor different from other Coulometric Sensors?

MOCON’s exclusive COULOX® Sensors analyze 100% of carrier gas, are intrinsic, and set the benchmark for the ASTM D3985 standard because they are not affected by temperature, flow or barometric pressure. The COULOX® Sensor requires no calibration with the advantage of the longest sensor life when compared to other Coulometric Sensors.

Coulox Coulometric Sensor (top) vs. Generic Electrochemical Coulometric Sensor (bottom)

MOCON Coulox Coulometric Sensor


Generic Electrochemical Coulometric Sensor


More than Just Coulometric

At the heart of AMETEK MOCON’s OX-TRAN line of OTR Permeation Analyzers is our patented COULOX® Sensor. More than just Coulometric, the COULOX® offers proven accuracy and reliability while providing absolute OTR detection without the need for calibration.

For a deeper dive on the COULOX®, check out our recent Technical Note which follows an R&D study on the accuracy of test results with both legacy and Next Generation instruments using the sensor.

Interested in learning more about AMETEK MOCON OTR Analyzers utilizing COULOX® Sensors? Watch the video below on the OX-TRAN 2/40 which uses the COULOX Sensor® for high accuracy package testing.

OX-TRAN 2/40 OTR Permeation Analyzer for Testing of Packages & Films - YouTube