Non-Depleting Oxygen Sensor Technology Versus Electrochemical Oxygen Sensing Technology

In this interview, AZoSensors talks to Andrew Cummings of SST Sensing about the differences, and respective benefits, between electrochemical oxygen sensing technology and non-depleting oxygen sensor technology.

Can you give a brief overview of SST Sensing and the work you do?

At SST Sensing Ltd. we design to manufacture, customize and configure sensors and switches for gases and liquids, specifically for oxygen monitoring and liquid level detection.

Established in 2002 and based in Scotland, UK, we produce over 29,000 sensors per month at our 1,400m² facility and have an impressive 98.5% on-time delivery. We export globally with strong supply into Europe, China, United States, South America, and Japan. As an ISO9001 accredited company, we produce products that are designed and manufactured to exacting standards, with some of our sensors approved for use in aerospace applications.

Our engineering team has a real passion to continually advance their expert knowledge and innovate our technologies into new fields. As a result, SST offers TWO different oxygen sensor technologies to cover a wide customer base.  We also have the widest range of single point liquid detection sensors on the market.

The primary aim has always been to solve the customers’ sensing problem. We pride ourselves on being highly focused and responsive to customer requests. Working closely with customers, established and new, allows us to understand how our oxygen sensors fit into a wide variety of niche markets and applications. This can only be done by offering high-quality sensor solutions and high levels of technical support.

Our sensors are used in many diverse applications within industries such as medical, industrial, domestic, transportation, marine, aerospace, and telecommunications.

Could you please explain the basic theory of how the two Oxygen Sensor technologies work?

Both technologies employ non-depleting sensing principles that measure the partial pressure of oxygen (ppO₂):

Zirconium Dioxide Sensors (up to 100% O₂)

The sensor employs a well-proven, small zirconium dioxide (ZrO₂) based element at its heart and due to its innovative design does NOT require a reference gas. This removes limitations in the environments in which the sensor can be operated with high temperatures, humidity, and oxygen pressures all possible.

At the core of the sensor is a cell that consists of two ZrO₂ squares that are constructed to form a hermetically sealed sensing chamber. Heated to > 650oC, stabilized ZrO2 exhibits two mechanisms.

• ZrO₂ partly dissociates and becomes a solid electrolyte for oxygen. The first ZrO2 square has a constant DC current source that allows ambient oxygen ions to be transported through the material. Alternating the direction of the current allows for evacuating or re-pressurizing the sealed chamber.
• ZrO₂ behaves like an electrolyte. If two different oxygen pressures exist on either side of a piece of ZrO₂, a voltage (Nernst voltage) is generated across it. The second ZrO2 square measures the Nernst voltage as the sealed chamber pressure varies.

The result generates a time-based waveform. This is measured and interpreted allowing the partial oxygen pressure around the sensor to be inferred. This unique closed-loop measurement system provides a diagnostic function. Generating a “heartbeat” signal and confirming the healthy operation of the sensor.

Due to their rugged stainless-steel construction, these oxygen sensors are used in sometimes harsh and extreme environments to monitor boiler efficiency, controlled environments, in aviation, and other high and low oxygen measurements.

The ZrO₂ sensors can cross into some electrochemical applications, offering longer life and greater stability.  However, the limitations for using ZrO₂ sensors lies in their high operating temperatures and power consumption.

The LuminOx is the main competitor of electrochemical sensors.

LuminOx, optical oxygen sensor (0-25% O₂)

The LuminOx series is based on a proprietary luminescence quenching technology.

They employ the properties of a specific dye which changes its luminescence behavior depending upon the amount of oxygen around it. A built-in LED excites this dye, causing it to produce light (luminescence) and the O₂ in the environment “quenches” this excitation. The rate of decay of luminescence, which is inversely proportional to the oxygen level, is interpreted and linearised by the sensor’s firmware and so allows the ppO2 to be calculated.

The LuminOx sensor has built in temperature and barometric pressure compensation. A barometric pressure sensor measurement along with the ppO₂ measurement allows an oxygen concentration (O₂%) value to be calculated. LuminOx is a digital sensor with a USART output, meaning there is no need for signal conditioning circuitry. The sensor can serially communicate the values of O2%, ppO2, barometric pressure, internal temperature, and sensor status.

How does this differ from Electrochemical Oxygen Sensors?

Electrochemical sensors are completely different. This type of device comprises a self-powered, galvanic cell that operates as a metal/air battery. The cell constantly depletes in the presence of oxygen and the lifetime can be restricted by components like the liquid electrolyte and a consumable lead anode. As it is a chemical reaction using an oxidization process, some of the oxygen being measured is actually consumed.

Electrochemical sensors are more restricted in regards to environmental sensitivities and applying them can sometimes require a bit of specialist knowledge.

What are the advantages of a non-depleting Oxygen Sensor?

The LuminOx sensor is typically positioned against electrochemical sensors. Both technologies frequently overlap in suitability to the environment of the application, but LuminOx is able to offer a number of advantages over what has been available in the oxygen sensor market.

Lifetime. LuminOx is a non-depleting sensor with a 5-year life.

Electrochemical sensors typically start to deplete as soon as they are exposed to any O₂, whether a measurement is being taken or not.  This depletion is sensitive to several environmental factors such as temperature, pressure, and other gases present in the atmosphere, making total sensor lifetime difficult to predict.

Minimal cross-sensitivity. The formulation of LuminOx is highly selective to oxygen.

Electrochemical sensors are cross-sensitive to numerous gases.  This could result in significant positive or negative measurement errors. This limits the environment and ultimately the number of markets such a product can be sold into.

No external signal conditioning circuitry required. LuminOx is a digital sensor that offers multiple output parameters.

Electrochemical sensors require an amplification stage to obtain a usable signal that is then typically processed by a microprocessor. LuminOx just connects to the micro via two UART pins.

Every LuminOx is factory calibrated across the operating temperature and oxygen pressure ranges.

The electrochemical sensor typically needs the user to manage temperature and pressure compensation.

LuminOx is REACH and RoHS compliant

To this day, exemptions in the RoHS legislation allow electrochemical sensors to contain harmful materials like lead (Pb).  This situation cannot continue forever and these sensors will have to be outlawed eventually. LuminOx was designed to be RoHS/REACH from the start.

What are the reasons a customer may replace their Electrochemical Sensor with an SST O₂ sensor?

Many of the customers approaching SST for LuminOx are already familiar with oxygen sensors. Often the customer will have been limited to using an electrochemical cell in the past. A lack of options in technology has dictated their design, how the product has to be used, and the markets available to them.

Primarily, these customers are interested in improving their sensor lifetime. Customer comments indicate a typical figure of approx. 18-24 months (if not less) before needing to replace an electrochemical sensor.

Cross sensitivity to other gases such as helium or background carbon dioxide has proven problematic when out in the field and has attracted customers to SST.

Customers designing new products take advantage of its easy to use nature. LuminOx removes some of the traditional design considerations and correction factors needed with electrochemical sensors.

What makes the LuminOx Optical Oxygen Sensor unique?

There is no other similarly packaged luminescence sensor on the market. The sensor’s firmware, factory calibration, integrated pressure, and temperature compensation ease the usual depth of knowledge sometimes required in designs.

The LuminOx output is a 3.3V UART that can stream several parameters (O₂%, ppO₂, temperature, barometric pressure, and sensor status). The sensor can communicate directly to a microcontroller making for a quicker and easier design.

SST luminescence quenching sensors have minimal cross-sensitivity to other gases, are RoHS compliant, comparatively low powered and competitively priced.

LOX-EVB LuminOx Evaluation Interface Board

What industries will benefit from the LuminOx Oxygen Sensor?

Numerous industries will benefit. The integration of LuminOx can give a competitive edge to a customer’s product and in turn, open up to new markets and benefits in the management of incumbent sensor replacement. Where traditional electrochemical sensors have struggled; LuminOx now offers new advantages.

Examples of the industry LuminOx is already applied in are oxygen monitoring; controlled atmosphere control for perishable goods; leak detection in inert environments; alerting in area monitoring for depletion or O₂ and in O₂-rich environments; bioscience cell culture; oxidization prevention in additive manufacturing, to name a few.

What’s next for SST Sensing?

SST is experiencing significant and consistent growth, especially in both of its oxygen sensor technologies. Both types are greatly succeeding in the transportation of perishable goods and controlled atmosphere markets.

Customer demand has driven a great amount of development into the LuminOx technology. One focus has been for a part per million (ppm) range sensor. Other ppm technologies available have some severe limitations that LuminOx will significantly improve upon. SST will offer a long-awaited solution in applications such as nitrogen and other gas generation, inert storage and oxidization prevention, glovebox purge and leak detection, modified atmosphere packaging lines, centrifuges, and inert ovens.

In answer to customer requests, two new housing variations of the current LOX-0₂ sensor (0-25% O2) have been launched. A sealed base diffusion type and a flow-through housing take a step further to making the mechanical design and gas management easier for our customers.

A global sales network will further our ability to supply and support customers locally. The next step for SST is to expand our global sales coverage with existing and new partners who have the expertise required to serve the gas sensing market. Places such as the Americas territory has a large potential with our products.

SST will continue to offer customers a high level of technical support and customization service that we are well known for. 

About Andrew Cummings

Andrew has worked at SST Sensing for 3 years in the role of Sales Engineer. He is SST’s primary contact to customer technical queries and support requirements. Based in Glasgow, Scotland, he originally studied Electronic Engineering BSc(Hons) where his career began in the semiconductor industry. He has over a decade’s experience that has focused on the customer and technical support.