Can you give a brief overview of SST Sensing and the work you do?
At SST Sensing Ltd. we design, manufacture, customise 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 which are designed and manufactured to exacting standards, with some of our sensors approved for use in aerospace applications.
Our engineering team have 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):
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 which consists of two ZrO2 squares that are constructed to form a hermetically sealed sensing chamber. Heated to > 650oC, stabilised 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-pressurising 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 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 a 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 to electrochemical sensors. The LuminOx series is based on a proprietary luminescence quenching technology.
They employ the properties of a specific dye which changes its luminescence behaviour 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 like 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 oxidisation 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.