The optical sensor, developed, patented, and manufactured by PST, is low-cost, low power and long-lasting oxygen sensor that measures the partial pressure of oxygen, ranging 0-3000ppO2, and temperature as well as the optional barometric pressure allowing the oxygen concentration to be calculated ranging from 0 to 25% O2, with a high accuracy of <2% fully scale of ppO2.
The sensor itself contains no lead of liquid electrolytes and therefore 100% RoHS compliant. With simple connections allowing connections directly to a microcontroller with UART communications so additional signal conditioning circuity required.
The sensor is built into a few compact options, flow through sensor body that allows push on fittings, a diffusion bases sensor with a PTFE filter membrane that allows the passive movement of O2 through as well as OEM options.
PST’s Zirconium Oxide Sensors
A dome shaped block of Zirconium dioxide (ZrO2) is coated on the inner and outer sides with a thin porous platinum layer. The Zirconium dioxide acts as a solid electrolyte and is doped with yttrium oxide which enhances the thermal and mechanical stability, and electrical characteristics. The platinum porous layer acts as an electrode, allowing oxygen ions to pass through into the ZrO2 electrolyte.
One side of the assembly is exposed to the sample gas, whereas the other side is exposed to a reference gas (typically air).
The entire assembly is heated above 600°C to maximise the ion conductivity of the ZrO2 electrolyte. This allows fast movement of oxygen ions from a higher concentration of oxygen to a lower one. The movement of oxygen ions across the zirconium oxide produces a voltage between the two electrodes, the magnitude of which is based on the oxygen partial pressure differential created by the reference and sample gas.
MSRS Sensor Cells
MSRS stands for Metallic Sealed Reference Sensor, and is a technology unique to PST’s oxygen sensors. The equilibrium state of solid metal oxide is used as a reference. This allows accurate operation irrespective of the quality of the ambient air (which is typically used as the reference gas), and negates the requirement for a “zero” calibration gas.
MIPS Sensor Cells
PST’s MIPS (Micro-Ion-Pump Sealed reference) cells are a perfect economical solution for percentage level oxygen measurement. The sealed reference cell conveniently does not require a supply of reference gas.
The cell is constructed from two Zirconium Dioxide (ZrO2) squares, each coated with a thin porous layer of platinum which serves as the electrodes. The platinum electrodes provide the catalyst necessary for the measured oxygen to dissociate, allowing the oxygen ions to be transported through the ZrO2.
The two ZrO2 squares are separated by a platinum ring which forms a hermetically sealed sensing chamber. At the outer surfaces there are two further platinum rings which, along with a centre platinum ring, provide the electrical connections to the cell.
Two outer alumina (Al2O3) discs filter and prevent any particulate matter from entering the sensor and also remove any un-burnt gases. This prevents contamination of the cell which may lead to unstable measurement readings.
A heater coil surrounds the sample cell, heating it above 600°C to maximise the ion conductivity of the Zirconium Dioxide. An outer sintered stainless steel cap filters larger particles and dust, and protects the sensor from mechanical damage.
PST’s Thermo-paramagnetic oxygen sensor offers excellent measurement stability in combination with a robust construction without moving parts.
The sample chamber has a strong central magnetic field, which attracts paramagnetic gas components, drawing the sample gas in and causing a localised increase of pressure. Two pairs of high accuracy temperature sensors measure the sample gas temperature at different points in the chamber.
A temperature gradient within the chamber creates an area of low pressure towards the edges of the magnetic field, inducing a flow of sample gas referred to as “magnetic wind”. The magnetic wind has a cooling effect on the temperature sensors, increasing the temperature difference between each pair as it passes across them.
Higher oxygen concentrations in the sample gas result in increased pressure inside the magnetic field. This in turn results in stronger magnetic wind, further increasing the temperature difference between the temperature sensors in each pair.
The oxygen concentration in the sample gas is a function of the temperature difference between the temperature sensors in each pair.