OXYGEN SENSORS

GENERAL OVERVIEW

All electrochemical oxygen sensors are of the self-powered, diffusion limited, metal-air battery type comprising an anode, electrolyte and an air cathode as shown.

An oxygen cell can simply be considered as an enclosure (either a metal can or a plastic moulding) which holds two electrodes: a flat PTFE tape coated with an active catalyst, the cathode and a block of lead metal, the anode. This enclosure is airtight apart from a small capillary at the top of the cell which allows oxygen access to the working electrode. The two electrodes are connected, via current collectors, to the pins which protrude externally and allow the sensor to be electronically connected to an instrument. The entire cell is filled with conductive electrolyte which allows transfer of ionic species between the electrodes (see figure 1).

The rate at which oxygen can enter the cell is controlled by the size of the capillary hole at the top of the sensor. When oxygen reaches the working electrode, it is immediately reduced to hydroxyl ions:

O₂ + 2H₂O + 4e^-  4OH^-

These hydroxyl ions migrate through the electrolyte to the lead anode where they are involved in the oxidation of the metal to its corresponding oxide.

2Pb + 4OH^-  2PbO + 2H₂O + 4e^-

As the two processes above take place, a current is generated which can be measured externally by passing it through a known resistance and measuring the potential drop across it. Since the current produced is proportional to the rate at which these reactions occur, its measurement allows accurate determination of the oxygen concentration.

As the electrochemical reaction results in the oxidation of the lead anode these sensors have a limited life. Once all the available lead has been oxidised they no longer work. Typically oxygen sensors have 1 - 2 year life times, however this can be lengthened by increasing the size of the anode or restricting the amount of oxygen that gets to the anode.

CAPILLARY AND PARTIAL PRESSURE OXYGEN SENSORS

City Technology produces two types of oxygen sensor differing by the mechanism that limits the diffusion of gas into the sensor. In one type the gas access is via a small capillary hole in the top of the sensor and the other type uses a solid membrane through which the gas diffuses. The capillary type measures the concentration of oxygen and the solid membrane sensors measure the partial pressure of oxygen.

The current generated by a capillary controlled oxygen sensor is proportional to the volume fraction (i.e. volume %) of oxygen present and this is independent of the total pressure of gas. If, however the pressure of gas is changed suddenly, then the oxygen sensor will produce a transient current which can cause problems if not correctly controlled. This can also occur where the CiTiceL® is subjected to repeated pressure pulses, for example, with a pumped gas supply. This behaviour can be explained as follows:

PRESSURE TRANSIENT

When a capillary oxygen sensor is subjected to a sudden sharp pressure increase or decrease, gas is forced through the capillary barrier (bulkflow). This results in an enhanced (or reduced) flux of gas into the sensor and hence a current transient on the measured signal. This transient quickly settles to zero once diffusion conditions are re-established and the pressure pulse is complete. These transients can send an instrument into alarm and so City Technology has actively sought methods to reduce this effect.

All City Technology capillary oxygen sensors are fitted with an anti-bulkflow mechanism, which is depicted in figure 2 below. Essentially, pressure changes can be 'dampened' by the addition of an additional PTFE anti-bulkflow membrane which reduces the magnitude of the transient effect seen.This membrane is held tightly over the capillary by a metal or moulded plastic cap. This design modification results in a considerable reduction in the signal transient.

Some stepwise pressure changes produce transients which are sufficient to overcome this in-built compensation, particularly in instruments using a pumped delivery of gas to the sensor head. Some pumps produce a gas delivery which subject the oxygen CiTiceL to a continual barrage of pressure pulses which can artifically enhance the signal measured. In these cases, it is often necessary to design an external expansion chamber into the gas flow which can minimise the pressure pulses to which the sensor is exposed.

PARTIAL PRESSURE OXYGEN SENSORS

Capillary control of gas diffusion is not the only method of limiting the rate of oxygen entry. It is also possible to use a very thin, plastic membrane over the top of the sensor - the membrane operates as a solid barrier in which the oxygen molecules must dissolve in order to reach the sensing electrode (figure 3).

The flux of oxygen to the working electrode is dependent on the partial pressure gradient of oxygen across the barrier. This means that the output signal from the cell is proportional to the partial pressure of oxygen in the gas mixture. Any changes in atmospheric pressure will therefore result in an equivalent change in the output current of the cell. It is important that this characteristic is considered when designing instruments to ensure that back pressure is not applied to the cell when using pumped gas feeds.

City Technology manufactures two types of partial pressure oxygen sensors for automotive (AO2/AO3) and medical (MOX) applications where the linear response and 0-100% range achieved with solid membrane cells is beneficial.

LINEARITY

The signal from a capillary controlled oxygen sensor is non-linear and follows the following relationship with the fractional oxygen concentration (C);

In practice, the output from the cells are effectively linear up to 30% oxygen and only oxygen concentrations higher than this cause measurement difficulties. In contrast, partial pressure sensors offer a linear output up to 100% oxygen (or 1.0 fractional oxygen concentration).

TEMPERATURE

Both capillary and solid membrane oxygen sensors are sensitive to changes to temperature; but to differing extent.

The effect of temperature on the performance of a capillary barrier oxygen sensor is relatively small, and typically changing the temperature from +20°C to -20°C will result in 10% loss of the output signal. In contrast, temperature has a much greater effect on solid membrane oxygen sensors. The diffusion of gas across the membrane is an activated process and as a result has a large temperature coefficient. Typically a 10°C change in temperature doubles the output signal from the sensor. Solid membrane oxygen sensors require temperature compensation as a result, and many oxygen CiTiceLs® have thermistors designed in.

ACTIVITY RESERVE

It is important in the design of any electrochemical gas sensor that the rate limiting step should be the diffusion of gas through the barrier (membrane or capillary) and all other stages should have rates which are significantly faster. To achieve this it is important that the electrode material has high catalytic activity for the electrochemical reactions of the sensor.

All CiTiceLs® have highly active electrodes resulting in sensors with very high activity reserves. This is an important factor in ensuring the long- term stability of the sensor and the low levels of drift.