11. WHAT APPROVALS DO THE SENSORS HAVE?

Our sensors have achieved various component approvals as follows: 

AO2	Automotive Oxygen CiTiceL®: PTB and BAR 97 approval
4P Range	Combustible Gas CiTipeLs®: CSA, UL, and SIRA certification
CDH300	Combustible Gas Head: CSA and SIRA certification

12. WHAT MATERIAL IS THE SENSOR HOUSING MADE OUT OF?

A number of different plastics are used which are chosen for their compatibility with the internal electrolyte system and for durability in the intended application. Typically ABS, polycarbonate or polypropylene are used. More details are in the individual sensor data sheets.

13. ARE THE SENSORS INTRINSICALLY SAFE?

Although they have not been certified intrinsically safe, instruments containing CiTiceLs® and CiTipeLs® can readily achieve intrinsic safety requirements. Contact City Technology for more help on achieving intrinsic safety approvals.

14. HOW DO I TEST MY CIRCUIT?

3 electrode and 4-electrode CiTiceLs are designed to run in a special circuit called a potentiostat. The purpose of this circuit is to allow the potential of the Sensing (and auxiliary) electrode to be controlled relative to the reference electrode whilst amplifying the current flowing into or out of it. The potentiostat circuit can be easily checked using the following method:

• Remove the sensor
• Connect a short circuit between the reference and counter terminals
• Measure the potential of the sensing (and auxiliary) terminal relative to the shorted reference-counter terminals. The value measured should be zero (±1mV) for unbiased sensors or equal to the recommended bias voltage for biased sensors.
• Connect a current source between the sensing (or auxiliary) terminal and the reference-counter terminal and confirm the voltage output is as expected.

The above steps will confirm the circuit is working correctly in most cases. The sensor may now be replaced and allowed to stabilise. The voltage measured between the sensing and reference terminals now should again be zero for unbiased sensors or equal to the recommended bias voltage for biased sensors.

15. HOW CAN I STERILISE A SENSOR?

In general it is not possible to expose sensors to typical sterilising systems without causing irreversible damage or affecting sensor performance. High pressures and temperatures will cause seal failures whilst aggressive chemicals such as Ethylene Oxide and Hydrogen Peroxide may damage the electrocatalyst.

Where sensors need to be returned to City Technology and may be a biohazard we recommend ethylene oxide sterilisation. We have not conducted any testing on gamma ray sterilisation but this may provide a way of sterilising a sensor without affecting performance.

16. WHAT HAPPENS IF I EXPOSE A SENSOR TO TEMPERATURES OUTSIDE THE QUOTED OPERATING RANGE?

Generally the effect of low temperatures is less of a problem mechanically as the liquid electrolyte in all the sensors except the Oxygen sensors will not freeze until around -70°C. Prolonged exposure to very low temperatures may cause problems with the plastic housing leading to cracking.

The Oxygen sensor electrolyte will freeze around -25 to -30°C although the high salt content means that it will not necessarily cause immediate damage. It is possible however that the sensor will fail as a result.

Temperatures above the top limit put stress on the seals on the sensors that will ultimately cause leakage of electrolyte to occur. The plastic bodies used for most of the sensor moulds will soften above 70°C causing sensor failure quite quickly.

17. WHAT HAPPENS IF I EXPOSE A SENSOR TO PRESSURES OUTSIDE THE QUOTED OPERATING RANGE?

The sensors all use a similar sealing system which relies on the hydrophobic properties of ptfe to prevent liquid passing out of the sensor even though there are holes to allow gas in. If the pressure at the sensor inlet is suddenly raised or lowered by more than the allowable limits the pressure across the internal membranes and seals may become large enough to cause leakage to occur.

If the pressure change takes place slowly enough it is possible to use the sensors over wider limits but the advice of the technical support group at City Technology should be sought.

18. WHAT ARE THE OPTIMAL STORAGE CONDITIONS FOR SENSORS?

Sensors are very robust and will not normally deteriorate to any significant extent when stored in their original packaging for extended periods. Where long storage periods are anticipated we advise the user to avoid placing the sensor in very hot environment such as in a window exposed to bright sunlight.

If a sensor has been removed from it's original packing it is important to store in a clean area away from solvent fumes, which might be absorbed onto the electrodes causing subsequent performance issues. Oxygen sensors are a special case since their life is consumed when they are put on load. For this reason they are either shipped off-load or in sealed packages where the oxygen level is held at a reduced level.

19. WHAT ARE THE POWER REQUIREMENTS FOR SENSORS?

Two electrode sensors such as the oxygen sensors and 2 electrode carbon monoxide sensors are self powered and thus have no power consumption themselves. Three and four electrode sensors must be run on a special potentiostatic circuit, which thus requires a power supply. In essence the sensor still doesn't require power as it generates the output current directly from the oxidation or reduction of the target gas however since the amplifiers in the circuitry are not perfect they consume some standing current. This can generally be reduced to a very low level if necessary.

20. HOW LONG DO INBOARD FILTERS LAST?

Certain sensors have chemical filters inside the sensor designed to remove gases that might otherwise cause an interference signal. Because the filter is placed behind the diffusion barrier the rate at which gas passes through it is much lower than if it were in the main gas path, hence a small mass of active chemicals can last a long time.

In general we design such filters to last the expected life of the sensor in the intended application however in some arduous applications such as emissions monitoring this can be difficult. For these applications we recommend using a sensor such as the 5 series design which features a replaceable inboard filter.

For certain contaminants the filter works by sorption rather than chemically reacting and in these cases it can be easily overloaded by high concentrations. This is often the case with organic vapours. City Technology's technical support team can provide more information on specific cases.