Frequently Asked Questions

1. What does the date code mean?

Most of City Technology's sensors have an eight-digit serial number followed by a three-digit date code. The first two digits of the date code represent the month of despatch whilst the third digit represents the year.

Certain sensors such as the Medical sensor range are marked differently and have either a 4 digit date code or a warranty expiration date. In the case of the 4-digit date code there are two numbers for the month followed by two for the year.

In the case of the warranty expiration date, it is normally marked on boxes on the label. In this case the serial number can be used by City Technology to check the date of manufacture.

For all sensors the serial number is used to trace the manufacturing data, test results and materials data for the sensor.

2. Why is the NX1 CiTiceL better than the 3NF/F for automotive applications?

The NX1 Nitric oxide CiTiceL is recommended for use in automotive applications, having been specifically designed to withstand the rigors of a garage environment. It shows little or no cross-sensitivity to the gases typically found in a vehicle exhaust gas sample and has an extremely fast response time. The standard 3NF/F CiTiceL has been designed for monitoring nitric oxide in flue (stack) gas emissions.

3. How often should sensors be re-calibrated?

The time period between initial calibration and successive re-calibrations depends on many factors, commonly the operating temperature/humidity/pressure in which the sensor is used, the gases it is exposed to and the length of time the sensor is exposed to gas. In general, however, CiTiceLs® provide very stable signals over time and for most applications, instruments containing CiTiceLs® would only require periodic calibration, often as little as once a year. In strenuous applications involving extremes of operation, or for sensors used in safety applications, more frequent instrument calibration may be required.

4. How variable are the cross-sensitivity values quoted?

The cross-sensitivity values we quote are based on tests conducted on a small number of sensors. They are intended to indicate sensor response to gases other than the target gas. Sensors may behave differently with changes in ambient conditions and any batch may show as much as 50% variation from the values quoted.

5. When you use a pump in front of the sensor, will the response time speed up?

The use of pumps will not quicken the response of the sensors themselves but pumps do enable samples of gas to be drawn across the sensors quickly and efficiently, from inaccessible locations. As a result pumps can affect the overall response time of the complete instrument.

6. Can I put a membrane or filter in front of a sensor?

Membranes and filters can be placed in front of CiTiceLs® for additional protection but care is needing to ensure "dead spaces" are not created that will tend to increase sensor response time.

7. What are the factors to consider when designing a suitable sampling system?

It is important when designing a sampling system to use materials that prevent gases from adsorbing to the surfaces of the sampling system. The best materials to use are fluoro-polymers, PTFE, TFE and FEP. Moisture condensing from the gas stream can cause blockages and flooding so it is important to remove any water condensing out by using a suitable water trap. Alternatively, water can be removed in the vapour phase by the use of Nafion tubing.

For gases at high temperatures the sample stream should be cooled to within the operating range of the sensor and any particulate matter should be removed from the sample by using appropriate filters. Additional in-line chemical filters can also be incorporated into any sampling system design to remove any effects from cross-interfering gases.

8. What happens if the gas itself has a different temperature to that of the sensor?

The zero offset current of the sensor is dependent on the temperature of the sensor itself and as such the temperature of the gas stream being measured will have little effect on the zero offset signal.

The span signal of the sensor is dependent on the rate of diffusion of gas molecules through the capillary hole to the sensing electrode for subsequent reaction. A gas diffusing through the capillary hole at a different temperature to that of the gas inside the sensor could have a small impact on the sensitivity of the sensor. It may also cause small shifts or transient currents until equilibrium is fully established.

9. What manufacturing traceability exists for your sensors?

City Technology has achieved ISO 9002 accreditation and has a formal procedure of sensor traceability via unique serial, product, and date codes attached to each individual sensor manufactured.

10. Can sensors tolerate continuous exposure to target gas?

Oxygen CiTiceLs® are available to monitor concentration of oxygen continuously in the range 0-30% oxygen by volume, or partial pressure continuously in the range 0-100% oxygen by volume.

Our Toxic Gas CiTiceLs® have been designed for intermittent monitoring of target gases and are generally unsuitable for continuous monitoring applications, particularly those involving high concentrations of gases or extremes of humidity and temperature. Continuous monitoring may sometimes be achieved by cycling two (or even three) sensors in and out of the gas stream, such that each sensor is only exposed to gas for up to one half the time, being allowed to recover in fresh air for the other half.

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.

21. What happens if I exceed the maximum overload?

The maximum overload is specified in terms of maintaining a linear response over a 10-minute exposure and recovering quickly thereafter. At higher levels the sensor will progressively become more non-linear and take increasing longer to recover as the sensing electrode is unable to consume all the gas diffusing to it.

If the level is increased still further gas will build up inside the sensor and diffuse into the internal spaces where it may interact with the reference electrode, altering it's potential. If this happens the sensor may take a very long time to recover once placed in clean air (several days)

The design of the circuitry also plays a role in ensuring a quick recovery from high levels since it is important that the amplifiers in the potentiostat circuit do not run into current or voltage saturation at the signal levels generated during the exposure. If the amplifiers do limit the current flow into the sensor this will limit the consumption rate of gas by the sensing electrode the allowing an immediate build up of gas inside the sensor with the potential effects described above.

Finally the load resistor connected to the sensing electrode should be chosen to ensure the voltage drop across it is never more than a few mV's at the highest gas concentration likely to be seen. If larger voltage drops are allowed to take place across the load resistor this will cause similar changes in potential of the sensing electrode, which will take time to recover from once the gas is removed.

22. How much Oxygen do sensors need to function correctly?

Sensors which generate an output by oxidising the target gas such as carbon monoxide sensors need a supply of oxygen to the counter electrode to support the balancing oxygen reduction reaction which takes place there. Generally a few thousand ppm at most is required to support this and this may be provided by the level of oxygen in the sample gas. Even where the sample gas is anaerobic there is enough oxygen inside the sensor to support short exposures.

The reference electrode also requires a small supply of oxygen for most sensors so that if a sensor is operated continuously in an anaerobic environment it will eventually start to misread.

23. Why is my sensor reading lower than the quoted specification?

There are many reasons why our customers measure different sensitivities to us so it is important when designing equipment based on these sensors to allow generous margins for calibration adjustment to allow for these as well as the natural decline in output over the life of the sensor. Some of the reasons we have found in the past are:

  • Using a different flow rate
  • Placing extra diffusion barriers in front of the sensor e.g. flame arrestors or ptfe membranes, especially if there is a large dead space between them and the sensor
  • Use of absorbent tubing or brass regulators with 'sticky' gases such as chlorine Contaminated gas cylinders e.g. NO cylinders degrading due to O2 ingress
  • Use of cylinders outside manufacturers recommended minimum pressure or life
  • Contamination of 'air' cylinders used to dilute mixtures
  • Using sampling systems without properly damping out pressure pulsation's
  • Test fixture designs can affect the measured signal of combustible sensors quite significantly.

24. How do I connect to the sensor?

Connection to the majority of City Technology's sensors is made via PCB connection pins; some sensors use other connections such as data sockets, jack sockets, and Molex connectors which are detailed on the appropriate product data sheet.

Connection to sensors with PCB pins; It is important that the PCB connection pins of CiTiceLs are not directly soldered onto. Soldering directly to the pins has the potential to damage the CiTiceL's housing and also the potential to cause internal damage which will go unseen. For this reason soldering to directly to the PCB connection pins on a CiTiceL will render your warranty void.

The best method of connection is to use sockets - which also give the additional advantage of allowing quick and easy replacement. The order codes for recommended sockets ar:

4-series and 5-Series CiTiceLs… 450-3326-01-xx-00
All other CiTiceLs… 450-5301-01-xx-00

The xx part of the code varies according to the finish of the sockets.

A supplier for these sockets is:

Cambion Electronics Ltd
Hope Valley
S33 8WR
Tel: 01433 621555
Fax: 01433 621290

Also available from:

See Distribution List on Cambion Web site

25. Do you have Temperature Data?

Temperature data is available for the majority of CiTiceLs, and can be found on the appropriate product data sheet. (For MediceLs this data is contained in the datapak appendix to the product data handbook). City Technology operate an ongoing test programme, so if there's no data shown on a product data sheet please contact us - we may have conducted additional tests since preparing the product data sheet.

26. Do you have cross-interference data?

CiTiceLs are designed to be highly selective, with minimal cross-interference from other reactive gases. The product data sheet for each CiTiceL shows the results of cross-interference testing with commonly occurring gases. City Technology operate an ongoing test programme, so if there's no data shown on a product data sheet please contact us - we may have conducted additional tests since preparing the product data sheet.

27. What is the recommended storage period?

The maximum recommended storage period for CiTiceLs is six months. During this time the CiTiceLs should be stored in the containers in which they were supplied in clean dry areas between 0°C and 20°C. CiTiceLs should not be stored in areas containing organic solvents or in flammable liquid stores. Under these conditions CiTiceLs may be stored for up to six months without the length of their expected operating life being reduced.

CiTiceL's may be stored for longer periods but we are unable to give data on how they will then perform.

28. Why is a minimum flow rate required?

A minimum flow rate is required to ensure accurate calibration - it also means that the response from a CiTiceL is equivalent in configurations where gas is flowing over the sensor and those where the sample is allowed to diffuse to the sensor. The minimum flow which is required will be different depending on the CiTiceL type; these are shown in the table:

Gas CiTiceL Minimum Flow
Rate mls/min
Carbon Monoxide CO All CiTiceL types 150
Hydrogen Sulphide H2S 4H, 4HS, 3H, 7H
3HH, 7H
Sulphur Dioxide SO2 4S, 3ST/F, 7ST/F
3SH, 7SH
Nitric Oxide NO 4NT
3NT, 7NT
Nitrogen Dioxide NO2 4ND
Chlorine Cl2 All CiTiceL types 1000
Hydrogen H2 3HYT, 3HYE, 4HYT, 7HYT 150
Hydrogen Chloride HCl All CiTiceL types 1000
Ammonia NH3 All CiTiceL types 250
Ethylene Oxide C2H4O All CiTiceL types 1000
Ozone O3 All CiTiceL types 1000

The reaction mechanism of CiTiceLs, consumes target gas - this means that the concentration of target gas will be depleted immediately in front of the CiTiceL. The minimum flow rates are set so that, the CiTiceL is exposed to a constant concentration of target gas - the flow rate is great enough to ensure that this depleted concentration is immediately replaced. This mimics the situation where the sample diffuses to the CiTiceL; there will be a large volume of target gas so that the depletion is immediately replaced - via diffusion.

29. What substances can kill a sensor?

Electrochemical CiTiceLs

CiTiceLs are designed for operation in a wide range of environments and harsh conditions. However it's important that exposure to high concentrations of solvent vapours is avoided, both during storage, fitting into instruments, and operation.

Formaldehyde is known to temporally inhibit the operation of Nitric Oxide sensors. Other solvents are known to create false high baselines. When using sensors with printed circuit boards (PCBs), decreasing agents should be used before the sensor is fitted. Do not glue directly on or near the CiTiceL as the solvent may cause crazing of the plastic.

Catalytic bead CiTipeLs

Certain substances are known to have a detrimental effect on catalytic sensors. CiTipeLs are designed to be resistant to these substances - there are CiTipeLs available which offer excellent resistance - however certain substances should be avoided. The detrimental effect can occur through two mechanism:


Some compounds will decompose on the catalyst and form a solid barrier over the catalyst surface. This action is cumulative and prolonged exposure results in an irreversible decrease in sensitivity. The most common of these substances are: lead or sulphur containing compounds; silicones; phosphates.


Certain other compounds, especially Hydrogen Sulphide and halogenated hydrocarbons, are absorbed or form compounds that are absorbed by the catalyst. This absorption is so strong that reaction sites can become blocked and normal reactions are inhibited. The resultant loss of sensitivity is temporary and in most cases a sensor will recover after a period of operation in clean air.

Most compounds fall into one of these categories, although some will exhibit both mechanisms to a greater or lesser extent. In applications where any of these compounds are likely to be present, CiTipeLs should be protected from exposure to compounds which they are not specifically resistant to - details of resistance are found on each CiTipeLs product data sheet.

30. How do I prepare a CiTiceL for use?

Standard Operation CiTiceLs

CiTiceLs are kept in a 'ready-to-work' condition whilst being shipped, by the shorting link, which connects the reference and sensing terminals. This performs the same function as the J-FET in the recommended operating circuit. (CiTiceLs which operate with an auxiliary electrode, will also have a shorting link connecting the auxiliary and reference terminals). The shorting link is removed just prior to placing the CiTiceL into its operating circuit; the sensor is now ready for calibration in the instrument.

Biased Operation CiTiceLs

Biased operation CiTiceLs, require the sensor's electrodes to be at different potentials. This means that Nitric Oxide (NO), Hydrogen Chloride (HCl), Ammonia (NH3) and Ethylene Oxide (C2H4O) CiTiceLs are not shipped with a shorting link; since shorting the terminals on these sensors can cause permanent damage.

Some biased operation CiTiceLs have the option of being shipped connected to a special PCB which maintains the bias potential whilst the CiTiceL is being shipped. This keeps the sensor in a ready to work state; if the sensor has been 'off' bias then the start-up time will be increased. Applying bias potential to a bias operation CiTiceL which has been 'off' bias will produce a large rapidly decreasing baseline. After several hours the baseline should be sufficiently stable for measurements to be made; two to three hours for Ammonia, CiTiceLs. Hydrogen Chloride Ethylene Oxide, or Nitric Oxide CiTiceLs require much longer and can take several days. The baseline will continue to stabilise slowly during the following three weeks, after which time it should be fully settled.

It's strongly recommended that the bias potential is maintained at all times - even when an instrument is switched off. If the bias potential is not maintained, a very long start up time will result when the instrument is switched on.

31. How do I know when to replace my sensor?

Expected operating lives can be found on the product data sheet for each CiTiceL. With time the sensitivity of toxic gas CiTiceLs slowly declines - as an example the decline in sensitivity for a Carbon Monoxide CiTiceL will typically be less than 5% of signal per year. It's this decline in sensitivity that ultimately determines the sensor's operating life and when it will need to be replaced. The point at which a toxic gas CiTiceL needs to be replaced, is when the instrument in which is installed, can no longer be calibrated.

CiTipeLs behave in a similar manner; their sensitivity will slowly decline over time. Again this means it's best to design instruments with a high degree of gain; CiTipeLs are replaced when calibration can no longer be completed.

Oxygen CiTiceLs however have a finite life; the length of their operating life will be determined by the total exposure to Oxygen. This is because the sensor has a metal anode which is consumed by the reaction mechanism. The characteristic of Oxygen CiTiceLs is that they have a stable signal output, which experiences minimal decline, right up until the end of the sensors life. At the end of the sensor's operating life there is a sudden and rapid drop-off in output as the sensor's anode is finally consumed.

32. Can I test CiTiceLs with a surrogate gas?

CiTiceLs should be calibrated with their target gas to ensure maximum accuracy. The calibration is best made using a gas mixture with a concentration where most measurements will be made; where this is not possible a gas mixture towards the top of the CiTiceL's measuring range should be used. Calibration gases that exceed the CiTiceLs measuring range should not be used since this may lead to an inaccurate calibration.

CiTiceLs are built and tested for their response to their target gas - prior to despatch all CiTiceLs are tested to ensure their response is within strict tolerances. Cross-sensitivities are tested on a batch basis; this means that there is inherently more possibility of variation. It is for this reason that it is strongly recommended that the use of surrogate gases for calibration be avoided.