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.