The FCX oxygen sensor series comes with 4 different measurement ranges; 0…1000ppm, 0…5%, 0…25% and 0…95%. The construction of the individual sensors with respect to the measurement range has been done to meet requirements such as accuracy, signal stability/repeatability, response time and operational lifetime of the sensor, but all sensors are based on the zirconia principle. The sensor output signal is in the µA range and it follows a logarithmic curve with increasing oxygen content.
Zirconium oxide (with proper dopant elements) when heated > 350 °C is penetrable for oxygen ions. The FCX oxygen series has it’s working point at 450°C. The heating element and the oxygen sensing element are bonded together ensuring a perfect thermal match and minimum power usage within the working point. A voltage applied to the oxygen sensing element, pumps the oxygen out of a closed inner chamber. At a constant gas pressure, the quantity of oxygen pumped out is equal to the quantity of oxygen molecules diffusing in through a small capillary hole. It is independent of the voltage applied between the electrodes within a certain range. The measurement current is proportional to the quantity of oxygen molecules pumped away. The relationship between the oxygen partial pressure pO2 and sensor current Is is given by the formula Is = constant x In (1 – pO2 / pt), where pt is the total partial pressure in the system.
The FCX-ULL oxygen sensor measures partial oxygen pressure in the range from 0…1000 ppm pO2, and also works under normal atmospheric conditions. Sensor lifetime is limited only by the total current through the sensor element. The accuracy of the FCX-ULL sensor can be very high, but it is very much dependent on the application and how the sensor is applied. The standard accuracy of +/-50 ppm can be improved by calibrating the sensor before each measurement or by measuring in a continuous mode. In the latter case, the resolution can be as good as a few ppm.
The FCX-UL oxygen sensor measures partial oxygen pressure in the range from 0…5% pO2, and also works under normal atmospheric conditions. Sensor lifetime, like the FCX-ULL sensor, is limited only by the total current through the sensor element. For oxygen concentrations in between 0 and 10.000 ppm the accuracy of the FCX-UL sensor is very good (+/-100 ppm). For oxygen concentrations above 10.000 ppm (i.e. between 1% and 5%), the accuracy is +/-1% of the signal. As with the FCX-ULL ppm oxygen sensor, accuracy and resolution can be improved based on the applied measurement procedure by the customer.
The FCX-ULL and FCX-UL sensors are almost linear in their response to the oxygen concentration. This can be seen from the equation above, because pO2/pt << 1. Both sensors are also very fast responding with a T90 response time in order of seconds in forced flow mode.
The FCX-UC oxygen sensor measures partial oxygen pressure in the range from 0…25% pO2.The response as a function of the oxygen concentration can be considered quasi-linear with a maximum error of 0.5% in the range from 13-17% oxygen. The FCX-UC oxygen sensor has a very long operational lifetime in air, and has a very stable signal output, which in turn minimizes the need for calibration. It can be calibrated in a single point and is therefore a very popular choice for oxygen transmitters in non-Ex environments.
The FCX-UWC oxygen sensor measures partial oxygen pressure in the range from 0…95% pO2. It is by far the most used sensor for medical oxygen concentrators, but is also being used to measured oxygen quality/concentration in processes requiring high oxygen concentrations, like, for instance, in some controlled incubator applications. The signal stability and the accuracy at high oxygen concentration are very good, ensuring safe operation of any device based on the FCX-UWC sensor.
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