OEM Potentiometric Oxygen Gas Sensor Modules

The potentiometric measuring principle for oxygen sensing covers the widest range of concentration range among all other oxygen sensing technologies – from 10-35 bar O2 and up to 100%V O2. The broad measurement range and high resolution of the sensing principle makes it interesting for many different application areas such as, additive manufacturing (AM) or Metal – 3D printing. In such application areas, oxygen sensors can be used to detect irregularities in the process, allowing for faster troubleshooting and improved efficiency. 

The measurement principle of potentiometric oxygen sensors is based on a heated zirconia element which is exposed to the sample gas with an unknown oxygen gas concentration and on the other side to a known reference oxygen gas concentration. The difference in oxygen partial pressure between the two electrodes generates an equilibrium constant voltage (charge balance). For accurate and reliable measurements of the unknown oxygen gas concentration, the temperature of the heated zirconia element needs to be controlled very precisely and the reference oxygen gas concentration, PO2 (Pabs,ref, Tref, Href), needs to be stored in the module.

Angst+Pfister Sensors and Power has developed two different types of OEM potentiometric oxygen gas sensors and gas sensor modules. First module type PZA MC-N sensors/modules is developed for inline, diffusion-based measurements. And the second module type PZA-MC-P is developed for forced flow measurements. Subsequently, the module can be separated into the sensor head typically called as A19-N/P, with N being representative for inline or diffusion-based measurements and P being representative for forced flow measurements.

A19-N sensor head has M27 x 2 mm thread which can be easily screw-mounted, and gas tightened into a process chamber or a process line. Alternatively, A19-P sensor head has a flow housing with a gas inlet (G3/8”) and an outlet (G1/4”) which ensures a very fast response to changes in the oxygen concentration and no pressure build up over the zirconia element. Both sensor types A19-N/P can be designed-in for temperatures up to 200°C. 

For controlling and regulating the temperature, as well as also conditioning the output signal, the MC electronics (full module PZA-MC-N/P) is recommended. The electronics controls the temperature very accurately and the conditioning electronics amplifies the mV output signal into a 4-20 mA output signal that can be managed easily with a PLC module. Another noteworthy characteristic of PZA MC N/P oxygen gas sensor module is that it has two measurement ranges. The reason for having two measurements ranges is that most applications require process control in a low oxygen concentration atmosphere (less than 10.000 ppm). Subsequently, the same application also require to control the oxygen concentration during start-up of the process or would like to check the output signal in ambient air (~21%V O2) before starting the process. The standard configuration offered by Angst+Pfister Sensor and Power PZA MC N/P oxygen gas sensor module is 0-1000 ppm and 0-25%V measurement ranges with a possibility of automatic switchover between the measurement ranges by the use of a potential-free contact.

As mentioned in the beginning of this newsletter, the most popular application, among many, for the PZA-MC-N potentiometric oxygen sensor module for inline, diffusion measurements is in additive manufacturing (AM) applications for controlling the inert gas conditions in powder bed based build chambers. The first measurement range (0-25%) is used for getting a good-for-go for the build job to be started, because the oxygen concentration is lower than a preset value for safe operation. The second range (0-1000 ppm) is used to control and monitor the oxygen concentration level during the build job. Most reactive metals, like titanium or aluminum, must be build in a very low oxygen atmosphere – preferably below 200 ppm O2.

Alternatively, one of the most popular applications for the PZA-MC-P potentiometric oxygen sensor module for forced flow measurements is in metal powder atomizer/separator machine for securing oxidation free processing in a closed chamber machine. Because of the conditions in the chamber, the gas-to-be-analyzed is extracted and analyzed using the PZA-MC25-P and with process limits as low as 10 ppm. The PZA-MC-P not only monitors the conditions before the start of the process but during the process it measures the quality of the process, for instance by being able to measure leakages within the closed chamber.

The mV output signal from the potentiometric zirconia oxygen sensors is logarithmic. The MC electronics linearizes the signal and deliver per default a linearized 4-20 mA signal. For some applications however, for instance those where a reducing gas can interfere with the oxygen gas on the surface of the zirconia, it is recommended to switch to the logarithmic measurement mode. In the logarithmic mode the concentration range can be very broad, but the output signal still lies within 4-20 mA. Thus, making the measurement more accurate.

All changes, be it an adjustment of the cable length between the sensor head and the electronics over a customer specific setting of the measurement range to a change between linear and logarithmic mode, is easily done using the Windows App and the Bluetooth connection in the electronics. All PZA-MC25-N/P are delivered with a very precise calibration and a calibration certificate, but it is recommended, and often needed, to make an adjustment/calibration in the applications. Small changes in pressure, flow or humidity can causes rather large deviations from pre-calibrated values. 

Fig. 1. PZA-MC25-N product consisting of electronics unit (left) and A19-N potentiometric oxygen sensor for inline, diffusion measurement. Left on the electronics unit are the status LEDs (incl. BLE communication LED), in the middle the 4-pole connector for connection to the A19-N sensor head and to the right the terminal connection block for power and output. The A19-N sensor head has a M27 x 2 mm thread for screw mounting. The diffusion flow to the sensor element is entering through large openings that are protected with a sinter filter.

Fig. 1. PZA-MC25-P product consisting of electronics unit (left) and A19-P potentiometric oxygen sensor for forced flow measurements. Left on the electronics unit are the status LEDs (incl. BLE communication LED), in the middle the 4-pole connector for connection to the A19-P sensor head and to the right the terminal connection block for power and output. The A19-P sensor head has a M27 x 2 mm thread for screw mounting. The forced flow to the sensor element is entering through a G3/8” gas inlet connection and exiting through a G1/4” gas outlet connection.