Mobile Sensing Technology

quadstat
The ‘QuadStat’ is an array of four potentiostats used for electrolytic type sensors. The ‘QuadStat’ has headers which can accomodate:

The ‘Quad-stat’ is also designed to handle the B4 line of Alphasense sensors, which incorporate an ‘auxillary’ electrode.
quadstat_pcb_anotated copy

Features

  • The ‘Quad-stat’ uses a 2.5V precision reference between the 0V and 5V inputs. As such, it can accomodate sensors which both oxidize and reduce the target gas. i.e. which produce positive and negative voltage changes.
  • A voltage divider allows the user to offset the reference electrode for sensors which require the working electrode to be at a different voltage than the reference electrode (eg. Alphasense Nitric Oxide sensor).
  • Two mcp3424 differential 18-bit analog-to-digital converters monitor the signal output from the working electrode and, when applicable, the auxiliary electrode.
  • An Analog Devices AD22100 analog temperature sensor can be used to monitor the ambient air temperature around the electrolytic sensors. Most applications require temperature compensation in the post-processing of electrolytic sensor data.
  • Jumper headers are also available should users prefer the RHT03 humidity and temperature sensor.

    • Addressing

    The solder jumpers on the mcp3424 ADCs set the hardware address necessary for proper communication. If the ALPHA_ARRAY flag is enabled on the UPOD, then the solder jumpers on the QuadStat need to be soldered such that:

  • ‘A0′ on ‘ADC#1′ is jumped to ground
  • ‘A1′ on ‘ADC#1′ is left unsoldered (floating)
  • ‘A0′ on ‘ADC#2′ is jumped to V+
  • ‘A1′ on ‘ADC#2′ is jumped to V+

    • quadstat_pcb_addressing

    Gain and Bias

    The QuadStat can be configured with different gain resistors. The gain resistor translates the current output from the working electrode into the measured voltage. We found the best stability was achieved with a 100K 0.1% precision resistor. The bill of materials (BOM) comes standard with this resistor. pstat_pcb_cropped
    The user may also populate the ‘bias1′ and ‘bias2′ resistor in order to offset the reference voltage with respect to the working electrode. The working electrode, in all cases, is held at V+==2.5V. In standard mode where the reference voltage equals the working electrode voltage, then ‘bias2′ is left empty, and ‘bias1′ is jumped with a 0ohm resistor (or solder bridge).
    The Nitric Oxide Alphasense B4 sensor, for example, needs the be held at 300mV above the reference electrode. Populating ‘bias1′ with a 634ohm resistor and ‘bias2′ with a 10kohm resistor will achieve this bias.     		reference_work_schematic

    Data Format

    The signal in micro-volts (uV) of each electrode is captured by the UPOD and written to file/streamed via serial. The output contains 8 values, which correspond to the sensors moving from left to to right across the board. Thus:
    …,AUX1,MAIN1,AUX2,MAIN2,AUX3,MAIN3,AUX4,MAIN4
    Where ‘AUX’ is the auxillary electrode (only applicable to the B4 series of Alphasense sensors) and ‘MAIN’ refers to the main working electrode voltage.

    Additional Features

    The QuadStat can be outfitted with an aluminum top-plate and fan assembly. The fan assembly has the added benefit of reducing the boundary layer on the surface of the sensor, resulting in increased sensitivity and less variability due to local airflow patterns. The aluminum plate, which is secured to the PCB via aluminum standoffs, acts as a Faraday cage, grounding the entire assembly and helping to reduce noise due to transient EMF.
    The build files for the QuadStat include the *.dxf drawing for the aluminum plate, which be cut at any waterjet facility. The bill of materials for the electronics include the four fans. A separate parts list includes the appropriate stand-offs and fasteners. The fans are powered through screw terminals on the QuadStat board.
    UPOD_w_quadstat
     

    Creative Commons License
    QuadStat by www.mobilesensingtechnology.com is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.