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Hydros21 Reporting -9999 on Mayfly 0.5b

Home › Forums › Mayfly Data Logger › Hydros21 Reporting -9999 on Mayfly 0.5b

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    • 2026-06-09 at 7:49 AM #19700
      caroline@wachusett
      Participant

        Hello,

        We have a Mayfly station with a version 0.5b board and a Hydros21 CTD sensor attached. The sensor had been working for years at the site but recently, the CTD measurements were being reported as -9999 for all three parameters. I tried troubleshooting and ended up swapping out the sensor for a new Hydros21 CTD sensor. Before deploying, I changed the address of the new sensor to 1. I installed the new sensor and plugged it into the 0.5b board, but we are still getting -9999 on the 0.5b board. I tried bringing out a version 1.1a board to the field to test it out and the sensor is reporting results on that board. Has anyone run into this issue with the 0.5b board and found a solution?

      • 2026-06-10 at 2:45 PM #19702
        Shannon Hicks
        Moderator

          All Mayfly logger board from version 0.3 to the current v1.1 (and including your v0.5v) have a separate 3.3v regulator that powers external sensors, and also the 5v boost circuit.  This secondary 3.3v regulator is turned on or off by controlling pin D22.  When the regulator is turned on, a red LED in the lower left corner of the Mayfly should turn on.  There’s a possibility that perhaps the sensor died and damaged the 3v (or 5v) output circuitry on your Mayfly.  Do you see the red LED (labeled LED3 on the Mayfly v0.5b) turning on during samples?  Have you tried putting a voltmeter across the ground (GND) and V pins of the SDI12 grove jack during readings to see what voltage you’re sending to the sensor?  You can set the voltage selection jumpers to either 3.3v or 5v for the Hydros21 sensor, it should work with either.

          How old is the sketch that you’re using on the v0.5b board?  If your old board was programmed before May 2022, then the old code won’t work with the newer Gen2 Hydros21 sensors because the SDI12 library for communication with the library had to be adjusted for some slight timing changes in how the Mayfly communicates with the sensors.  Your v1.1 board is newer and probably has the updated SDI12 library and therefore is able to successfully talk to the sensor.

        • 2026-06-11 at 1:48 PM #19705
          caroline@wachusett
          Participant

            Hi Shannon,

             

            Thank you for the response. I ran the board attached to my computer in Arduino and the red LED3 light did come on when the sample was run. I put a voltmeter across the GND and V pins and the voltage was 3.3v when the sample was collected. It still was reading -9999 when this test was run (with the old sensor attached to the board).

             

            The sketch on the 0.5b board is older than 2022 so that would explain why the new Gen2 sensor did not work. The sketch on the 1.1 board is from 2025.

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