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Hydros 21 CTD sensor calibration question

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    • #17348

      Please help to clear up my thinking about these sensors.

      This has probably been asked before, but after searching thru the forum I am not finding  answers to this question.   We have the Hydros 21 CTD sensor.  It looks like this sensor is calibrated at the factory and the sensor reports engineering values to the Mayfly datalogger.

      Is this assumption correct?

      So there is no way to calibrate a sensor int the field?

      If we need to change out a sensor there should be no need to calibrate anything in the Mayfly datalogger?

      Thanks for your help in understanding this device.

    • #17349

      @rogers1313 – its a good “engineering question”. speaking as an electrical engineer and some one that has blogged on understanding traceability,  measurement range, measurement resolution. measurement accuracy and  total error band of measurement being different aspects of the  measurement.

      In short the Hydro 21 has calibrated measurements that can be read digitally (SDI-12) with defined accuracy.

      Data sheet for Hydros 21 https://library.metergroup.com/Integrator%20Guide/18468%20HYDROS%2021%20Gen2%20Integrator%20Guide.pdf

      A more complex answer on accuracy is what are you trying to measure accurately?

      See Hydros 21 CTD  ” 3.1 specifications”  and I queried  the manufacturer about some aspects of the specifications (and afterwards the specifications where updated and easier to understand). The nice part of the Hydros 21 CTD is the “low cost”, SDI-12 interface, combined  Depth, Temperature and ElectricalConductivity in one physical unit.

      Water Depth units are mm an accuracy is ±0.05% of full scale at 20 °C, for full scale of 10,000mm that is +/-5mm @ 20C

      However there is the real world “NOTE: Depth measurement accuracy assumes no abrupt temperature variations.”

      No abrupt temperature variations is not defined  – and I asked them about it – is that a 1C shift in 15minute period, or 10C shift in 15minute period. If you are measuring the last 100mm of a stream drying out in a mountain pool, with say a 10C temperature diurnal change then there may be a temperature dependency on the measurement. If you are measuring the way a water body peaks at 10m (33ft), from 1m(3ft) or well water depth changes its probably good accuracy.  The engineering challenge for this type of water depth sensor is that the pressure sensor that converts  water depth to an electrical parameter is  highly optimized resistor, which has a large temperature coefficient. Generally speaking for accuracy in the component sensor, its usually stated as between 10% and 90% of range – but this is complicated engineering. Sufficient to say again, if you want to measure the last 100mm of a stream drying out which we do in California accuracy is challenging.

      Temperature is one of simples aspects of measurement and reasonable accuracy at +/1C and resolution at +/0.1C. Units are C

      EC is stated as accuracy  ±0.01 dS/m or ±10%, whichever is greater, Resolution 0.001 dS/m

      There are some users on this board who have extensive experience with CTD21, however the people I work with in drought areas with large temperature changes don’t use it, and I’ve only looked at the specs, never tested a unit.

      When it comes to sensors I have tested for accuracy and temperature dependency a number of low cost sensors, and also Keller Levelgage (lousy temperature dependency)  Keller Acculevel (better ) and the gold standard that TU Oakland uses Insitu LT500. I’ve also returned some LT500 to the manufacturer for further calibration of the temperature coefficient as it was outside their stated range.



      The LT500 has the ability to define the units, as well as few other parameters.

      Hope that is useful – BTW I’ve also started a thread on using Scaling Modular Sensors https://www.envirodiy.org/topic/geographically-scaling-modular-sensors/

    • #17351

      <div class=”bbp-reply-author”> @rogers1313</div>
      <div>I have used the Hydros-21 for several years now and can add a bit to what Neil has mentioned. I also asked the engineers at Meter Group about temperature compensation and found their response unhelpful. My Hydros21 sensors output water depth that is highly affected by temperature (I can be more specific if you like). I run most of my Hydros21 sensors side-by-side with HOBO sensors (made by Onset Computer Corp) and I have come to the conclusion that a post-processing temperature compensation results in a much tighter match with the HOBO output and nearly eliminates the sinuosity imposed by the diurnal temperature fluctuations.   If you are running these in small streams where small change in water level can result in a large percent increase in flow, then these fluctuations can really make a difference in the measured discharge. As for conductivity, we clean off the electrodes regularly and perform corrections for drift due to fouling and apply offsets to match regularly calibrated YSI sensors which provide sanity checks a few times per month. We find the Hydros21 performs fairly well with conductivity so long as it is cleaned regularly.</div>

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