@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.
https://www.envirodiy.org/comparison-temperature-dependency-in-situ-lt500-and-keller-nano-level/
https://www.envirodiy.org/measuring-low-water-in-streams-accurately/
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/
Welcome to EnviroDIY, a community for do-it-yourself environmental science and monitoring. EnviroDIY is part of 
