I look at the measurement accuracy, and vented depth sensors. Especially at lower water levels vented depth sensors are I believe the most accurate.
Good measurement accuracy starts at about 0.25% of range and optionally can be of 0.1%
I’ve been using the Keller Acculevel for ranges over 10′, and characterizing the Keller Nanolevel for ranges under 10′ – both of these use a Modbus interface – RS485 physical driver and excitation voltage of about 12V.
The accuracy including temperature compensation can be selected to be 0.1% – ie for 10’range is +/-0.01′(+/3mm) – across temperature variations. That is if you have a smaller temperature range the accuracy can be better than 0.1%
https://www.instrumart.com/products/33962/keller-acculevel-submersible-level-transmitter (Start ~$527)
However if accuracy is important, you have to dig a bit deeper in to the specification, I can explain if interested.
The do have a Keller Digilevel SDI-12 which has the same spec – but haven’t seen anybody interface to it. Shouldn’t be too hard I would think, but there isn’t much value to their SDI-12 as it still requires 6V excitation.
We initially used the Keller LevelGage sensors, but in streams, they started failing after a couple of years and where replaced by Acculevel then Insitu LT500.
The hydrologists I work with have good experience with Insitu LT500 series for accuracy (0.1%FS and 0.01%FS) and stability. They are expensive (base $2K+) and are standalone, they have battery and loggers, the cables are highly engineered devices and cables. Recently I’ve had two of the pig-tail connectors fail (at$350each). There isn’t a ModularSensors library for them yet.
I’ve blogged about accuracy on sensors (but not specifically the Keller AccuLevel).
I recently got a quote for the CTD-10 (10m or 30′ – they only do the 10m now) with a fixed accuracy at 20C of 0.05% Full Scale or +-/5mm and no statement of how it varies with temperature (called temperature compensation).
UpdateJan9th: I got a reply from firstname.lastname@example.org “We typically evaluate this from 0 to 50C and we see somewhere around +/- 2 cm in that range. It does vary sensor-to-sensor and some sensors have lower sensitivity.”
So practically they are saying temperature compensation is challenging, and they could have the readings varied by ±25mm or 0.25% of scale – and they wouldn’t take the instrument back.
https://www.metergroup.com/environment/products/hydros-21-water-level-monitoring/ Quote $491 per sensor and with a 10-meter cable.
The CTD-10’s value seems to be its widely used in ModularSensors, and can be interfaced to simply with the base Mayfly, though I don’t have any experience with it.
Hope that’s useful, and not too much info!! There are a few people using Acculevels, so if you have any other questions, maybe you can describe the expected site and also the accuracy you are aiming for 🙂