Very interesting – I like that conductivity measurement port up from the depth measurement port – out of the mud so to speak. It looks to be at least 30mm up the sensor column. And that’s a difficult mechanical feat. I wonder where you have a quote from. I’ve got Keller sensors from https://www.instrumart.com/brands/1051/keller – but they aren’t showing this model.
I would say the short answer is RS485 and a working +12V (IHMO boosted from 4.0V) ~ as Shannon was saying . Partly it depends on the distance from the measurement place to the Mayfly Enclosure. 36XiW-CTD RS485 protocol identifies 1,400m, which can go through intermediate wiring connection points. Mayfly SDI-12 is unspecified
I’ve implemented the Keller RS485 protocol ~ See KellerXxxLevel.h on https://github.com/EnviroDIY/ModularSensors/tree/master/src/sensors and used it with a +12v boosted from +4V.
I’d be interested in helping, and would need one of the instruments, but in the end its new development for ModularSensors and would have some risk until the first message had been successfully polled for.
I found the data sheet at https://keller-druck.com/en/products/level-probes/multi-parameter-probes/series-36xiw-ctd
Practically Mayfly 1.0A3 doesn’t have a working +12V, and hopefully the new version of Mayfly 1.x will have a power specification to the +12V so it can be matched to what the Instrument requires. https://github.com/EnviroDIY/EnviroDIY_Mayfly_Logger/issues/33
Hopefully Shannon’s RS485 will have some visibility soon – https://www.envirodiy.org/topic/rs485-schematic/ and if it all works, its a very nice combination.
However there are two other RS485’s board with boost to +12V, one of which I’ve implemented https://github.com/EnviroDIY/Mayfly-Modbus-Wing/tree/master/knh002-MayflyWingShield
A perspective, from working with Insitu LT500, the SDI-12 implemented on the Mayfly is a subset of the SDI-12 Specification. https://sdi-12.org/ . That’s technical speak for saying you’ll have to try it to see if it works, for an instrument range and for each instrument thereafter.
As Shannon said a level shifter is needed – from out-of-spec 3.3V to specified 5V – and there has been a lot of discussion about the “Half-duplex” level shifter elsewhere, which with the SDI-12 software is complex to get right.
The Mayfly software implementation for SDI-12 protocol doesn’t include the CRC option, and the RS485 version has that implicitly built into. I’ve been looking at this, https://github.com/neilh10/ModularSensors/issues/68 and got a version of hardware to test it against https://vegetronix.com/Products/SDI-12-Sensor-Translator/ (its lower cost than an LT500 for regular testing.)
The Mayfly SDI12 subset is not protected from voltage (lightening, ESD) as per the recommendation in the spec.
As ever, hope this isn’t too much info, The 36XiW-CTD looks very interesting . I’m happy to chat on 707.780.1569 .
Also I’ve listed it at https://github.com/neilh10/ModularSensors/issues/100