The moisture measurement tool shown is the same ones sold for other projects and I have used them. The problem faced with them was that they wear down over time fairly quickly. I always liked the idea of the ones that use to copper rods with a current drop measured through the soil. But those too will wear down/corrode.
So the tool I thought of would be two copper (or whatever) probes that are coated in hard plastic. The metal and plastic would be crimped at different intervals and when snapped off it would expose new metal each time. The tool holder would have a form fitted section at the bottom that locks onto the ends when pushed into.
The arm would then perform a twist or sideways movement to snap the old leads off (or the locking process could shear it off or the tool could just scrap off an outer layer) which would reveal a new fresh set of leads (think about how one of the old push through pencils fit into each other, like that if the lead was attached at the center)
The arm would then hit another lever that would reset the tool catch and release the old end to fall down into a bin.
This is someone building there own version. This video also shows that the capacitive sensor gives more information. http://www.mait.com.au/products/aquaflex-soil-moisture-sensors-soil-moisture-probes/
This is one of the commercial versions people deploy on farms. There is another commercial one where you manually walk up to a pre made hole and shove the capactive sensor up and down to get map of the water profile in the soil.
Basically cheep style of soil moisture sensor being resister because it does not need calibration but it also has limited range of reading. Capacitive Soil sensors reading vary based on your soil mix so need calibration but they can be built like a absolute tank as there is no need for direct contact between any of the metal parts and the soil. Out casing can be made replaceable particularly thinking out of casing on a capacitive sensors contain no electronics. Yes a Capacitive soil sensor will give you exact reading once calibrated on soil percentage of moisture.
Really in my mind there is no point attempting to work around resistive base soil moisture sensors when you can use capacitive instead and avoid the issues.
bagginsdada basically you did a lot of good thinking that might be usable else where but there is more issues to putting a probe in soil and it corroding than what has been considered as well. http://www.coppersulfatecrystals.com/killroots.html
Yes copper sulfate can be produced with copper reacting with soil. So a cheep made resistance based sensor can be contaminating your complete bed with light grade plant killer as it corrodes. Lot of resistance based soil moisture sensors are toys should not be used for anything serous around plants. The only resistance based soil sensors that are not toys are like the gold plated resistance sensors(corrosion resistant coating) they are half the price as a capacitive pre-built but even those are not built to be shoved in and out instead designed as insert and leave because inserting and removing all the time will remove the coating. Insert and remove soil moisture sensors that last are all capacitive of some form. Yes good resistive might be used at a capacitive calibration point.
I see the problem with the soil moisture sensor is incorrectly cost cutting sometimes things are cheap for a reason.
Great reply with awesome information You converted me completely. Had I known about the alternatives I would have never brought it up.
Should there be a running list of items that should be used once somebody wants to implement them?
Fore example without digging through all of the information on the site, I started that thread because of the sensor I saw in the video. But suppose there was an area with the sections of farmbot laid out and talking points on each section, Then under soil moisture sensor there would be both (or all) options with pros and cons.