monitoring moisture level in a soil type substrate

[ghostman11]

i have a substrate similar to soil its made from treated coconut fibre, hemp fibre,wood pulp and a tiny amount of real peat, i need to keep it at a fairly stable moisture level, we normaly only use small amounts so in the past its been done by "eye", the substrate is used for growing a type of segmented worm thats used exstensively in a small niche market. we have been given a 3 year contract to supply theese worms in large quantities so doing everything by eye isnt an option (well not cost effective option). how would i go about measureing moisture content? i have considered using say a pic CMTU modual or maybe CVD. or would it be better to say charge or discharged a capcitor and measure that? i would preferably like to build a solution as it could then easily tie in with existing equipment does anyone have any thoughts on how best to measure a substrate moisture content? it isnt just a question of not letting it dry out etc as theese worms are pretty fussy about there living conditions

 

[Sceadwian]

How about something **broken link removed**?
Cheap as can be, would be easy to calibrate, I'm thinking you'd have to replace the sensors every few months though, given the cost of the sensors though this isn't really an issue. It would be trivial to create a jig to get very consistent sensor construction after a little playing around with.

 

[ghostman11]

i dont realy understand why he used the gypsum? why didnt he just use a etched pcb board? i will have to see how inert gypsum is before i try something like that out, i dont want anything that will alter the pH of the substrate.

 

[ghostman11]

the more i think about it the i wonder is the answer isnt more simple than i first thought. i might try a few things out over the weekend if i get time. trouble is i dont actualy know very much about substrates and there electrical charateristics, i am thinking a pcb etched strip with thin traces close together in a kind of looped pattern forming the 'sensor' have the 'sensor' a set width of 1cm or so and use it to form one half of a voltage divider then use a high accuracy resistor (0.1%) as the other side of the divider and just read it from a ADC on a pic. My main problem is one of scale, at present theese things are grown in a small incubator that sits on a desk in my biology lab, they are housed in a small plastic container with a lid and i probaly add water by fine mist spray about every 3 days.
Now the problem with scaleing it all up to the size i need is the substrate containers have been custom made by a factory that normaly makes ABS water tanks and they are approx 7 meters long and 2 meters wide they also dont have lids! the worms themselves grow to a max of 4mm i need only the male worms wich grow to a max of 3mm. i am mainly concerned with evaporation loss etc. oh well time to try a few things out

 

[Sceadwian]

The conduction level between traces of a PCB with a given moisture level can't be known, PH for one will destroy the reading, not to mention total dissolved solids or metal content.

Gypsum is used because it will equalize with ambient moisture levels naturally. It will will lose it's accuracy over time as minerals build up in the gypsum making it harder for moisture to penetrate and release naturally.

You will not get .1% accuracy outside of lab quality equipment, meaning lab quality cost. It's not as simple a thing as you think. You can define moisture very neatly in a few words, how it's measured and it's effects on dynamic systems is totally different.

For containers as large as you're saying you'll need multiple sensors, there is no way to detect the moisture content of the tank as a whole practically from a single point.

 

[Sceadwian]

If you want high speed accuracy and resolution, this might work.
https://www.vegetronix.com/Products/VH400/

 

[ghostman11]

i had figured on a sensor every 18" or so, i had also thought of something along the lines of a TDS type monitor as the water composistion is known the water is purified through a RO unit then minerals etc are added back to it using a comercial product normaly used for aquariums. the Ph of the substrate is also known and is kept constant at around 6.6 -6.7 (idealy) the last link looks promising they look fast and accurate, as for accurate resistors that isnt a problem i have more than enough
I might buy in a few of those sensors and give them a try. the last tunnel was erected today and by the end of the weekend the bench's etc will be in place, i hope to have the light units fitted by wednesday and power to the units friday, that should then give me 6 full weeks before i need to 'seed' the substrate, so i guess at a push i have a little over 6 weeks to get this sorted. i have a fairly good budget for this (for a change) and i got a realy good discount for the tunnels in the end as they were meant to be black powder coated metal tubes for the frames and they came white! so the company that suplied and erected them gave me a good discount rather than come and take them down again and replace , the stupid thing is i picked the colour at random and didnt notice it was wrong! it was one the guys that put them up that noticed lol

 

[ghostman11]

i wonder how those probes actualy work?? anyone got any idea's? its not that i want to copy them i am just curious how its done

 

[Sceadwian]

The plaster ones? It explains everything on the page. Gypsum will naturally equalize to ambient moisture because it's porous, it's conductance changes with moisture content, so the electrodes are treated as a variable resistance. You could measure it with a meter or use a capacitor charge/discharge time on a micro controller to measure it.

Calibration would be as simple as determining what the sensor value is when your eye tells you the soil is good and when it's bad.

 

[Wp100]

Hi,

Had been considering buying one of these probes for my greenhouse, though no idea how effective they are, no micro really needed, but suppose you could connect a switch up to them ..?
**broken link removed**

Re your active probes, have you tested the worms on them - if they are as you describe sensitive beasties they might react against the electrical current, small to us but possibly dangerous to them ..??

From a backup point of view, if you are using a sealed enviroment, then perhaps a humidity sensor like the DHT11 or DHT22 might act as a fail safe incase the soil probes / wires corrode.

 

[Sceadwian]

Those don't measure moisture content Wp100, they just provide a constant source of moisture, if controlling the moisture in the substrate is as important as ghostman says he'll want to monitor the level somehow to control the watering system.

The current should only be flowing between the probe tips especially if the sensors are placed several feet apart so there will be no current outside of the unit.

Humidity sensors measure air moisture, air and soil moisture will not track directly related to one another, so humidity sensors won't be of much use.

Outside of direct conduction like the Gypsum sensors there are RF units that offer instant readout and high accuracy but they're EXTREMELY expensive.

 

[ghostman11]

the poly tunnels are having SHT75 sensors in them (3 in each) the containers are open not closed. i have found a sensor and sent off a email i cant find the link at the moment but soon as i find it again i will post it next job is too work out REDOX probes and Ph probes

 

[tauro0221]

Hi,
Check this site they explained how to read moisture and how to do it.

https://gardenbot.org/howTo/soilMoisture/#The_local_circuit_-_simple_voltage