Need a Moisture transmitter signal

[piscis]

The Moisture transmitter for my next project will take 6 weeks to arrive and I will like to begin the ladder logic (AD 260CPU) as soon as possible.



Does anyone here know to build a low cost Moisture transmitter? The project calls for checking the moisture of 15 blocks of an OASIS (the water absorbing foam used by florists)



I need to monitor its moisture content as the Oasis dries up due to the plant absorbing the water in it. An irrigation Pump must be turned on for 10 seconds each time a moisture level is reached.



Any help will be highly appreciated. I have a 4-20ma simulator but I would like to use the real thing or at least as close as possible.

How can I build a Moisture transmitter from Radio Shack parts and built one?

Any ideas?

Thanks

 

[10baseT]

I was always under the impression with that green stuff that you could keep a certain level of water (say 5 - 10mm ) , and it would suck (capillary) the water to give the correct saturation - however , this may be a load of cobblers !

Well , you could always try a pair of electrodes and a resistor bridge to generate an analogue signal , or if it was just the green stuff you wanted to check , you could do by weight :- (wet weight- dry weight)/wet weight for WWB calculations .

I seem to recall in the dim and distant past having a project like this - let me take a look .

 

[stasis]

you could try moist tech, they make products just for sensing moisture

http://www.moisttech.com/

but since you indicated you have oneordered...what are the specs of the unit?

 

[piscis]

Thanks guys I found this one for $45-


http://secure.sciencecompany.com/Davis-Soil-Moisture-Sensor-P6623C592.aspx

 

[juice76]

Thanks guys I found this one for $45-


http://secure.sciencecompany.com/Davis-Soil-Moisture-Sensor-P6623C592.aspx

That's a special order item that takes a minimum of 2 weeks. Also, this is just the sensor which must be connect to a transmitter - more $$. If I was just only concerned with writing a functional PLC program, I would use a 4-20mA simulator.

But if you have to read the moisture and can't wait, I once built a simple moisture meter using a DC power supply, a couple of electrodes and a resistor. This is a basic voltage divider circuit and the PLC was connected to read the voltage drop across the electrodes - (0-10 volts). The voltage reading has to be corelated to some real world moisture value. In my case, I had our lab do a bake and weigh moisture analysis so I establish a volts per moisture % ratio. This wasn't highly accurate, but it served my purpose.