ac half-bridge datalogger instruction [Archive]

premelec

29-01-2006, 02:23

The AC is necessary to prevent polarization - in order to log AC output on the PICAXE - you need to change the output signal to DC proportionate to the AC. I'm not sure what the AC exictation level is on the probe - A precision AC to DC converter can be made with an operational amplifier and diode.

If there is high AC voltage [relative to .8 volt diode drop] you can use just diodes to rectify the AC to a DC signal.

There are gypsum block soil humidity sensors as well. You might check irrigation supply sources.

So the short answer is there are ways to do it and it would take some extra signal conditioning circuitry rather than feeding the sensor signal directly to the PICAXE.

christree

29-01-2006, 13:49

Can you provide an example schematic or link to web site for an operational amplifier + diode to change ac to dc.

Does the picaxe need to output ac to the probe or just receive ac from the probe?

I am new at electronics and I'm learning from example.

Thanks,
Chris

christree

29-01-2006, 14:07

The link below shows an example of how to connect the watermark sensor to a datalogger
http://www.emesystems.com/pdfs/SMX.pdf

Do you think this devise would work with the picaxe-08 or is there a simpler devise that I can make that will work?

I have used the watermark sensor using a purchased datalogger and the sensor is superior over others. It lasts for years in the soil and is reliable.
________
Wendie 99 (http://www.lovelywendie99.com/)

BeanieBots

29-01-2006, 17:50

Are you trying to just use just the probe or do you have the control electronics as well?
If you are using the control electronics, then the output depends on which type you have. It can be either a voltage or a frequency. Both can be read by the PICAXE but you must know which type first.
If you do not have the control electronics, then you have to build some before the PICAXE can be used.
As mentioned earlier by premelec, the probe must be driven by an AC excitation to prevent errosion of the electrodes. This AC signal must also be isolated from earth to prevent leakage currents. (not very easy for an electronics newbee). Again, as mentioned earlier, you will need to use a precision rectifier (do a google to get circuit ideas, they are common enough) to convert the signal into something the PICAXE can read.
My advise is to get the drive electronics module which I assume can be purchased from the same source as the probe. The PICAXE can then log the data that comes from it. If you try to build your own and get it wrong, your probe will disintegrate very quickly due to the effect of electrolysis.

christree

29-01-2006, 20:27

Thanks for your suggestions.
I only have the probe. I tried directly interfacing it with my picaxe08. It gave a reading and I can use it to differentiate wet or dry.

Is it possible to alternate the polarity using the picaxe08 to the probe several times a second solve the problem with electolysis?

Thanks,
Chris

________

BeanieBots

30-01-2006, 08:08

I don't know enough about the chemistry to be able to suggest a drive method that will not destroy the probe but at your own risk you could try connecting the probe via resistors between outputs. Your program would need to alternate the outputs in antiphase in a symetrical manner to prevent eletrolysis. I have no idea what frequency you would need to use.
You could then connect an analog input to one end of the probe. When the output on that side of the probe is high, measure the voltage. Depending on the resistor value, you should get a voltage which is inversely proportional to the drive current which is proportional to the conductivity which in turn will be a function of the moisture level.

Edited by - beaniebots on 1/30/2006 7:43:57 AM

Michael 2727

30-01-2006, 09:26

Chris you only have the probe and not the SMX module correct ?

If you try to use the Picaxe only to drive the probe you will eventually damage it.

I have experience in these matters.

You need to connect the probe to an AC supply.
Use say a 5V to 12V AC transformer output via resistors.
Limit the current to less than 2 mA or so
and about 5 to 10 Volts.
Pass one AC line through the probes( first
probe, soil, second probe)
Then both AC lines are taken to a Bridge rectifier.
The output of the bridge has an elctrolitic
capacitor around 10µF 25V across it to limit
the ripple.
This is the voltage you send to the PicAxe.
Play with the resistor values to get as close
to 5V at the output of the bridge rectifier.
You can even make the bridge out of 4 x 1N4148 signal diodes.

Try using 2 x 1500 Ohm resistors on the transformer outputs, as a starting point.

Edited by - Michael 2727 on 1/30/2006 8:29:24 AM

Michael 2727

31-01-2006, 10:48

I'll try this new ASCII Art caper.

<code><pre>

1k5 Ohm
___
Trns |o----o--|___|-------------o o------o-------o
| | | | |
-. ,--- .-. | .-----. | | 10 µF 25V
)|( | | ?? Ohm o-----|~ +|---o +|
)|( | | o-----|~ -|---o ---
-' '--- '-' | '-----' | ---
| | ___ | | |
|o----o-|___|---o o-------o | |
| | o------o-------o
1k5 Ohm | |
| |
| | Probes
o o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) </pre></code>

Admin Edit:
You need to put the artwork between the code tags - see protocol details tab above.

Edited by - Admin on 1/31/2006 10:31:48 AM

christree

31-01-2006, 11:29

Hi Michael:
Does the power source come from the picaxe, i.e. the 12ac comes from the 5v dc via a transformer? To clarify, can you draw a diagram of the circuit you are suggesting?
Thanks again for your help,
Chris

Michael 2727

31-01-2006, 11:35

I tried to draw a circuit, still can't get
the ASCII Art part correct just yet.
I will send you a circuit.

Michael 2727

31-01-2006, 12:05

WOW, just saw the edit above, thanks.

Chris the power needed for the probes comes
from the AC transformer, it could also be used as the picaxe supply using an extra regulator etc.
The output from the Bridge is the input "signal" only to the picaxe input.

How to set it up -
Using the above schematic
The resistor ?? IS WRONGLY POSITIONED, should
be moved across >>> to the other side of the 1.5K resistors.
Whith the probes removed and the line shorted where they were.
Find a value resistor ?? that gives you 5V across
the capacitor on the output of the bridge.
This will be the max voltage you can ever get on the input.
Now having said that you should put a zener (5.1v, 100R)
diode and small value resistor between the
bridge output after the capacitor to prevent
any overshoot, noise or whatever.

You could replace resistor ?? with a 3k to 3k
small audio coupling transformer and reduce
the 1.5k resistors about by half or less
until you get 5V at the bridge output.
Depends what voltage the AC you are using.

BTW that circuit is copyright, so no commercial runs,,~ ;o)

Good luck.

You can mail me if you get stuck.

Michael 2727

31-01-2006, 12:43

<code><pre> 5V >>
1.5K
___ Audio Trns o------o----o--o
Trns o--|___|-o o----------o | | .-.
| | | | Bridge 10µF 25V| | |
-. ,---o o-. ,-o | .-----. | | | |100R
)|( 3K )|( o-----|~ +|---o +| '-'
)|( )|( 3K o-----|~ -|---o --- |
-' '---o o-' '-o | '-----' | --- z
AC | ___ | | | | | A 5.1V
o--|___|-o o---o o---o | | |
| | o------o----o--o
1.5K | |
| | To Picaxe input >>
| | Probes Neg >>
o o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) </pre></code>

christree

01-02-2006, 01:24

Michael, Thanks!

A few more questions, sorry...

Can I use a 9v battery to power the 12v ac transformer? Or should it be a 5v dc to 12v ac transformer?

Is the purpose of the bridge to convert the ac to dc? Can you explain how I can make this out of 4 x 1N4148 signal diodes? Or can I purchase this bridge somewhere?

Thanks again,
Chris

________

Michael 2727

01-02-2006, 07:53

Download this page -
<A href='http://www.allaboutcircuits.com/vol_3/chpt_3/4.html' Target=_Blank>External Web Link</a>
And go to figure 6 "Full-wave rectifier circuit" (Bridge Design).
You can make this using 4 x 1N4148 signal diodes.
If you need a bridge for larger currents
you should use 4x 1N4004 or 1N4007 etc, but
in this case signal diodes will perform better.
Or just buy any (small) or 1A Bridge rectifier,
voltage is not a concern in this case.

You can NOT USE DC on any Transformer.
They only work with AC. If you do you just
end up with a heater or an electro magnet.

Use a Plugpack/Wallpack transformer with
6V to 12V AC output. (240V AC in 6V AC out)

Any DC across the probes will eventually eat
them away due to electrolysis (Google it).
(galvanic errosion, electrolysis)

Read the above web page it explains a lot.

Michael 2727

01-02-2006, 08:27

The AC part of the circuit is only there to
drive the PROBES.
The output from the probes is then converted to DC so you can use it as an
input to send to the picaxe.
If you send any AC to the picaxe you will probably kill it.

The end product of the bridge rectifier should not exceed 5V MAX.
When setting up the voltage the probes
should be removed and a short or link put in their place.
This means that even if the probes get shorted
out the voltage to the picaxe will never go
above 5V.

christree

01-02-2006, 12:52

A ha! Your link and description clarified everything.

The only issue I have left is using the probe in the field where there is no access to a wall plug! How can I generate AC in the field to power the probe?

Thanks,
Chris

BeanieBots

01-02-2006, 16:15

That is why quite a while back on this thread I suggested connecting the probe between two outputs and alternating them to give bidirectional drive. What I don't know if the wave shape is important to probe life or what would be a good frequency to use.

Michael 2727

02-02-2006, 05:07

You need to drive the probes with a true AC
signal, e.g. current goes --->> for half the
cycle then reverses <<--- for the other.
This is difficult to do from a battery, it can be done though.
And I can't supply you with a circuit for this sorry.(try Google maybe)
The frequency does not really matter as long
as the bridge can get a reasonably steady output of DC when rectified.
But any DC component in the signal AC
should be avoided if possible.
Even a current as low as 2mA (regardless of voltage)
will eventually eat away the probes.
BTW these probes can be home made from any
non ferrous metal, copper, brass, aluminium,
stainless steel works well.

andypro

02-02-2006, 05:26

making an AC source isn't that hard really. First thing you need is an inverted voltage. There are many parts to do this, I'll dig somehting up if you cant find anyhting.

Next you need a comparator. Most any dual supply op amp will work. Connect the positive and negative (inverted) voltages to thier respective inputs, run the inverting input to ground, and pulse the other one at the frequency you want the AC to be (can use PWMOUT form picaxe if you want). This will make a square AC waveform.

follow this with a filter...can just be an RC network, designed to work at the frequency of the output of your comparator. This will round off the square waves and give you a relatively good sine wave.

AC couple the sine(ish) wave to the probes, otherwise when you have no picaxe signal (or any other signal) youll get DC at the probes in the form of the negative voltage rail from the comparator.

There. Now you have AC for your probes...and it only takes a few components ;)

--Andy P

Edited by - andypro on 2/2/2006 4:30:48 AM

christree

06-02-2006, 01:29

Thanks for the dc to ac conversion technique. I'll need to research these components and may post a followup reply.

christree

06-02-2006, 15:20

I'm googling for "inverted voltage" and can't seem to find a simple description or method on this.

Would you be able to note some parts as you suggested?
Thanks,

Chris

andypro

06-02-2006, 19:43

You can try the TC962 from microchip or the LT1026 or LT1054 from linear tech. There are many others that other companies make, but these are the only ones I've experience with. Both can be had in a dip-8 package.

Searching for "Inductorless DC-DC converter" or swap "converter" with "inverter" should turn up more devices. Check the major manufacturers, and also the distributors like digikey, mouser, arrow, etc.

--Andy P

andypro

06-02-2006, 20:05

Just realized a problem with the circuit I described...I have to stop thinking when I should be sleeping!! =)

Make sure youve got a potential slightly higher than 0v on the inverting input of the comparator. You dont HAVE to, but you could end up with problems if you dont. Something as low as .1 volts should work...jsut want to keep it from switching around if you dont want it to, and you want to make sure it switches to the negative side of it when needed, rather than staying positive.

--Andy P

christree

06-02-2006, 21:12

Can you explain what an inverted voltage is?

Thanks,
Chris
________

BarryP

07-02-2006, 00:18

Assuming the Device doesn't need much AC current, You might consider driving it off a General Purpose OP amp setup as a wein bridge.
If you power the OpAmp with 4 battery's , you can create an earth/common.
I.E. +3V 0V -3V
Check out the wein bridge here <A href='http://www.interq.or.jp/japan/se-inoue/e_ckt18_2.htm' Target=_Blank>External Web Link</a>

Michael 2727

07-02-2006, 06:38

You will probably need a device, op amp,
oscillator, signal generator, whatever using
a split rail supply e.g. +pos, 0zero, -Neg.

Don't worry too much about the frequency,
anything in the range of 20 to 200Hz should be fine.
And a sine wave or triangle wave would be better than square wave preferably.

And it does not need to deliver much current
either.
Try to keep the current across the probes as
small as possible, under 2mA or 10 x less,
as long as you get enough to charge the
capacitor after the bridge rectifier and
keep it topped up. You should include a
very slight drain to earth on the picaxe
input from the capacitor so it is not just
measuring a "held charge" on the capacitor.

I can't suggest any circuits or approach
you should take, although I had planned to
develope my own portable device later on
this year, ( bit busy at the moment).

It took me 2 years to develope my 240v model
and iron out all the bugs and pitfalls to
get it working exactly how I heeded it to.

Measuring soil moisture content is a
science in itself, there are 100 different
ways to do it.
Then when you work all that out there is soil
salinity, PH levels, mineral content and structure to take into account also.
As far as I know there is no single device
on the market that can tell you everything
accurately and all at once, they are getting
close though.(and you will need a very Fat sugar daddy to buy one).

Good luck.

christree

07-02-2006, 16:57

Thanks.
Earlier in this thread was discussion on using two outputs from the picaxe, one supplying power to a different lead on the soil moisture probe, thus creating an alternating current. One of the probe's leads would be connected to the analog input while the other would be connected to the -ve rail. (The wiring seems odd, am I right?)

The picaxe would be programmed to switch between outputs to create a cycle of 60x/second. I'm assuming this would prevent corrosion of the probes.

Will this truly work or is the more complex approach to generating AC as discussed above the only way to get the probe working without corrosion?

By the way, I have been using the watermark soil moisture probe daily for the last year with the manufacturer's data logger and it works excellent and paints a true picture of soil moisture conditions. I highly recommend this probe.

I am trying to make my own datalogger since these can be quite expensive and I would like to have a number of them in the field, hence this extensive thread.

Any further advice would be appreciated.

Thanks,
Chris

BeanieBots

07-02-2006, 18:44

There is no negative rail needed with that approach and also no rectification because the PICAXE generates the AC and hence can synch the ADC readings.
It works as follows:-
Connect a resistor in series with the probe, I have no idea what value should be used but ideally for the ADC it should be about half the resistance of the probe when it is measuring mid-point moisture. It may need to be higher than that to keep probe current down but that will decrease the effective range of the ADC.
The probe lead that does not have the resistor is connected to one PICAXE output (let's say 0). The resistor is then connected to another output (let's say 1).
The other end of the resistor (the end connected to the probe) is then connected to an ADC input.
A program then needs to be written which will drive the outputs at whatever frequency is decided upon. They should be driven in antiphase. When one is high, the other is low and so on. Thus the AC drive is created.
When output 1 is high, output 0 is low and the probe has current flowing through it via the resistor. When the outputs are in this condition, the ADC is read.
If the probe is completely dry, it will have extremely high resistance and there will be no volt drop across the resistor. Thus, the ADC will read full scale.
If the probe is then moved to wet soil, its resistance will reduce and current will flow. Thus a voltage will be developed across the resistor making the end connected to the ADC lower than the voltage on the high output. The ADC will read a LOWER value.

The difficult bit will be making the software generate a symetrical signal to prevent probe errosion. It might be worth putting a high value non-polar capacitor in series with the other leg of the probe to prevent any net DC from getting through.

A program like this would do the job

main:
high 0
low 1
pause 20
low 0
high 1
readADC 0,b0
'code to do some thing with reading here
pause 19 'reduce this value depending on time taken for your code.
goto main

I cannot gaurantee that your probe will not errode using this method but it will last a lot longer than trying to use DC.

christree

07-02-2006, 19:53

Thanks,
I'll try that and get back soon with the results.

Just a note, I only anticipate reading the probe a few times a day with each read taking a few milliseconds. I wonder if probe corrosion is a huge issue at such a low use rate?

Chris

BeanieBots

07-02-2006, 20:25

I'm sure it will be fine at such a low useage but no gaurantee. I'd take a few readings and average the results then set both outputs to low.
Probably worth fitting the cap just in case.
Only time will tell for sure.

bobrayner

07-02-2006, 22:40

I think the picaxe method will only produce a signal from zero volts to +5 volts (up and down DC). Cathodic protection of the probe will probably require a true AC signal, ie; one leg to go negative while the other goes positive. Passing the up and down DC through a capacitor removes the DC component. By using a resistor to ground and using the correct time constant a good approximation of a sine wave is possible. Capacitor value is important as if it is too small you will end up with a sting of positive and negative going spikes.
Regards BobR

Michael 2727

08-02-2006, 06:33

Bob you are right.
I think people are missing the point.

AC, True AC , not a DC Biased sine/triangle/square wave, whatever.
Must have equal parts where the current flows in
one direction then travels in the reverse
direction on alternate halves of the cycle.

POS >>>>>>>>>>>>>
ZERO ----------- 0 volts
NEG <<<<<<<<<<<<<

Google AC and DC current then DC biased sine wave.

<A href='http://www.ibiblio.org/obp/electricCircuits/AC/AC_1.html ' Target=_Blank>External Web Link</a> AC.
<A href='http://www.ibiblio.org/obp/electricCircuits/AC/AC_1.html' Target=_Blank>External Web Link</a> Generating sine waves.

A DC biased sine wave is NOT a true AC signal, as it never goes below negative or above positive, depending which way you bias it.

BeanieBots

08-02-2006, 08:25

bobrayner, Michael 2727,
Shame on you both. There is NO net DC with the method described as long as the wave is symetrical. With a cap in series there CANNOT be any net DC.
And what exactly is 0v, positive and negative on a floating supply? 0v with respect to what? The probe sees current in one direction for a period of time and then the same current in the opposite direction for the same period of time. Readings are only taken during one phase so they are always the same polarity.
Two outputs are used. The probe will see 10v TRUE AC when the PICAXE is powered at 5v.

Edited by - beaniebots on 2/8/2006 7:28:46 AM

Michael 2727

08-02-2006, 11:43

Beaniebots not so much shame.
In a glass of water on the table ok.
Place the probes in the backyard for 5 years
is a different story.
I will let you know in 3 years the effect of
this, when I have some more data.

There are also strange capacitive effects
with DC build up in the soil which can give
interesting readings. As I said, this field
is a science in its own right.

BeanieBots

08-02-2006, 12:38

Michael 2727, I agree that it is still possible for the electrodes to errode because I have absolutely no idea about the chemistry and how reversable the electrolysis effect is. My point relates purely to the production of an AC signal that will NOT send any net DC through the probe. I think we all agree that the probe must be driven with AC. The described circuit will do just that. (50% duty square wave).
The circuit must NOT have any connection to ground other than the probe or DC could flow. It was described earlier as needing to be battery powered in the field and hence could not use AC from a transformer.
I also agree that "on the bench" and "in the field" are very different things. Only time will tell and even then it could be more about the probe material and soil constitution than whether or not errosion was due to sqaure wave drive rather than sine wave.

Michael 2727

08-02-2006, 13:58

So depending on what you want to use it for -

For short duration sampling, almost anything will work
even DC. (will drift over a few minutes, and eat you probes in no time)

For a more serious reading, stable meter, an oscillator/op amp approach will work fine
for short term readings.

If you want something to last for many years
in the ground without destroying the probes
use an AC plugpack/wallpack and stainless
steel probes. This is by far the simplest
method to use and the cheapest.

The front end (Probe and Driver stage) will
make or break the design and fine tuning it
may take many hours of experimentation.

Keep the probe current as low as possible.

Those "cheapie" under $5.00 moisture meters
mounted on a probe (VU Meter style ones)are
only a simple electro/mechanical meter which
have a zinc tip which reacts with the probe
shaft to form a battery in the presence of
moisture, so in fact a cup of coffee with
sugar in it should appear wetter than water,
due to the acid in suar being a better
conductor and contributing to the battery effect.

I once spoke to an irrigation company owner,
they tried for years to design their own
moisture meter, they could never get the probes right, even to the point of having
Gold Plated Probes made up. They had DC in
the circuit, it was doomed to fail.
They gave up in the end.

A picaxe using the readadc/10 command should
give a reasonable resolution which can be
sent to either a PC, LCD or even the new 7
seg display I have, I should give it a try
for curiousity sake.

Calibration is a whole nuther story.
I have a headache just thinking about it.

Anyway it will keep you off the streets for
at least 10 minutes, have fun.

BeanieBots

08-02-2006, 15:53

Michael 2727, I thought the zinc tipped ones with VU meter and no power source were meant to be pH meters. I've seen something like that in Garden Centers. I'm quite sure the scale was 1 to 14 which sounded to me like pH rather than moisture.

I've got some small stainless rods. I'm damn well going to build one these things and pass DC through it just to see how quick they errode;) (I'll try bicarb and vinegar solutions as well)

Dippy

08-02-2006, 18:38

I've been reading this thread with great interest hoping the gurus would come to agreement.

As I can't afford a neutron moisture meter I was wondering about design too.

So, I've got some questions, which bother me;
1. Surely any contact type soil sensor is going to have problems in the soil over a perdiod of time? Not only the corrosive effects of electrolytes but the dry/wet/dry cycle might affect the 'contact' of probe to soil? (i.e. in fine soil it might lose contact?)

2. Could you alternate the pos/neg current by switching and perhaps take an average impedance in each direction? And also reducing electrolytic corrosion?
As someone said quite rightly, pos and neg are relative.
3. Could you use a non-contact inductive method using the conductive charactersitics of the soil? (Nothing to wear out corrode/erode take your pick).
4. As mentioned just now there's the galvanic effect too. Does this mean you nead to pass enough currect through a contact sensor before you can get a sensible impedance measurement. Cf. bias on a photodiode to 'overcome' the p/e effect.
And perhaps averaging as in 2 above would indicate any effect? Depends on soil of course.
5. Would the application of a phase sensitive recifier in an AC driven circuit provide benfits? (That'll get Google working)

Any ideas?

BeanieBots

08-02-2006, 19:09

Hi Dippy, welcome to this thread.
I hope christree doesn't mind it being somewhat highjacked.
With any measuring device there will always some compromise and effects of other parameters that will spoil the parameter you are trying to measure. I like your idea of non-contact perhaps using magnetic resonance or similar but what happens if your probe is near a ferrous deposit?
Anyway, Michael 2727 is the only one who appears to have practical experience of using these probes so hopefully he can answer some questions that have been bugging me while I prepare to try this out myself.
How big/long should the probes be?
How far apart?
Could a single (portable) probe be realistically be made from stainless steel tube. I'm thinking along the lines of two lengths of 10mm steel tube fixed to a non conductive inner so that the gap between the two peices is constant. The whole assembly is then pushed into the ground when you want to take a reading and removed and wiped clean afterwards.
It doesn't sound like it would be very consistant but I'm prepared to give it a go.
Trouble is, I wouldn't know if I had an inconsistant result or the moisture had really changed. Maybe a "control" probe as well?

Dippy

08-02-2006, 21:15

Good point BB.
I was only daydreaming about inductive methods as I remember from metal detecting that one of things that upset them was 'ground effects' which was the effect of damp mineral bearing soils upsetting the balance. I was wondering if an 'unwanted' property could be used?
I also played about with soil moisture measuring several years ago. Anyway, results were reasonable but drifted over time. And the thing corroded. I was just hoping someone had a non-contact answer.
I'm afraid neutrons are a bit expensive.

Michael 2727

08-02-2006, 21:17

BeanieBots, those VU meter types, there are
2 that I know of, one for moisture, then
later they came up with the PH version.
That one has an extra electrode from memory,
e.g. Ring, Tip, Sleeve, like a phono plug.
You need 3 electrodes to measure PH in this way, above and below "7" on the PH scale.
Hi Dippy,
In my unit I use 3mm 316 stainless steel rod
which is spaced around 20mm apart (10 to
30mm works well) they are covered with 6mm
standard air line nylon tubing which fits
neatly over the rod, I then have 20mm of
exposed rod at the ends which are the probe
ends(contact points)in the soil.
I have had a set as described in the (audio transformer design) circuit above running
off a standard 24V AC irrigation plugpack,
outside my back door for nearly 2 years now
and except for running them over whith the lawn mower 3 months back, they are as good
as the day I put them in, no corrosion
whatsoever.

In the last few years many companies have millions looking for a better way to crack this nut.
They have capacitance type meters now ( non
contact in a way ) which fit inside a tube
drilled into the soil, they work basically
like an RFID Tag, they use the soil moisture
to increase/decrease the capacitance value of the transmitted RF signal.

There are also now TDR types Time Domain
Reflectometry, which bounce a microwave signal through the soil and compare it
to a fixed wire, basically underground Radar.
And they are all very expensive.

In some cases it can take several weeks to
register any changes in soil moisture
content
The soil seems to breathe or have tides you
would not imagine posible, drawing water up
from down deep like a sponge etc.
A lot happens down there we don't even know
about or realise, and it happens very slowly so we never really notice it.

It is an interesting subject.

Dippy

08-02-2006, 21:32

Phewy!

christree

08-02-2006, 22:20

This is great. I'm glad so many people have joined into this thread.

Just some background info, the watermark soil moisture probe has been tested now for many years where it originated in California. There, everything is irrigated and many watering systems are controled by PLCs connected to these probes. They seem reliable and provide consistent readings. I have 40 in a forest setting right now and it is amazing how closely they change with rain events.

I am currently testing a probe connected to the picaxe, using the picaxe to supply the power to each leg of the probe (my novice and incorrect attempt at AC). I'll let you know what I find in the next couple of days.

There was a device with web link that I noted in one of my earliest replies that can be purchased to connect the watermark moisture probe to a dc power supply and datalogger. The schematic was shown in the web link. It is over my head though right now. Perhaps someone else can interpret it.

Thanks and keep up the discussion.

Chris

BeanieBots

09-02-2006, 08:05

Thanks for the detailed info. This could easily become a very in depth topic.

Michael 2727

09-02-2006, 09:20

Christree don't waste any more money buying
expensive probes.

Stainless Steel wire, Welding filler rod, or
nails will do just as well.

Solder your wires to the SS then waterproof
the joins any way you can, glue, epoxy etc.
Space them 1 inch apart and they will be as
good or better than any bought product.

SS is very hard to solder.
Use a few grains from a block of Sal Ammoniac
or make a solder puddle in a depresion in
a block of it and tin the SSteel there.

Sal Ammoniac Blocks are used to clean Large
copper gas heated plumbers soldreing irons.
Ammonium Chloride.

It eats away the copper so use a crappy iron
if you need to do a lot of work.
Don't use a good electronics iron for this.
(or wash the tip well in hot water afterwards)