Conductivity Sensor Thanks you!
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Thanks to all that had input on my problem. Tracy's ideas have help
allot. I put it all together and am allot closer to what I need.
There still seems to be some unusual readings at times, but I'm
looking into it.
If I take reading in a small plastic drinking glass all is very
stable. But, the tank I want to do the readings in is a larger say 20
gal tank (15" x 24" x 10") with nothing else in it but one mag drive
pump that sits in the corner not running 95% of the time, otherwise
there is nothing in it. When the same probe is in this tank the
readings wander from time to time, no flow in the tank at the time.
I'll be tacking down the cause in the next few days I hope.
One thing of note that I found odd. During some testing I changed the
length of the exposed part of the probes, the probes are 1/8" rods
with a type of heatshrink tube over them to keep solution off all but
the 1/2 end in the water, that way level doesn't mater. When I
changed the amount exposed from 1/2" to 1" I would have thought the
reading would have gotten more sensitive, but it didn't, it was less.
I'm not sure I understand why that would be.
Anyway thanks again for everyone helping.
Mike
>Hi Mike,
>
>Include two capacitors in series with the electrodes, as shown here:
> http://www.emesys.com/OL2mhos.htm
>The article explains that the capacitors will give better galvanic
>isolation and more certainty there will be no DC current through the
>electrodes. (I have some circuit boards for that circuit--contact me
>off list if you are interested in obtaining one). For simplicity, the
>Earth Measurements units did not include the capacitors, but in a
>revision I would, because think it is really important in practice.
>
>Also, try a carbon electrode instead of stainless steel. Either coat
>the stainless steel _heavily_ with carbon black by holding it in a
>candle flame, or use a fat pencil lead for the electrode. Carbon has
>a lower electrochemical activation energy than iron and fewer
>electrochemical interactions.
>
>The configuration of two parallel rods is sensitive to nearby
>obstacles, such as container walls or materials like glass or roots.
>Maybe encase the two electrodes inside a porous shield? Is it
>possible that there is not enough flow to circulate the solution
>quickly throughout the growing medium that houses the plant's roots?
>Have you done these measurements "in vitro" too, without the growing
>medium?
>
>It sounds like the influence of the motor is not electrical pickup,
>but something related to the flow of the solution.
>
> -- best regards
> Tracy Allen
> electronically monitored ecosystems
> http://www.emesystems.com
> mailto:tracy@e...
>
>
>>Has anyone had any luck working with conductivity of hydroponic solutions?
>>
>>I'm working on a doser for my home system. The probe I made is two
>>1/8" stainless steel rods covered in a way so that only about 3/4" of
>>each is exposed to the solution so that level doesn't matter. The two
>>rods are about 1/2" apart. The probe is used as a resistor across
>>pins 2 and 3 of a 555 timer. Much like is done in the Stamp Earth
>>Measurements book (pg 108-116). The stamp counts the pulses from the
>>555.
>>
>>I also have a LM334 temp chip in the solution to track temp and
>>compensate for the change in conductivity with temp. The temp readout
>>works great.
>>
>>I seem to have two problems with the conductivity part though.
>>
>>1. The measurements of the change of conductivity don't seem to
>>change enough that I can get good results. As I change the
>>conductivity the reading may go up, but not by much. If I raise it by
>>say 20% the number of pulses I read may change by only 30 or 40
>>pulses out of 15,000. Too close to the noise level of the normal
>>world to really be sure it changed.
>>
>>2. I have a 110V pump in the tank that has the probe in it. When it
>>runs the counts on the probe slowly drops, then slowly raise after
>>it turns off. The pumps run 10 times a day, so I need to filter this
> >out.
>>
>>If anyone has any ideas of better ways to run and sense the probe or
>>way to filter the noise I would love to hear.
>>
>>Thanks for you help.
>>
>>Mike
>
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>Subject and Body of the message will be ignored.
>
>
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allot. I put it all together and am allot closer to what I need.
There still seems to be some unusual readings at times, but I'm
looking into it.
If I take reading in a small plastic drinking glass all is very
stable. But, the tank I want to do the readings in is a larger say 20
gal tank (15" x 24" x 10") with nothing else in it but one mag drive
pump that sits in the corner not running 95% of the time, otherwise
there is nothing in it. When the same probe is in this tank the
readings wander from time to time, no flow in the tank at the time.
I'll be tacking down the cause in the next few days I hope.
One thing of note that I found odd. During some testing I changed the
length of the exposed part of the probes, the probes are 1/8" rods
with a type of heatshrink tube over them to keep solution off all but
the 1/2 end in the water, that way level doesn't mater. When I
changed the amount exposed from 1/2" to 1" I would have thought the
reading would have gotten more sensitive, but it didn't, it was less.
I'm not sure I understand why that would be.
Anyway thanks again for everyone helping.
Mike
>Hi Mike,
>
>Include two capacitors in series with the electrodes, as shown here:
> http://www.emesys.com/OL2mhos.htm
>The article explains that the capacitors will give better galvanic
>isolation and more certainty there will be no DC current through the
>electrodes. (I have some circuit boards for that circuit--contact me
>off list if you are interested in obtaining one). For simplicity, the
>Earth Measurements units did not include the capacitors, but in a
>revision I would, because think it is really important in practice.
>
>Also, try a carbon electrode instead of stainless steel. Either coat
>the stainless steel _heavily_ with carbon black by holding it in a
>candle flame, or use a fat pencil lead for the electrode. Carbon has
>a lower electrochemical activation energy than iron and fewer
>electrochemical interactions.
>
>The configuration of two parallel rods is sensitive to nearby
>obstacles, such as container walls or materials like glass or roots.
>Maybe encase the two electrodes inside a porous shield? Is it
>possible that there is not enough flow to circulate the solution
>quickly throughout the growing medium that houses the plant's roots?
>Have you done these measurements "in vitro" too, without the growing
>medium?
>
>It sounds like the influence of the motor is not electrical pickup,
>but something related to the flow of the solution.
>
> -- best regards
> Tracy Allen
> electronically monitored ecosystems
> http://www.emesystems.com
> mailto:tracy@e...
>
>
>>Has anyone had any luck working with conductivity of hydroponic solutions?
>>
>>I'm working on a doser for my home system. The probe I made is two
>>1/8" stainless steel rods covered in a way so that only about 3/4" of
>>each is exposed to the solution so that level doesn't matter. The two
>>rods are about 1/2" apart. The probe is used as a resistor across
>>pins 2 and 3 of a 555 timer. Much like is done in the Stamp Earth
>>Measurements book (pg 108-116). The stamp counts the pulses from the
>>555.
>>
>>I also have a LM334 temp chip in the solution to track temp and
>>compensate for the change in conductivity with temp. The temp readout
>>works great.
>>
>>I seem to have two problems with the conductivity part though.
>>
>>1. The measurements of the change of conductivity don't seem to
>>change enough that I can get good results. As I change the
>>conductivity the reading may go up, but not by much. If I raise it by
>>say 20% the number of pulses I read may change by only 30 or 40
>>pulses out of 15,000. Too close to the noise level of the normal
>>world to really be sure it changed.
>>
>>2. I have a 110V pump in the tank that has the probe in it. When it
>>runs the counts on the probe slowly drops, then slowly raise after
>>it turns off. The pumps run 10 times a day, so I need to filter this
> >out.
>>
>>If anyone has any ideas of better ways to run and sense the probe or
>>way to filter the noise I would love to hear.
>>
>>Thanks for you help.
>>
>>Mike
>
>To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
>from the same email address that you subscribed. Text in the
>Subject and Body of the message will be ignored.
>
>
>Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
Comments
>stable. But, the tank I want to do the readings in is a larger say 20
>gal tank (15" x 24" x 10") with nothing else in it but one mag drive
>pump that sits in the corner not running 95% of the time, otherwise
>there is nothing in it. When the same probe is in this tank the
>readings wander from time to time, no flow in the tank at the time.
>I'll be tacking down the cause in the next few days I hope.
As systems scale up in size, the body of water and everything
attached to it become an antenna to pick up 60hz from nearby
powerlines. (It is a big deal with fish ponds and aquaculture sites
that typically have a lot of power equipment around.) If the motor
is a 60hz motor with a power cord, see if the problem goes away if
you unplug it (it does not have to be running to generate 60hz
fields). Just a thought. It is sometimes possible to put a grounded
shield around the instrument. In severe cases, a sampler dips out
some solution and in effect tests it in a small cup. pH is the
toughest one, as the electrode has to be in galvanic contact with the
solution.
>One thing of note that I found odd. During some testing I changed the
>length of the exposed part of the probes, the probes are 1/8" rods
>with a type of heatshrink tube over them to keep solution off all but
>the 1/2 end in the water, that way level doesn't mater. When I
>changed the amount exposed from 1/2" to 1" I would have thought the
>reading would have gotten more sensitive, but it didn't, it was less.
>I'm not sure I understand why that would be.
The Stamp reading (555 frequency) should go up in proportion to the
conductance (1/resistance). Not true?
-- Tracy