Conductivity Sensor for Hydroponics
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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
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
Comments
mdavey@n... writes:
> 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.
>
I used to use the same process for measuring the purity of water in PC board
rinsing baths, except I used two gold pins. Just to check your system,
measure the differnece between distilled water and a salty solution. As I
recall the resistance of distilled water was something like 500 megohms. You
may have to use a 50 to 1 op-amp to increase the observed change.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
>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.
Use a series of probes of increasingly longer length with resistors (R)
between each as shown below:
to Stamp pin--|-R-|-R-|-R-|-etc if needed-
| | | |
| | | |
| | |
| |
|
As the solution gets higher another probe enters the liquid and the
resistance drops because one more resistor is effectively shunted out of
the circuit. If the nature of your liquid changes so it's resistance
becomes *very* high, this won't work as well since the resistance of the
liquid will be so much greater than any of the "R" values that changing "R"
values won't make much difference. Pick "R" to give the change you want to
see.
View or print the above using a fixed pitch font if it looks confusing.
Jim H
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