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Topic: Arduino + PH probe in Aquarium. Ground Loop. Please help. (Read 16 times) previous topic - next topic

vrgomes

Hi, I'm making a project with arduino that monitor my reef aquarium. So one of the things that I have is PH monitoring.
To do that, I have the Atlas Scientific Ph Stamps:

http://atlas-scientific.com/product_pages/embedded/ph.html

My connection is like this:


When I mesure the PH inside a bottle or a glass it measures the PH very precise, but when I place the probe inside my aquarium it give me wrong values. I also noticed that when I turn off my watter pumps in the aquarium the reading is fine, but when I turn the pumps on the problem appears again.

I have read a lot about ground loops (http://www.all-about-ph.com/ground-loop-problems.html) and interference with pumps and noise that is inducted into the PH probe but I don't understand what I have to do/buy to solve this.

So for my understanding until now I have to buy  DC-DC power converter and an optical data isolator but I don't understand what chips to buy and how to connect them im my arduino conection like I showed.
For example this other controler show this:

http://www.splatco.com/sx10507.htm
Quote
This uses a transformer isolated DC-DC power converter, and an optical data isolator, to ensure there is no metallic (ohmic) connection between the pH probe and the controller. This isolation eliminates the interference that can be caused by ground loops.


Please give me some guidance about this. If what I think it is, is correct and how can I solve this issue.
Thank you vey much.

Krodal

#1
Dec 07, 2012, 12:02 pm Last Edit: Dec 08, 2012, 12:28 am by Krodal Reason: 1
Could you add a link to the PH-probe ?
I read that a glass-PH-probe could be 800MOhm, is that right ?

Do you use a good shielded cable, which is fully shielded at both ends ?

Some noise of the mains voltage is probably passed on to the water via inductive/capacitive coupling in the pumps. It is almost impossible to get rid of that (only if batteries are used to power the pumps).

An opticial isolation could work. A DC/DC converter with an transformer removes the ground loop, but also passes on noise. An input circuit with a battery could also do, and with a optocoupler to pass the digital signal to the Arduino.

You could do a test. Use a battery for the Arduino and sensor board. Don't connect the usb or anything else to the Arduino, and store the PH-value in EEPROM. After that, read the EEPROM and check the values. If those values are good, your circuit is good and the problem is the noise and the ground loop.

I'm sorry, but I couldn't find a good solution for a PH-sensor. You might have to buy such an isolated circuit as the SX10507. Using your Atlas Scientific Ph Stamps circuit board and making your own isolated data transfer is not easy.


Added: Now that I have thought about it, the best way to isolate it, is at the serial connection. You could use two optocouplers for the RX and TX signals. The Atlas Scientific Ph Stamp could be powered with an galvanically isolated DC-DC converter.
There are ic's for RS232 isolation, but two normal optocouplers is better. Like these: http://yusisukmalia.wordpress.com/2009/06/11/rs232-make-optocoupler/
http://www.circuitsonline.net/forum/view/106111
An isolated DC-DC converter is cheap if they have unregulated output:
http://www.ebay.nl/itm/DC-DC-1W-isolated-converter-5V-5V-OUT-MORNSUN-F0505M-1W-/130674774325
But you have to test it, the output could be too noisy.

vrgomes

Hi, first of all thank you very much for the answer.

My PH probe is this one:
http://www.marinedepot.com/American_Marine_Pinpoint_pH_Probe_pH_Replacement_Probes-American_Marine_Pinpoint_Monitors-AM1411-FITERPPH-vi.html

I think the cable and connectors are shielded so i don't think that's the problem...
So in your opinio I should power up the PH stamp with an  isolated DC-DC converter like that one you showed on ebay.
Second I should isolate the RX - TX that connects the arduino with the stamp with 2 optocouplers. Can you plese show-me a model available on ebay so that I'm able to buy it or the reference to search on my local store.
I also don't know how to connect the opctocupler but that will be another stage.

Thanks again.

Krodal

For someone familiar with electronics, two optocouples like the schematics I linked, is not hard.
I'm sorry but I can't find a ready-to-use board on Ebay.

vrgomes

#4
Dec 10, 2012, 10:51 am Last Edit: Dec 10, 2012, 10:57 am by vrgomes Reason: 1
Hi what about this optocoupler:
http://arduino.cc/blog/2010/09/22/optocoupler-demystified/

Does it will work?
And I have to buy 2 of these, on for Rx and other for TX.

I also said that there are some RS232 IC's that do this isolation. Can you point me any that i can find on ebay?

Thanks.

Edit:
Fund this:
http://www.ebay.com/itm/1x-MAX251EPD-5V-Isolated-RS-232-2-Driver-2-Receiver-MAX251-/110753204175?pt=LH_DefaultDomain_0&hash=item19c967c7cf

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