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Topic: pH controller unstable in field measurements (Read 6 times) previous topic - next topic

Androxys

#20
May 15, 2012, 11:56 pm Last Edit: May 16, 2012, 12:11 am by Androxys Reason: 1
Hello again.

To my disappointment, the turbulence pipe didn't do anything. I got a 50 cm long pvc pipe, 4 cm diameter, closed on one end, and drilled 4 little 3 mm diameter holes on that end, to allow water to enter, and kept the rim above water level. All measurements done with the probe inside still suffered from the same variance, despite all tries (once again, when I take the pipe out of the water, that sample is 100% stable, if I immerse it again, the variance shows up). That makes me believe it's not PSU related noise.

I am using star grounding, my teacher at the instrumentation course made perfectly clear that daisy-chaining was bad pratice. One of the sides of my double-sided copper pcb is the ground plane and all ground terminals are soldered on that side. Then, that plane is connected through a wire to the transformer's center tap.

I will attach a picture of the important bits of the EAGLE project. Maybe I commited a horrid design flaw and haven't realized it.
IC6 is my instrumentation amplifier (an INA111), IC5 is merely a buffer ampop, and the different power supply components give 9v (IC1), -9v (IC2), 5v (IC3), 2.5v (IC4)

http://imageshack.us/photo/my-images/526/schematicx.jpg/ (R4 actually doesn't exist anymore, if I recall correctly)

Thank you.

trendski


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Don't put any wire into this thing if there are animals in it.  Copper is poisonous to marine life.


But it is an essential micro-nutrient also. Probably OK to use stainless steel though or gold plated electrodes.

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Since the pH value should not vary quickly


Seawater pH should not vary much second to second, unless exposed to extreme pollution - it has it's own buffering capacity. Freshwater (eg river) has little buffering capacity compared to seawater and pH can vary dramatically with pollutant loads. Aquatic plants can alter pH even on a day/night cycle.


Craig Turner, blog: http://gampageek.blogspot.co.uk/ It helps with my learning if I write things down, esp. for others to follow (constructive comments welcomed to improve)

Docedison

I think You are dealing with ground leakage currents, In other words there is a galvanic connection between your probe and the Mains return or neutral and the reason that I said that the PSU output cannot have any connection to earth ground, even an electrostatic one, thus the shielded transformer. Your Power Supply Must look like batteries to the instrument while following standard mains wiring isolation and safety, considering possibly reversed wiring, open Ground's and other "Unknown" catastrophes. Without that precaution the measurement must be done externally, That is not an issue however... You can always 'pump' a sample into a receiver and dump it when you are through. To test this hypothesis is simple in order to eliminate ground interaction (s)... connect a set of batteries to the device and try again, where the instrument is floating, not connected to anything else except the measurement sample. Your PSU Primary ground (Green Wire) should only connect to the transformer frame, Nowhere Else. I do need to say that you have to of course follow the applicable regulations in your country of origin. This device due to the nature of it's intended application is most very similar to a "Medical" Device in it's possible interactions with currents flowing from any other source, much like the necessary line isolation required for an EKG or EMG as whatever line leakage current flowing is also a part of your measurement current. The fluctuations might possibly be sampling artifacts of a big 60 Hz leakage current flowing between your probe and the local ground (the reservoir).  IMO

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Androxys

#23
May 17, 2012, 05:50 am Last Edit: May 17, 2012, 06:30 am by Androxys Reason: 1
Doc, I have a feeling you might have stroke the problem right on the head.

I've mentioned before that the PCB ground was connected to the transformer center tap. What I think I haven't mentioned yet, was that I also connected the center tap to the transformer frame, and thus to the green/yellow mains ground wire (I only did this because I thought it was the easier way to set the tap at 0V, because I was getting up to a 4V drift without it, thus getting the 12V and -12V I needed out of the two outputs, it's a 240V-24V transformer). So, my PCB ground (where the probe reference is also attached) is connected to center tap, which in turn is connected to the mains ground. Could this be the cause for that leakage?

I'll test tomorrow with some batteries just to test that hypothesis, and hopefully that will be the end of it. And if it does work, I have to find a way to have a stable center tap at 0V. If not, I think I'll just try to implement a way to pump a sample in a cup and cut the corners.

I'll keep you posted. Thank you.

Docedison

Change your transformer to a shielded type ground the frame to earth ground for fire/litening safety through grounding. You must I think keep the PSU and earth grounds separated, sometimes it is necessary to use a transformer with a high voltage type insulation for safety reasons particularly in sensitive situations where other equipment is connected as well, data loggers, RS485 Comm, 4 - 20mA loops... all are possible sources of interaction. It thus unfortunately becomes incumbent on the designer to make sure that the equipment's design isn't a part of someone else s faulty or shortsighted design work. Much the same should be said of software as well there are many obvious issues with software engineers trying to design hardware without a good idea of how the hardware must be supported. or how to condition analog as well as digital signals. I don't think much more need to be said about good by-passing methods ass these are required parts of any design. IMO

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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