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unstable PH readings

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unstable PH readings

Postby gtipc » Fri Sep 04, 2009 6:37 pm

Hi

I have just started using my DrDaq data logger, and I was taking PH readings from my fish tank, but the readings fluctuate from 6 to 7. constantly going up and down, on the graph it looks like a wave going up and down. anyone have any ideas why the reading is so unstable. all the equipment is brand new.

Thanks
Gareth
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Postby ziko » Mon Sep 07, 2009 9:59 am

Are you using a temperature sensor to take into account the temperature variations of the water?

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Postby gtipc » Mon Sep 07, 2009 12:02 pm

The external temp sensor no1 is in the water, but I'm not sure if the software is set to compensate, I couldent see any settings to do this.

If I set it to display the average reading it displays exactly what my handheld devise displays. is this what I need to do always.

Thanks
Gareth
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Postby ziko » Tue Sep 08, 2009 4:03 pm

Hi when you connect the external sensor it will automatically compensate. Could you send me your settings file? Also have you tried to do some kind of calibration with three buffer solutions?

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Postby gtipc » Tue Sep 08, 2009 6:55 pm

Hi not sure what settings file you require all there is is a bunch of .dll's.
I have not as yet, done a calibration.
I have tried using a diferent probe but get the same results.
I have also shorted out the terminals and I get a stable reading of 6.9

Thanks
Gareth
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Postby ziko » Wed Sep 09, 2009 8:29 am

Hi Gareth,

Which terminals did you short out? When I mean settings file, you can send a data file instead. The data file will have your settings in it.

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Postby gtipc » Wed Sep 09, 2009 6:44 pm

I read in another post, to test the board short out the ph terminal and if you get a reading of around 7 then it's ok. I have done a calibration look-up tabe, but it made no diference. the only way I can get a reasonably stable reading, is if I set it to display the average, is that what you are suposed to do anyway?
I have attached the settings file
Attachments
ph standard.psd
(12.95 KiB) Downloaded 280 times
ph with average.psd
(18.92 KiB) Downloaded 152 times
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Postby ziko » Thu Sep 17, 2009 9:56 am

Hi,

I just had a look at your data, it seems to me that the fluctuations are 50Hz, which is possibly mains hum! I also noticed that you are using PicoScope 5, have you tried using PicoLog recorder?

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Postby gtipc » Thu Sep 17, 2009 2:12 pm

Yes I have tried using pico log and I get the same fluctuation. You mantionded the fluctuations are 50hz wich is what mains is, but the board is powered by the computers com port. So how do we pinpoint the fault, is it a fault within the board or a fault with the supply to the board?
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Postby ziko » Thu Sep 17, 2009 5:11 pm

Hi, it could be coming through the earth, there could be a ground looping issue.

Take a sample of the water of your fish tank and measure the ph that way rather than directly in the fish tank.

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Re: unstable PH readings

Postby gtipc » Thu Dec 24, 2009 8:06 pm

Hello and a merry christmas to you all and a happy new year.

I have tried just measuring a water sample, but it made no difference. I have also tried diferent probes but still no better.

could the drdaq board itself be faulty?
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Re: unstable PH readings

Postby gtipc » Sun Dec 27, 2009 6:17 pm

Hello again

Ok my mistake. It is the pump in the tank that is causing the problem. when I remove the pump or put the probe in a sample like you said, I get a stable reading

Is there a way around this problem? earth the water or something?
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Re: unstable PH readings

Postby Pico Stuart » Tue Dec 29, 2009 9:08 pm

Hi Gtipc,

Thankyou for your observations with regard to your Ph
measurements. Looking through your reports looks like you need some filtering on your pump supply.

It may be worth looking at some form of isolation transformer. This could possibly be a solution to your interference.

Hope that helps.

Kindest regards

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Re: unstable PH readings

Postby niel_malan » Sun Apr 11, 2010 8:49 pm

I'm not surprised that you have problems measuring the pH of a fish tank. The conductivity of fresh water is very low, which makes the impedance of the measurement very high, so it becomes sensitive to electrical noise, and the measurement will easily pick up the hum from the motor. Add to that the low ionic strength of the solution, which makes the reading very sensitive to motion in the water, and you have recipe for unstable readings, especially if the sampling rate is low and you get aliasing.

To measure the pH of nearly pure water with a glass electrode, remove a sample and add some potassium chloride. This increases the conductivity without changing the pH.
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