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Old 03-26-2011, 04:21 PM   #1
nilo
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Default Using a pH meter

Recently bought a Milwaukee MW102 and I have a question on using it.
I sent this question to their tech support but didn't get anything back.
Meter is calibrated as suggested on the manual.
When I place the PH probe in sample with the temp probe, I stir for 5 sec with the PH probe and leave it alone to complete the reading. Display starts showing 6.5 (example) and slowly increases and after 30s it is at 7.2 when the reading is shown to be completed. Thing is, leaving the probe in the sample further will make it re-read the PH and it keeps increasing, specially if I stir a bit like the first time. After several reading cycles, it reaches 7.9
So what is the deal with this thing? Is the first reading the actual PH (7.2) or do I need to keep stirring the probe in the sample during all reading process?


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Old 03-26-2011, 04:30 PM   #2
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Is the sample water?

If so the pH is rising over time as co2 leaves solution. Stirring it accelerates this.


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Old 03-26-2011, 04:38 PM   #3
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Quote:
Originally Posted by remilard View Post
Is the sample water?

If so the pH is rising over time as co2 leaves solution. Stirring it accelerates this.
yes, I was just testing my water PH. So should I stir it while testing?
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Old 03-26-2011, 05:09 PM   #4
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pH electrodes do take time to equillibrate. The older the electrode the longer it takes. Also, quick response is one of the things you pay extra $ for.

CO2 leaving the solution may be an explanation. If the water you are testing is oversaturated with CO2 then CO2 will leave and pH rise. OTOH if it is undersaturated pH will fall. If the water is RO or DI it is of very low conductivity and pH measurement is complicated. In any case, water probably isn't the best thing to test on. But pH 7 buffer is. If you stick the electrode in that and it continues to drift as much as 0.7 pH then you have a problem with the electrode. The first place you always look is the junction - in particular plugging or fouling of it. If you look at the business end of your electrode there should be three things sticking out of it. The most protuberant is a glass bulb - that's the actual pH electrode. The second most prominent thing is obviously metallic and is the RTD for the ATC. The remaining thing is the junction - often a bundle of fibers or a frit of some sort. Try directing a strong stream of water against that or dip it in an enzymatic cleaner or, better still, follow the manufacturer's recommendation for electrode cleaning.

Stirring introduces an error called, imaginatively enough "stirring error". If you stir the buffers and samples in the same way then stirring error gets picked up in the calibration. If you calibrate without stirring but stir the sample, it doesn't. It's probably best not to stir while waiting for the meter to stabilize but you should stir when you first place the electrode in the buffer or sample. Of course you have rinsed the electrode with distilled water and blotted it before moving it from buffer to buffer or buffer to sample but despite the blotting a little water will still adhere. The quick stir at the beginning of a measurement or calibration step washes that water away from the sense bulb and the junction thus given a more reliable reading.
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Old 03-26-2011, 05:29 PM   #5
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Thank you guys for the quick response. I'm fly sparging right now as I type this.
I surelly do wash the probe with DI everytime before using on a sample. Then I shake it a bit and dry with a paper towel.
My PH meter has two separated probes, one for PH and one for the temperature reading/ATC.
Although the specs say that the ATC is from 0 to 70C, I'm trying to get the sample at 25C for reading. It seems that when I test a wort sample, it is much more stable and it doesn't behave like when testing water, so my water may be saturated with CO2 and you said. I run the water thru a carbon filter, not sute if that has anything to do with it.
Thanks again!
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Old 03-26-2011, 05:38 PM   #6
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Quote:
Originally Posted by nilo View Post
Thank you guys for the quick response. I'm fly sparging right now as I type this.
I surelly do wash the probe with DI everytime before using on a sample. Then I shake it a bit and dry with a paper towel.
My PH meter has two separated probes, one for PH and one for the temperature reading/ATC.
Then the junction is the second thing sticking out the bottom of the pH electrode piece but it sounds as if you don't really have a problem.

Quote:
Originally Posted by nilo View Post
It seems that when I test a wort sample, it is much more stable and it doesn't behave like when testing water, so my water may be saturated with CO2 and you said. I run the water thru a carbon filter, not sute if that has anything to do with it.
There's your answer. You should get similar stability in buffer.
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Old 03-26-2011, 06:02 PM   #7
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Here's the new lab
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Old 03-26-2011, 06:11 PM   #8
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Too tidy. How do you find things?
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Old 05-26-2011, 02:43 AM   #9
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Here's the new lab
You have a close up of that log sheet? I am looking to make one and would love some "inspiration."
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Old 06-07-2011, 12:14 AM   #10
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Yeah for sure. This pen and college ruled note pad doesn't cut it. Although I do find some pretty rad notes in there after a good brew session. It's too disorderly.


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