[jjflash]

I have use this Milwaukee MW102 pH meter for the last three or more years.
I immerse the pH probe and temp probe into the solution, wait for the hour-glass symbol to stop blinking, take the pH reading.
I have noticed if I leave the probe in the solution, the reading will drift upward in all solutions I test.
Checked my water pH last night, initial pH 6.6, hour later 7.2
Checked my sparge pH this AM, initial pH 5.7, hour later 6.2
The electrode probe was change out early this year.
Meter is calibrated before every brew session.
Not sure what to think about this.
Is first reading correct, or is last reading correct? 

[PORTERHAUS]

What temp are the samples when you first test them compared to what they are 1 hr later? The samples would have to get cooler for the ph to rise but I don't know if I can see it being that much of a difference. Just throwing out the only thing I can think of.

[jjflash]

Milwaukee MW102 has ATC, that is why there is a "temp probe".
All samples are at room temperature.

[mainebrewer]

I have the same pH meter.
It will change by .01 after the pH measurement stabilizes.
How old is the probe?
I find I need to replace mine about every 2 years.

[PORTERHAUS]

Milwaukee MW102 has ATC, that is why there is a "temp probe".
All samples are at room temperature.

There would still be a change in ph with change in temperature, despite ATC.

[Stevie]

Milwaukee MW102 has ATC, that is why there is a "temp probe".
All samples are at room temperature.

There would still be a change in ph with change in temperature, despite ATC.

Exactly. The pH will change with temp. From my understanding, the atc is for the probes sake not for the sample.

[JT]

If you got 3 years from your probe, that's pretty good!  Time to replace.

Edit: sorry, I somehow skipped the part about the probe being replaced earlier this year. 

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[Wort-H.O.G.]

PH of your tap water can drift.....possible. You getting consistent reading in test solution?


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[narvin]

As I understand it, the probe works through ion exchange.  Is it possible that after an hour in water the solution is actually leeching stuff from the probe?

Not an expert, but I think this is why they want you to store it in buffered storage solution.

[Wort-H.O.G.]

Curious also if water had brewing salts or not. Not expert, but o2 absorption I thought impacts ....not sure on this.


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[narcout]

With my MW101 (which I believe uses the same probe as the MW102), I noticed that the pH reading would slowly drift upward over the course of 5 minutes or so, and it never really seemed to stabilize.

I purchased a new probe, and it solved the issue - though I still find that I need to do a bit of stirring to get a good reading.

I've never let it sit for a full hour though to see what would happen. 

As I understand it, the probe works through ion exchange.  Is it possible that after an hour in water the solution is actually leeching stuff from the probe?

I think I remember reading that, but I cannot remember where.  Maybe it was a post on this forum.

[pfabsits]

There can be  a number of issues as to why instability in a reading. Below is a link that as you scroll down you will find a lot of information.


https://www.homebrewersassociation.org/forum/index.php?topic=24548.0

A pH meter is a very sensitive volt meter. A voltage is generated by a pH electrode when it is placed in a solution.
There are two wires that are in the measuring circuit. They are separated by the glass pH bulb (indicating membrane) and the reference junction. The link below brings you to a pH electrode page and if you scroll down there is a diagram comparing a single junction vs. double junction pH electrode.  Understanding the parts will help in determining the problem


http://shop.hannainst.com/hi1043b-refillable-double-junction-ph-electrode.html

For a reading to drift there has to be a change to the measuring circuit. These changes can be from:
1) Probe is not properly hydrated. A hydrated layer forms on the glass pH membrane over time. It takes 3-4 hours to fully form. It is one of the reasons that we store the pH electrode in a storage solution. A dry electrode will generate a completely different voltage than a hydrated one. So if the probe is dry you will read one pH value and then it will drift over time until the hydration layer is formed.

2) Related to storage solution is also the junction potential. The junction (barrier from reference cell to sample) has its own potential that is part of the measured potential. It is important that a liquid junction potential be maintained. Also if the junction is allowed to dry then some of the reference electrolyte will precipitate clogging the junction. Any clogging of the junction will result in erratic/unstable readings.

3) The probe had a slight build up on the glass at the beginning that came off as it was in the sample. Again any changes to the glass will result in a different measured potential

4) Most likely, a build up is forming on the glass as it is left in the sample. If the probe was hydrated and clean in the beginning a a slight layer forms on the bulb as it sits in solution then the reading will drift because three is a change to the bulb that has not been accounted for.

5) The sample is not conductive enough to carry the flow of current. Most likely not the problem but it is still important to know that a standard pH electrode needs 100 uS/cm of EC to work. Without salts present then the electrical current cannot flow. RO water equals 20-30 uS/cm. Lake Michigan is around 300 uS/cm. Groundwater varies by location. In Chicago suburbs upwards 1500 uS/cm. If using RO water with Bru'n additive then this is not a problems since the salts will increase EC.


If you take a paperclip and short the BNC connector on the meter then it is possible to do a default calibration that can be helpful in determining if there is a build up on the probe.

1) Place one end of the paperclip into the center hole and then wrap the other end around the outside of the connector. This will cause a short that = 0 mV

2) Calibrate the meter to pH 7.01 with the connector shorted. Theoretically pH 7.01 = 0 mV (known as offset). It is the deviation from this point that we compensate for by calibration.

3) After calibrating to pH 7.0 then reattach the pH electrode and place in fresh pH7.01 buffer. Record the pH value.

4) Determine the mV from the pH value displayed. The probe should read between pH 6.5 and 7.5 (+/-30 mV). Outside this value indicates the probe might need to be cleaned.

A pH electrode theoretically generates 59.16 mV/pH unit at 25 oC. This is known as slope.  A pH of 6 = +59.16 mV and a pH of 8 = -59.16 mv.  So each 0.1 pH = 5.916 or basically 6 mV. A pH reading of 6.5 is 0.5 pH units away from pH 7 which means that 0.5 pH x 6 mV = 30 mV. If the meter displayed pH 6 then that would be a 60 mV offset error.

It is important that the pH 7 buffer is fresh. If the solution is not fresh then it is possible that it is actually not pH 7 but a different value. It would make the test above irrelevant since there is not a known standard to compare to.

Lastly, manufactures will allow an offset  of +/-60 mV or greater to be accepted for a pH 7 calibration. Accuracy will suffer if the value is outside +/- 30 mV. The same for slope. when a slope is less than 90% or 53 mV/pH (59.16 x 0.9) then accuracy will suffer.

If you have questions then feel free to message  me.

[narvin]

That's a lot of great info.  Thanks!