Author Topic: MW102 pH meter issue  (Read 1515 times)

Offline mainebrewer

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MW102 pH meter issue
« on: July 03, 2016, 11:07:58 AM »
Lately, when checking mash pH I've noticed that my meter's reading was .2-.3 lower than the Brunwater estimate. After checking a few other things, I used the meter to check the pH of the 4.01 and 7.01 calibration solutions.

The meter reads the 7.01 correctly but the reading on the 4.01 solution is 3.75.
The probe is less than a year old, always stored in the storage solution.
The calibration solutions are both new bottles.
I've noticed that the last few times I have calibrated the meter that it takes much longer to lock onto the 4.01 reading than the 7.01.

This is a first for me with this meter. Do I need to replace the probe or is there something else going on?


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Offline Slowbrew

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Re: MW102 pH meter issue
« Reply #1 on: July 03, 2016, 11:38:38 AM »
Mine did that pretty quickly after I had purchased it.  It got to the point it wouldn't lock at all.  I replaced the probe and everything works like a champ again.

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Offline pfabsits

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Re: MW102 pH meter issue
« Reply #2 on: July 03, 2016, 01:09:10 PM »
It is not normal to see a slope change from one calibration to the next. Usually a slope change is the result of a change in the pH buffer. The slope of a pH electrode will degrade over time but it is gradual change. To have a change in only one point usually indicates a problem in the buffer. If there was a change in both points then it would indicate that the probe needs to be cleaned.

The purpose of calibration is to compensate for changes to the pH electrode. After calibration with the buffers the pH should display the temperature compensated pH value. These are the values found on the bottle of pH buffer. The table represent the pH of a solution at a given temperature. A pH meter will display pH 4.01 if the temperature is 25 oC.

Since the pH value is less than pH 4.01 by 0.25 pH then that means that the slope actually increased, which typically is not  bad thing. A new pH electrode will have a slope of 95-105% of the theoretical voltage of 59.16 mV/pH unit @ 25 oC. A 0.25 pH is equal to about 15 mV (59/4) and over three pH units would equal 5 mV/pH unit. So if a pH probe had a slope of 85% or 50 mV/pH and the slope increased by 5 mV/pH = 54 mv/pH then the slope increased to 91%.  This information is only provided as an example since the actual slope is not known. The mV response for a pH electrode in a given buffer is what we are compensating for.

Overall the best recommendation from a manufacturer is, if you have cleaning solution then clean the electrode, store in storage solution for 3-4 hours, then re-calibrate with new pH buffer. That should cover all bases.



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Offline mainebrewer

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Re: MW102 pH meter issue
« Reply #3 on: July 04, 2016, 10:13:09 AM »
Thanks for the advice.
I do have cleaning solution and I'll pick up some new pH buffer (4.01, 7.01).
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Offline mainebrewer

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Re: MW102 pH meter issue
« Reply #4 on: July 09, 2016, 11:13:09 AM »
Quick update: fresh ph buffer arrived a couple days ago.
Gave the probe a 10 min soak in cleaning solution, then calibrated the meter.
The meter now reads both ph buffer solutions correctly.
I'll be brewing again next week, so I'll see if the pH measurements go back to being close to the Brunwater estimate again.
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Offline Sam G

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Re: MW102 pH meter issue
« Reply #5 on: July 14, 2016, 02:51:38 PM »
Happened to my MW102. The probe is only guaranteed for 6 months of regular use and when stored properly. Beyond that, the company won't cover a replacement for the probe itself. You can get a replacement for 1/3 of the cost of the entire unit.

http://safety.katsent.com/saf-ma911b2-milwaukee-ma911b2-double-junction-ph-electrode
http://www.testequipmentdepot.com/milwaukee/probes/ph-epoxy.htm

Offline mainebrewer

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Re: MW102 pH meter issue
« Reply #6 on: July 14, 2016, 04:28:52 PM »
I find that I can get around 2 years out of a probe before it starts to take forever to lock onto a reading or has some other problem. I'm on my third probe now.
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Offline brewnz

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Re: MW102 pH meter issue
« Reply #7 on: October 25, 2016, 08:58:23 PM »
One potential cause for the changing pH over time is that proper use of a pH meter requires a gentle stirring motion.  This is because the gel inside the pH meter is gradually seeping through the porous glass bulb at the end of the electrode. Without stirring it will cause a localized imbalance and you'll get a false reading.

So, for example, if I take a wort reading while I am gently stirring the electrode in cooled wort, pH meter stabilizes and I write down that value. Then if I was to simply leave the electrode sitting in that same wort - the value will drift over time. By stirring again, it should come back to the initial reading (repeatable measure). If it does not, then there is more going on. As I read the initial post, just sounds like incorrect method.

This thread has been a good read for me, thanks!

BrewnZ

Offline brewinhard

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Re: MW102 pH meter issue
« Reply #8 on: October 25, 2016, 09:28:07 PM »
Lately, when checking mash pH I've noticed that my meter's reading was .2-.3 lower than the Brunwater estimate. After checking a few other things, I used the meter to check the pH of the 4.01 and 7.01 calibration solutions.


I was going to ask if you are taking the pH readings at room (or close to room) temperature and not mash temps? That can cause a .2-.3 difference right off the bat.

Offline Steve L

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Re: MW102 pH meter issue
« Reply #9 on: October 26, 2016, 01:35:25 AM »
I've always been curious and I honestly never paid attention to the probes I've purchased in the past.... do they come with a date of manufacture? and if not... how long have they been sitting on a shelf waiting to be purchased? is it that they don't really begin to degrade at an appreciable rate until they see use outside of their original storage solution? Just curious.
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Offline mabrungard

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Re: MW102 pH meter issue
« Reply #10 on: October 26, 2016, 01:39:36 PM »
There should be no degradation in a probe that has been sitting on the shelf, as long as the junction doesn't get dried out. The main degradation is due to the fouling of the junction. I've found that the gel filled Milwaukee probes are pretty durable. I have something like 30 brew days on my Milwaukee MW-101 in the nearly 5 years I've had it. Still going strong. It's always kept in storage solution.
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Offline mainebrewer

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Re: MW102 pH meter issue
« Reply #11 on: October 26, 2016, 07:49:08 PM »
Lately, when checking mash pH I've noticed that my meter's reading was .2-.3 lower than the Brunwater estimate. After checking a few other things, I used the meter to check the pH of the 4.01 and 7.01 calibration solutions.


I was going to ask if you are taking the pH readings at room (or close to room) temperature and not mash temps? That can cause a .2-.3 difference right off the bat.

The pH readings were done at 25C. I also do the "gently stir" while measuring the pH of a solution.
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