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Old 10-09-2010, 12:06 PM   #1
hedonist91
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Default Drifting Ph Meter

Hey all,

Last brew day we had a few problems trying to get our mash Ph where we wanted. I had already read up a lot on what I thought I needed to do, but one thing is for certain-our Ph meter is drifting quite a bit. I got the one Kai recommended, I don't have it in front o me, I'm at work-it was somewhere around 85 bucks from NB. Anyway, drifting like crazy. Even drifted in the neighborhood of .2 Ph with tap.

I calibrated it as per the instructions, maybe I messed up calibrating?

Thought maybe you folks have had a similar issue. Thanks!

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Old 10-09-2010, 12:44 PM   #2
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The first question would be as to how old the meter is and how you have stored it, how much use you have had with it, whether you have put it into hot mash etc.

It is essential that a pH electrode be properly hydrated (per the manufacturer's instructions) that it be stored per the manufacturers instructions (usually in a particular storage fluid, most often a saturated solution of potassium chloride), that it be kept full of electrolyte (if it is a refillable electrode in which case it is also important that the fill port be open while the electrode is in use), that the reference junction be renewed as necessary (if it is a renewable junction electrode), that the sensing bulb be kept free of protein deposits and that it not be subject to high temperature. In a nutshell, follow the manufacturers directions to the letter.

The usual cause of drift is a plugged reference junction. This is typically a small frit which allows electrolyte from a gel or liquid filled into the body of the electrode to flow into the sample. If you look into the end of the electrode you will see three items: the glass bulb (the sensing element), a slightly smaller metal rod (the temperature sensing RTD) and a third item (the reference junction). If inspection shows that it is covered with gunk, mold, etc then you need to clean that off. A stream of DI water from a wash bottle is probably the easiest but won't get anything too stubborn. In brewing the best thing to use is an enzyme cleaner such as Zymit (go to www.coleparmer.com) or an enzyme cleaner made especially for cleaning pH electrodes (ideally by the people that made the electrode). If anything is going to solve the problem this is most likely it.

In some electrodes you can pull out the frit with a tweezers and replace it. Obviously that is even better than cleaning but not many electrodes offer that feature.

Eventually, of course, gel filled electrodes run out of juice but they usually are loaded with enough to last a good long time (i.e. longer than the life of the bulb). If you store in the wrong storage solution that can draw electrolyte out of the electrode much faster than normal. In either case and electrode that is depleted needs to be replaced.

Finally, pH meters do drift quite a bit in solutions of low ionic strength like distilled or otherwise deionized water - perhaps even some RO waters. If your water is very pure it would not be unexpected that the electrode take a long time to come to equilibrium. If adding some table salt stabilizes the reading then low ionic strength was probably the cause.

Use of the mV mode with fresh buffers can be diagnostic. In pH 7 buffer the meter should read at most a few mV (positive or negative) and the reading should be stable even if the temperature isn't. In pH 4 buffer the reading should be close to +173 mV and should be stable if the temperature is stable but will drift with temperature. If you get drift in buffers then look at a plugged reference junction as the probably cause. Note that it is normal for it to take a couple of minutes for the reading to change from near 0 in 7 buffer to +173 in 4 buffer. As the electrode ages or as the bulb becomes coated with protein, this takes longer. Note that cleaning with Zymit not only cleans protein off the frit but the bulb as well.

If your meter tells you slope and offset as part of the calibration process it is a good idea to write these numbers down with the date of the calibration. An electrode has a finite useful life (can be a bit more than 2 years with proper care) and the history of change in slope (should be around 57 mV/pH but is sometimes given as a percentage) and offset (should be 0 but will increase in magnitude with age) will give you an indication as to how healthy your electrode is.

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Old 10-10-2010, 03:07 PM   #3
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Quote:
Originally Posted by ajdelange View Post
The first question would be as to how old the meter is and how you have stored it, how much use you have had with it, whether you have put it into hot mash etc.

It is essential that a pH electrode be properly hydrated (per the manufacturer's instructions) that it be stored per the manufacturers instructions (usually in a particular storage fluid, most often a saturated solution of potassium chloride), that it be kept full of electrolyte (if it is a refillable electrode in which case it is also important that the fill port be open while the electrode is in use), that the reference junction be renewed as necessary (if it is a renewable junction electrode), that the sensing bulb be kept free of protein deposits and that it not be subject to high temperature. In a nutshell, follow the manufacturers directions to the letter.

The usual cause of drift is a plugged reference junction. This is typically a small frit which allows electrolyte from a gel or liquid filled into the body of the electrode to flow into the sample. If you look into the end of the electrode you will see three items: the glass bulb (the sensing element), a slightly smaller metal rod (the temperature sensing RTD) and a third item (the reference junction). If inspection shows that it is covered with gunk, mold, etc then you need to clean that off. A stream of DI water from a wash bottle is probably the easiest but won't get anything too stubborn. In brewing the best thing to use is an enzyme cleaner such as Zymit (go to www.coleparmer.com) or an enzyme cleaner made especially for cleaning pH electrodes (ideally by the people that made the electrode). If anything is going to solve the problem this is most likely it.

In some electrodes you can pull out the frit with a tweezers and replace it. Obviously that is even better than cleaning but not many electrodes offer that feature.

Eventually, of course, gel filled electrodes run out of juice but they usually are loaded with enough to last a good long time (i.e. longer than the life of the bulb). If you store in the wrong storage solution that can draw electrolyte out of the electrode much faster than normal. In either case and electrode that is depleted needs to be replaced.

Finally, pH meters do drift quite a bit in solutions of low ionic strength like distilled or otherwise deionized water - perhaps even some RO waters. If your water is very pure it would not be unexpected that the electrode take a long time to come to equilibrium. If adding some table salt stabilizes the reading then low ionic strength was probably the cause.

Use of the mV mode with fresh buffers can be diagnostic. In pH 7 buffer the meter should read at most a few mV (positive or negative) and the reading should be stable even if the temperature isn't. In pH 4 buffer the reading should be close to +173 mV and should be stable if the temperature is stable but will drift with temperature. If you get drift in buffers then look at a plugged reference junction as the probably cause. Note that it is normal for it to take a couple of minutes for the reading to change from near 0 in 7 buffer to +173 in 4 buffer. As the electrode ages or as the bulb becomes coated with protein, this takes longer. Note that cleaning with Zymit not only cleans protein off the frit but the bulb as well.

If your meter tells you slope and offset as part of the calibration process it is a good idea to write these numbers down with the date of the calibration. An electrode has a finite useful life (can be a bit more than 2 years with proper care) and the history of change in slope (should be around 57 mV/pH but is sometimes given as a percentage) and offset (should be 0 but will increase in magnitude with age) will give you an indication as to how healthy your electrode is.

Wow! That was a long answer! Thanks!

The meter is brand new, I store the electrode in electrode storage solution. I clean it with electrode cleaning solution. I bought all these two weeks ago at the same time as the meter.

It's not dirty, or plugged up, I know that, and it's rated with ATC up to 60 C. I don't get a drift in buffers. It seems it only drifts when I need it not to. I think I'm going to calibrate it again, just play with it some more and see what the story is. Maybe it doesn't like the heat, even though it's accurate to 60C.

Thanks for all of the info, I'll be sure to reference it when dealing with my Ph meter issues.
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Old 10-10-2010, 04:36 PM   #4
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Given your responses I suspect it is simply that you are not used to the sometimes slow response of a pH meter. They do take some getting used to. You might want to try measuring the pH of various things around the house like milk, orange juice, lemon juice, bleach, ammonia, salt water, tap water etc. to see how long it takes for your instrument to respond. As I noted above you can expect quite slow response in water of low ionic strength (soft).

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Old 10-13-2010, 02:50 PM   #5
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Hm. Thing's drifting with tap water now. The model Ph meter is Martini Instruments Ph 56. Drifted from 7.6 to 8.31 slowly and steadily over 24 minutes. Disappointing. Going to rinse it with distilled water, recalibrate, try again.

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Old 10-13-2010, 03:21 PM   #6
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What is the tap water like? Where does it come from. pH shift over time can be normal as I've indicated a couple of times with very soft water. Also, if a water is highly saturated with CO2 that will escape over time and the pH will rise.

Does it drift in buffer? If it doesn't drift in buffer then the electrode is probably OK.

There is, of course, a possibility that you have a bad electrode out of the box. It can happen.

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Old 10-13-2010, 04:20 PM   #7
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Quote:
Originally Posted by ajdelange View Post
What is the tap water like? Where does it come from. pH shift over time can be normal as I've indicated a couple of times with very soft water. Also, if a water is highly saturated with CO2 that will escape over time and the pH will rise.

Does it drift in buffer? If it doesn't drift in buffer then the electrode is probably OK.

There is, of course, a possibility that you have a bad electrode out of the box. It can happen.
Hm. I like the CO2 thing. I'll investigate that in a few hours. I've noticed less of a shift with lemon juice, and other substances that don't use tap. Maybe it is CO2 escaping. I like that.

As for right now, I've called the manufacturer and the guy on the phone said it sounds "exactly" like a dried out or not properly hydrated probe. I don't think that's it, as I've followed the instructions that came with it.

Although, this gentleman said that the instructions are incorrect, and the hydration needs to go up further than what the cap can store. IDK, I filled the cap all the way up with solution, and gushes out the sides until I can close it, leading me to believe it's explored every avenue it can, hydrating where it needs to hydrate. I'm humoring the guy though, and hydrating it for a few hours and recalibrating.

As far as measuring the mash itself, in practice, I had a small cup of mash, kernels and all. Maybe I should be using just runoff, straining out particles? The meter is equipped with ATC, but maybe it would be easier if I cooled it off a touch. Experiments to come.
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Old 10-14-2010, 04:32 PM   #8
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Quick update, I humored the guy I talked to on the phone, and tested it with my tap.

It started a few tenths of a point low, but over 4 minutes settled in without drifting to exactly what my water report says is supposed to be the tap's Ph, all the way down to the hundredths place. Very, very cool. I guess it did get dried out, or was never properly hydrated in the first place.

I haven't had the chance yet to test it with lemon juice, bleach, or the like. Good stuff though. Definitely a step in the right direction.

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