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Old 07-07-2009, 04:52 PM   #1
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Default Buying Guide for a pH meter

I’d like to loose a few words regarding buying a pH meter and what to look for. Some have asked me about that via PM in the past, so I may as well write it here.

When you look for pH meters you’ll find that there are a lot of different models with a variety of features and prices ranging from $50 to $500 and once you justified the expense you’ll ask yourself what meter should I get as a brewer.

pH meters are glorified volt meters that measure the electrical potential produced by a special pH probe. What determines their quality and precision is the quality of the probe and the quality of the amplifier and analog-digital converter. The probe wears over time and you should expect that you’ll have to replace it every 2-3 years if you care for it well. The probe’s response to pH is linear. This means there is a linear function between the voltage it produces and the pH of the solution it is in. This linear function has an offset and a slope which are the parameters that are set when you calibrate it. When the probe ages the slope will deteriorate and get flatter. This means you’ll get less voltage difference for the same pH difference.

pH meters seem to come in 3 forms: pen-style, protable handheld and bench top. Bench top meters are generally the most precise, least mobile and most expensive.

When it comes to choosing a pH meter, these are the specs and features you should look for:

- precision: for brewing purposes you want to have at least +/- 0.01 .. 0.02 pH units. This is plenty for testing mash pH and good enough for detecting pH rises in beer that can be a sign of autolysis. Some meters provide only +/- 0.1 which I consider too little and you may quickly find that you would like more than that. But if the meter is cheap enough you may go with that precision if you are only interested in being in the correct mash pH range

- calibration: Two concepts exist here: manual and automatic. Manual calibration requires you to adjust 2 knobs (one for the offset (pH 7.00) and one for the slope (pH 4.00 or 10.00)) after you places the probe into the appropriate calibration buffer. Automatic calibration does this for you. It tells you what buffer to place the probe into or recognizes the buffer on its own. My first meter had automatic calibration and it failed to recognize the buffers fairly early in its life. From that point on I had to do the calibration externally by using the values read for the 4 and 7 pH buffers and the value read for the sample. I’m now a proponent of manual calibration as it is more hands-on and there is less and the meter won’t be able to refuse to calibrate

- temperature correction: pH meters come with no, manual or automatic temperature correction (ATC) . Besides the pH of the solution the pH probe is also affected by the temperature of the solution and in order to determine the pH the temperature of the solution needs to be known. But the need for temperature compensation is generally overstated since the actual pH of the solution may also change with temperature. The latter requires you to state the temp that the pH was measured at or always measure pH at the same temperature. The same is true for the calibration buffers which have their nominal pH only at 25C and this is where you should calibrate the meter. Otherwise you need to look up the pH of the solution at its actual temperature. Many buffer solutions have table that will show you these pH changes. The same is true for wort. I always measure all my pH values at 25C which means that beer needs to be heated to this temp and wort and mash samples need to be cooled. As a result I do not have a real need for a temperature compensation on the pH meter. pH meters with manual temp compensation will have a dial or other means of entering the sample temperature and ATC meters have a temperature probe in addition to the pH probe. That probe may be in the same housing as the pH probe or a separate probe altogether. The latter allows you to test the sample temp before you submerse the pH probe in a possibly hot sample.

NOTE: many brewers think that ATC means that you can test the mash pH at any temp within the pH meter’s tempo range. While this is true you still need to know the temp dependent pH shift in order to correct the pH temp to the standard temp at which the optimal pH ranges were published. Briggs and DeClerck cite a pH shift of -0.35 between a room temps and a mash temp sample while my own experiments showed only -0.18. There doesn’t seem to be much data about this shift out there and the majority of the pH values that are given for brewing are room temp pH measurements. So just b/c a meter has ATC it is not more accurate, especially if the temp to pH function is unknown.

- pH probe connection: the standard for connecting pH probes are BNC connections. If you have a meter with a BNC connection you may be able to shop around for replacement probes as you are not tied to a specific connector design. It is possible that incompatibilities still exists that I’m not aware of, so treat this statement with caution. BNC connectors are generally found on bench-top or portable meters but not on the pen-style ones.

- mV read-out: Since the slope of a pH meter can be uses to asses its age it’s nice if the pH meter also has a mV read-out. But I have not seen this on the entry level pH meters that are priced attractive to brewers

- price: I think that you can get a decent pH meter suitable for brewing applications between $50 and $100. You can pay more but the added benefit may not be worth the money and you may be better off spending this money on another neat piece of testing equipment: A dissolved oxygen meter.

After considering all these facts I personally settled on the SM101 from Milwaukee (http://www.milwaukeetesters.com/SM101.html)

You can find if for ~$80 on the web. It has an accuracy of 0.01, manual calibration and BNC probe connector. The temp correction is manual for the few cases where I want to measure non 25C samples. I have been using it for about 2 weeks now and am very pleased with it. After the initial calibration I have noticed an only very slight drift which means that I can trust the pH reading even w/o constant calibration. I check the calibration whenever I use it extensively but don’t calibrate it for occasional uses since I rarely have to change the calibration anyway.

I hope this summary helped and I didn’t misstate anything. If that is the case please correct me. After all, I’m a brewer and not a lab technician.

Kai
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Old 07-07-2009, 06:53 PM   #2
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I nominate this for one of the "best posts ever" award. We should create a section on this board specifically for buying guide information such as this.

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Old 04-17-2010, 02:06 PM   #3
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I'm going to ask what may be an incredibly dumb question. After reading this I thought to myself, "Couldn't I just use a volt meter and use my own chicken scratch formulas to get the pH?" I'm going to assume the probes need to be made of a specific material for this to work, or would clean non-oxidized copper work? If I were to use fine emery paper before each reading to ensure the copper is always clean with no oxidation would I always get accurate results? Obviously there is a big question of precision. Is my multimeter precise enough for pH measurements?

I'm really curious if this is realistically possible.

Scott

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Old 04-21-2010, 01:21 AM   #4
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Nice post!

In addition to brewing, I also have been in the reefkeeping hobby for about 25 years (more reefkeeping than brewing during that span). Hopefully you will have better results than I did with the Milwaukee Instruments pH probe. Not that they are bad per se, as I have had limited results with others such as Hanna pens. However, I did find the Hanna pen to be easier to use and less costly for a replacement probe. IMHO, the biggest PITA with pens and probes of any sort is the need to calibrate frequently for best results. I'm speaking strictly from a reefkeeping standpoint. Use of the probe for brewing may be far more reliable long lasting. I concur on the pricepoint; one has to decide what makes sense of accuracy, reliablility and longevity.

IMHO, it is just as easy to use a good quality titration kit which seems to last as long or longer than the probe depending upon frequency of use. Not sure this would work too well with wort due to the color, although I haven't tried it. Also, there may be good quality test strips suitable for brewing. The next time I'm at my local micro brewery, I'll bring up the question of what they use! :-)

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Old 02-01-2011, 08:06 PM   #5
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For those of us still referring to this post, note that Milwaukee has updated the SM101 model number to MW101. Any pros and cons to the new model? I'm about to purchase.

http://www.milwaukeetesters.com/MW101.html

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Old 02-01-2011, 08:30 PM   #6
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I had a milwaukee one, and it Sh!t the bed after 2 months, frequent calibrations were necessary with each use and it would lose its calibration if you let it sit for 24 hours. I thought it was junk.

All that said I might have had a dud, but I do know this one is awesome. Have had it for over a year, rarely needs callibration, waterproof and under $100 shipped (+- .05) and it comphensates PH with temp of liquid.

http://www.hannainst.com/usa/prods2.cfm?id=002003&ProdCode=HI%2098128
HI 98128 (pHep 5)

No reservations reccomending this - I am still leary of milwaukee - but just from my bad experience.

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Old 02-01-2011, 08:32 PM   #7
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How about one that would work great for a brewer and a gardener?

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Old 02-02-2011, 12:21 AM   #8
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I still don't get the ATC thing. I thought that this does take the pH/temp shift into account. Mine has a temp and pH probe on the same unit so I figured that it would take the temp into account and adjust the pH accordingly?? I emailed Palmer several years ago and his input was that with ATC I would be looking for mash pH in the 5.4-5.8 range. I've always been confused as to which mash pH I should be targeting with my meter.

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Old 02-02-2011, 12:36 AM   #9
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I have been using one of these for years HI 98128 (pHep 5) without problems the trick is to rinse well after use and store the tip in storage solution

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Old 02-02-2011, 07:47 PM   #10
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Has anyone looked at the Sper Scientific DO and PH meter? You have to add probes still, but really it's not that bad.

http://www.sperdirect.com/cgi-bin/item/850081/DissolvedOxyge/-Water-Quality-Meter

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