Cheapo pH Meter Experience

Systems, Gear, & DIY
1

Chasing pH problems lately, I decided to try out one of the cheap (13 bucks) pH meters that are on amazon.  I’m not advocating for this one in particular, I thought just I might share my experience with it.

First off, here is the link to the one I got:
https://www.amazon.com/Dr-Meter-0-1pH-Accuracy-Tester-Measurement/dp/B00PU0W35K/ref=sr_1_2?ie=UTF8&qid=1483931972&sr=8-2&keywords=dr+meter

When I got it, I followed the calibration instructions and noticed right off the bat that it was REALLY unstable.  I stood 6ft away from the meter and it would change readings as I walked toward it or away, and forget touching it.  Anyway, so I followed the calibration and it read correctly for the 6.89 and 4.01 buffer solutions if I gave it a many minutes to settle in and stood far away from it.  So, I washed it with distilled water per the instructions and put it away (wet).  The instructions are pretty minimal, and say nothing about storage solutions or anything.

Two days later, I am going to brew so I pull it out and do some sanity checks on it.  First, I noticed that the instability of the thing is gone and it reads nice an stable now.  I assume this is because the sensor in it got dried out and being in the buffer solutions and put away wet allowed it to rehydrate and so it plays nice again.  So I put it back in the buffers and recalibrated it again to the 6.89 buffer (it was +0.5 off at this point).  Now it was time to test some stuff and see if it gave the expected results.

First was the 4.01 buffer provided with the meter.  I found that it took about one or two minutes to settle to a number, and read 4.00.  Close enough for me.  Next, I did a distilled water mash of pilsner malt which should be 5.9 according to the malt analysis sheet, and after a minute of settling the meter read 5.89.  Again, close enough.

Next I brewed some beer (czech lager) and compared mash the results of the spread sheet to what I actually got (spread sheet provided by our LODO friends, thanks).  The predicted mash pH for this batch was 5.3, and the measured value with this meter came in at 5.29.  Again, there seems to be a systemic 0.01 error most likely from my calibration.  In any case, close enough for my needs.

So I can’t speak to the longevity of the thing, but at least at first whack it was well worth the 13 dollars.  When I bought it I was pretty skeptical that it wouldn’t be total junk but figured for the price I would try one it out just to find out (and let others know what I found as well).  If I have problems with it in the future, I’ll post it.

2

The Dr. Meter brand name does lend a certain panache.

3

Oh yes, none of that Mr. Meter crap…

4

As a chemist, I can’t believe I’m saying this but, honestly, they aren’t the worst as long as you calibrate them every time you use them.

5

I got a very similar one a few months back.  Worked really well until I submerged it about 2/3 deep in Starsan and some got inside.  That was it.

6

I have noticed the calibration doesn’t last too long (long enough though for what I do).  I’m curious if it would drift less if I stored it in a storage solution.

7

I just got my first pH meter for Christmas – it’s almost an identical $13 model compared to the one posted above… and I’ve got to say, I love it.  I didn’t even have to calibrate it, it was reading correctly from right out of the box, and the convenience of just sticking it in a sample and getting an instant result instead of having to interpret different shades of puke from the color strips… so much better.  If it breaks or goes too far out of cal, I’ll just buy a new one!

8

I’ve run a Dr. Meter vs one of the research bench-grade pH meters at work. When I calibrated it right before testing, the Dr. Meter was within +/- 0.02 of the research grade meter. The difference is that I could leave the research grade meter in storage solution for 6 months, then use it without calibration, and it would be just as accurate (not that I’d ever actually do that). You can’t do that with a $20 meter.

9

Is the sensor element similar on the cheap vs expensive ones?  As I mentioned in a previous post, they give little information in the instructions, and no storage guidance.  I would assume that it really should be stored in a storage solution (KCl), but that isn’t based on much.

10

I’m not certain but my guess is that the cheap ones are Ag/AgCl reference electrodes whereas research grade are typically calomel (Hg/Hg2Cl2). Calomel are significantly more robust reference electrodes within a pH meter. The glass electrode used as the hydrogen ion (H+, proton) conductor is probably of better quality on a high-end instrument as well. The electronics are probably about the same.

11

All pH meters have a reference junction. When a new meter is received most likely both the pH sensing bulb and the junction need to be hydrated. A meter shoudl be placed in storage solution or a pH buffer for at least 3-4 hours before use.

It is common to see the pH calibration change if the probe is not hydrated. The reason being is that calibration is compensating changes to the sensor. So a dry sensor has one characteristic as compared to a hydrated one. If the probe is calibrated when it is dry and then allowed to hydrate, the meter will no longer be calibrated. Seeing a 0.5 pH change as the hydration process occurs over time is not unusual.

The drying of the junction was most likely the cause of the sluggish response and instability issue. The junction is an electrical pathway from the internal reference to the sample. If dried out or clogged then current cannot pass and the result is erratic readings.

The vast majority of pH meters available are using pH electrodes with a Ag/AgCl reference. That is one of the wires in the pH electrode is coated with a silver chloride coating and then placed in a solution saturated with silver. A reaction occurs that produces a stable potential as part of the measuring circuit. Even though we still make pH electrodes with Hg/HgCl reference, they are not typically used due to the handling of mercury. In R&D the Hg/HgCl reference is used as a benchmark when testing new designs.

My guess the reason that the research grade benchtop held its calibration is that the probe was hydrated, calibrated and stored properly. I have had testers sitting in a instrument case for 6-12 months and pull them out to see that they are holding their calibration.

The $13.00 price is appealing for a pH meter but there should be many other considerations when choosing a pH meter than just price. For example, local warranty, support, and that the products are QC by the seller and not just passed along through a distribution channel. For the more advanced brewer that is investing in portable and bench meters then it becomes more important to understand the choices in sensors available including a sensor that can withstand high temperatures.

For the user on a fixed budget, we have just launched a new meter at $39.50. Has all the bells and whistles including battery percent level, stability indicators, temperature readout and automatic calibration. The feature that I like best is the extractable junction since if it gets clogged (i.e. solids in mash) then it can be cleared making work like new again. 

http://hannainst.com/hi98107-phep-ph-tester.html

Before Hanna, these testers did not exist. We are the one that brought the technology to market in 1986 and have been improving it ever since.

12

Interesting unit. Too bad it doesn’t report to the hundredth. That is helpful when checking wort samples.

13

Looks like the pHep+ unit ($10 more) reports to the hundredth.

14

I assume Nate linked the wrong meter, and actually meant to link to their 0.01 accuracy model which is also ~$13. I make this assumption because in his OP he is talking about measurements that are to the hundredths.

https://www.amazon.com/Dr-Meter-Backlit-Accuracy-Pocket-Measurement/dp/B00LNSEZH2/ref=sr_1_4?ie=UTF8&qid=1484163175&sr=8-4&keywords=dr+meter+0.01

15

The link provided to the Hanna product was for a 0.1 pH resolution. We do have the exact same meter with a 0.01 pH resolution for $10.00 more.

http://hannainst.com/hi98108-phep-ph-tester.html

It is important to note that unless you monitor the offset and slope of the pH electrode it is difficult to obtain a high accuracy reading that is better than 0.1 pH units.  I have been with my company for over 17 years and have worked with pH in pretty much every industry and with every format (pockets to process control).

Just because a pH meter reads pH 7.01 in a buffer after being calibrated does not mean that it will read the sample accurately. I have seen greater than a 0.5 pH unit difference between two probes in the same sample even though they read the same value in the buffers.  The reading I trust is based on the offset and slope. That is in pH 7.01 the probe needs to be +/- 30 mV and the slope (mV change from pH 7 to 4) greater than 90%. Manufacturers program the meters to accept +/- 60 mV or greater in pH 7.0 and a slope of at least 85%.

Below is a link to basic tips on pH electrode maintenance.
http://hannainst.com/ph-electrode-maintenance-calibration-guide

The electrode characteristic is very important. Our QC tolerance in manufacturing is +/- 10 mV in pH 7 and a slope greater than 95%. This is the area that is the biggest difference between companies like Hanna, Mettler, Metrohm, Hamilton and other reputable companies and the knock offs.

To truly understand in there is a difference between a $13.00 meter and a $50 one it would be critical to understand the electrode characteristic. The biggest challenge for a manufacturer is with the sensor. It is the most important part of the measuring system.

Personally, I prefer a 0.01 pH resolution but for the number to be meaningful would require a knowledge of pH electrodes and the behavior in the buffers. For the brewer measuring a pH less than 5.6 at 140-150 oF it would be more important to have ATC than the increased resolution due to the effect temperature has on the membrane potential.

Unfortunately, I cannot figure out to attach an article that was written for a publication that was based on the Orange County Water Department problem with pH readings that were 1 pH unit apart from a field meter versus a lab meter.  The problem was with a 60 mV offset (fixed by simple cleaning) and a low slope due to their pH 10 buffer actually being pH 9.5. If anybody is interested then just message and I will forward.

16

The electrode is not replaceable on this model, right?  Still only expected to have a 12 month life?