Saturday, November 25, 2017

Testing pH meters - let's compare a few!

I've been talking a lot about testing the pH of our products as it's so important when we start using different surfactants, conditioners, and active ingredients.

I purchased three pH meters from Amazon. I chose them according to the reviews I saw left on the site. My goal is to calibrate each of them and test them in the calibration solutions to see how well they measure pH.

When you're considering a pH meter, look at at the calibrations it can do. Two is good, three is better.

And look at what the pH levels are for each calibration. It's easy to get pH 4, 7, and 10 calibration solution from Amazon or vendors like Lotioncrafter. If you're using 4.01, 6.86, and 9.18, you'll have to order those powders from the company online somewhere.

Please note, any links you see are not affiliate links. They're intended as information for you about these meters. 

The orange one is the HM Digital pH meter PH-200. (This is my second one as the first one didn't calibrate properly.) It was $100 or so on Amazon. It has an accuracy of +/- 0.1 pH and can also measure temperature in Celsius or Fahrenheit. It can do three point calibrations, pH 4, 7, and 10. It came with two calibration solutions, pH 4 and 7.
(See the link here...)

The yellow one is the Etekcity I purchased from Amazon for around $23. It has an accuracy of +/- 0.05 pH. It can do two point calibrations, pH 4 and 6.86. It came with two calibration powders, pH 4 and 6.86.
(See the link here...)

The red one is the WeePro PHpro 814 I purchased from Amazon for about $20. It has an accuracy of +/- 0.01 pH. It doesn't measure temperature. It can do three point calibrations, pH 4.01, 6.86, and 9.18, and it came with powders for each of those.
(See the link here...)

I thought I'd test three pH meters using three different batches of pH calibration solutions.

I have a pH 4, 7, and 10 calibration solutions from Lotioncrafter and a pH 4 and 7 solution from a company called Growtek I purchased from a local hydroponics store. I also made up the 4.0, 6.86, and 9.18 solutions that came with the pH meters.

Different meters require different first calibration methods. In general, you'll dip them into the solution to cover the electrode- the little bit at the bottom - and alter them if they aren't measuring exactly the number you expect.

I'll post the results in a chart below for easy reference. 

The PH-200 comes factory calibrated to pH 7, but it's always a good idea to make sure this is accurate. For this one, dip the meter into a pH 4, 7, or 10 pH. Lightly stir the meter for approximately 30 seconds. Press and hold the "temp/cal" button, the meter will show you the pH, then you can calibrate it by pushing a button up or down to get exactly the pH range you are expecting.

I generally use the pH 4 fluid to start so I can go up to pH 7 next. I calibrated it using the Lotioncrafter 4, then 7 solutions.

It measured the pH of the pH 4 calibration solution from Lotioncrafter as 4. It tested the Growtek pH 4 calibration solution as 3.98, which slightly more than the +/- 0.1 difference we can expect,.

It measured the measured the 6.86 as 6.98 and 7.00, the Lotioncrafter 7.0 as 7.00, and the Growtek 7.0 as 7.07. It measured the package 9.18 as 9.17.

The Etekcity doesn't mention if it's calibrated or not. When you remove it from the package, submerge the electrode into distilled water for 10 minutes. Make up the solution, then calibrate.

It suggested that I try the pH 6.86 solution first. Submerge the electrode into the solution, stirring gently for a few seconds, then waiting until the meter calculated a stable reading. It was reading 6.71, so I had to adjust it using the included screwdriver to be pH 6.86.

If I may be completely honest, it was a massive pain in the bum to try to get to the tiny screw on the back of the meter while it was immersed in a container of liquid. I'd move it a bit, then check the front of the meter, then adjust it again. It was so annoying! 

It measured the pH of the calibration solution from Lotioncrafter as 3.98, which is more than the +/- difference we can expect. It tested the Growtek pH solution as 4.06.

It measured the measured the 6.86 as 6.71 and 7.05. the Lotioncrafter 7.0 as 7.05, and the Growtek 7.0 as 6.71 and 7.10.  It measured the package 9.18 as 9.47.

The WeePro also doesn't mention if it's calibrated or not. It suggests you make up the calibration solutions using 250 ml of distilled water, then immerse the pH meter in each of the solutions.

Start with the 6.86 solution. Press the "CAL" button on the front for 5 seconds, then release. The display will flash 6.86, and you have to wait until it's finished flashing, which means it's calibrated.

Then put it into the 4.00 solution, press the "CAL" button again for 5 seconds, release, then press and release it again to get pH 4. It'll flash, then it's calibrated.

I tried the 9.18 solution and did the same thing, only this time I hit pressed the "CAL" button for 5 seconds, released, hit it again, released, then hit it again for the 9.18 flashing.

It measured the pH of the Lotioncrafter 4.0 solution as 3.97, and the Growtek solution as 3.97, which is more than the +/- 0.05 difference we can expect.

It measured the pH 6.86 solution in which we calibrated the meter as 6.72 and 6.65, the Lotioncrafter 7.0 as 6.78, the Growtek as 7.03, and the 9.18 as 8.56.

For all these tests, I made sure I had a very clean container and used distilled water from Superstore that had been purchased the day before and opened just before the tests. After doing the first 6.86 test, I realized that the water was colder than it should be at 12˚C, which means the pH would be higher at between 6.86 to 6.92, so I heated it slightly to 20˚C. The pH would still be a little higher at 6.88 rather than 6.86, but that's not that bad.

As an aside, for something like this, you want to make sure you always have freshly opened distilled, reverse osmosis, or de-ionized water as the carbon dioxide in the air can acidify it. It won't be a huge amount, but it could be enough to cause the slight changes we see in this chart.

You can see that they all did relatively well for the two 4.0 tests. The PH200 was out by 0.01, which is within its usage guidelines, while the others were only out by 0.02 and 0.03, which is just fine. The PH200 and WeePro were a little under for the Growtek solution, but again, not by much. The Etekcity  was out by 0.06, but that's still not that big a deal.

The package labelled as 6.86 is quite out of whack, and this is concerning for one huge reason - this is the one I used to calibrate the Etekcity and WeePro meters. If it's out of whack by that much - if I use the PH200 findings, it's out by at least 0.12 around 7.00, this means I've calibrated my meters to read lower than they should, which I think is why the WeePro is reading the 6.86 as 6.72.

So I made up a second batch of this powder and ensured the temperature was around 20˚C - the results in the brackets - and my PH200 registered it as 7.00, the Etekcity as 7.10, and the WeePro as 6.65. (I have no idea what to make of this.) I honestly think this may be a pH 7.00 solution, not a 6.86 solution.

The Lotioncrafter 7.0 came up as 7.00 for the PH200, 7.05 for Etekcity, and the WeePro as 6.78.

Growtek 7.0 was a little higher for all of them, but not enough to worry about as it's less than 1% for each of them.

The 9.18 package was good for the PH200, but the others were really out of whack there, 0.29 higher for the Etekcity and 0.62 lower for the WeePro, which is a 7.24% difference. This worries me.

What to make of all of this? If I had to make a recommendation, I'd say go for the PH200 or the Jenco 630 I used to own. It's worked well for me for a while now, offering me what I consider to be very accurate measurements. The downside is that it's $100. That isn't a lot when you consider that I had a comparable machine for 7 years and only replaced it because the electrode needed replacing and I couldn't find a place to purchase it. I would have happily bought that meter again, but I couldn't find a place in Canada to buy it, and the PH200 was at Amazon.

Given that most of what we do has an acidic pH level - except for soap and stearic acid-TEA emulsifiers (like Lush uses) - and given the Etekcity tested well with with all but the alkaline or 9.18 solutions, I could suggest this pH meter if you're looking to spend less than $25. Ignore what the last number is on the meter - so if it registers as 7.14, just assume it's 7.1 - and you can get some good readings. Just know that if you're over pH 7, adjust the pH slowly and measure often. I really hate the way you can calibrate the machine by using the screwdriver, but that's only a small downside.

I can't recommend the WeePro at this time, but I plan some more tests with it. I need to try calibrating it again to see how it performs.

If you plan to use these machines, don't rely on their testing powders. Visit your local hydroponic store or vendor and get their version of the calibration solutions.

Join me tomorrow for more on pH meters as this is far too long!

Related posts:
How to test the pH of our products and more (updated for 2017) - loads of links in this post

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Clare J said...

Why does the temperature affect the pH? Overall do you think you get what you pay for with pH meters, so it's worth paying more? Interesting that the two cheap ones claim to be more accurate than the higher priced one... hmm.

Erica Echternach said...

I use Dr Meter ph-100. I haven't had any problems with it so far....but now I feel like I better go retest it. LOL

Ashlee said...

Very timely! Thank you for this comparison, it really narrows down the choices for me :)

Desiree van Schaick said...

I would definitely advise a Ph meter; I make shampoo with decyl glucoside and bring the Ph down with lactic acid. When I did this for the first time and used strips, they did not indicate any difference between no acid, a little acid or a lot of acid so I do not trust them. Ph meter is a bit of an investment but so easy and accurate to work with!

Marcia said...

Thank you, now I had better invest in a Ph meter! -following