After 6 months of moderate use, my Hanna pHep-5 meter developed a problem in its electronics making the display hard to read. Hanna has a couple of aspects of poor design, so I've decided to buy another meter from a different maker. I'll first buy a new meter, and then return the Hanna for warranty-replacement so I won't lack a meter during the return process. I'll end up with two meters.
I'd like a meter that has an accuracy of +/-0.02 pH, and that uses a standard BNC connector. Price around US$200-300. Any recommendations?
There are many meters on the market, and here are a couple that appeal to me:
Cole-Parmer meter: http://www.coleparmer.com/Product/Co...al/WU-05996-60
This meter lacks bells and whistles, but looks solid with good resolution. Probe must be purchased separately.
Sper 850056 kit: http://www.sperdirect.com/cgi-bin/it...d-pH-Meter-Kit
More features than the Cole-Parmer, and includes a probe, for a lower price. But I wonder if the quality of this is good enough. I'm leery of anything that looks like mass-produced junk.
Oakton sells many lower priced meters, but my concern as always is the quality.
Opinions are encouraged!
Why in the world do you need that extreme level of accuracy?
Because I'm creating new developers, and I need better accuracy than +/-0.1 pH to insure good consistency and repeatability.
Originally Posted by RattyMouse
When I mix developers, I am consistently within +/-0.02 pH. When the difference reaches 0.05 pH, I know something went wrong. But that requires a two-digit meter.
if you really want to use the extreme accuracy of +/- 0.01 pH-values, you have to use the best possible electrode for your work. You should contact a laboratory pH-meter manufacturer or a supplier for laboratory equipment for the best recommendation for your work. Only professional laboratory pH-meters (several haundred $ or €) can use different types of dedicated electrodes. Furthermore it is important that your meter is temperature-compensated and can be calibrated by at least two, better three calibration buffer solutions. The shelf-life of these calibration standards is limited. When not in use the electrode has to be stored in a specified storage-solution in order not to get dried.
The cheap Hanna meter broke! I wanted to be able to run experiments this weekend, so I built my own based on this web site that describes a simple DIY meter. Its heart is a TL082 op amp. After spending a few USDs at Radio Shack, I built this (inside view):
Attachment 55262 Attachment 55263
Here it is taking a measurement of a developer. Connect a voltmeter to it, and then calculate pH based on millivolts.
You calibrate it any way you'd like, which gives you a slope and offset. Having those, the actual pH calculations are quick; much quicker than the time it takes to measure and mix the chemicals and wait for the probe to settle.
I want a better probe than the one that came in the Hanna (pictured above). Do you suggest single- or double-junction? Refillable versus gel? I've heard that refillable probes are faster than gel-electrodes. Mine takes about 5 minutes for the tenths of millivolts to stabilise -- that's slow! jochen pointed out that good probes are important. Any opinions about these two:
I don't have any personal knowledge about the items you linked, but I read quite a few photo chemical samples in my younger days - beyond 20 years ago. We used info from Kodak for our guidelines. Similar to what Kodak's motion picture group has available here (if you haven't already seen it): http://motion.kodak.com/motion/uploa...3_ulm191-2.pdf
I think you'd be fine with a single junction reference. Double junctions are mainly for special cases where something you're doing will cause interference. We typically used separate electrodes - this lets you rotate out the pH electrodes. And if something goes bad, you only replace that electrode.
I never personally used gel electrodes; they were either frit or anular (sleeve) junctions. A frit junction is harder to troubleshoot for clogging; with sleeve junctions, you can just open them up to drain, then refill. So if there are any clogging issues, a sleeve junction is good. Note that we were testing replenished and regenerated solutions; if you only test scratch-mixed B&W developers, I suspect that these are less problematic.
Whatever you use, you'll probably get a little faster response by gently stirring the sample while reading. It should be a constant speed, so a small magnetic stirrer works great. Calibrating (with buffers) should be done the same way.
Mark, are you taking requests? :)
Originally Posted by albada
At least you didn't ask me to build a pH meter for you. :)
Originally Posted by Michael R 1974
Try looking for Beckman Zeromatic model. Again the accuracy as someone mentioned is in the probe itself. Get your buffer solutions to calibrate and you're set.
When I was doing photo chemical analysis in the 80s under military spec requirements, we used Beckman pH meters as the standard. Lots of Beckman stuff on eBay cheap.
And, in best practice you leave them running all the time with the probe in a proper buffer solution.
Thanks for the suggestion. I'm realizing that I can't go cheap on the probe, so I'm looking at a refillable model from Thermo Scientific. Interesting that the Beckmans are cheap on eBay. I'd guess those meters were well north of US$1000 when new.
Originally Posted by kreeger