Hach replacement sensor

monkeyman1000 · Apr 18, 2014

My replacement sensor arrived from Hach today. They were a bit backlogged it seems. Running a stability check on it now. I did notice that the bulb on this one was clearer and had less liquid in it than the old one. I tried to clean the old one per the manufacturer's instructions and with the solution they provided about a month ago.
Calibrated it per manual. Fresh buffers used. Time listed is counting down.
at 1 hour 30minutes 3.99 at 70.9F
1 29 3.99 at 70.9
1 28 4.01 at 73.0
1 24 4.03 at 76.1
1 19 4.04 at 77.0
1 17 4.04 at 77.0
1 14 4.04 at 76.8
1 04 4.06 at 75.9
52 4.06 at 74.7
46 4.06 at 74.1
42 4.07 at 73.8
29 4.07 at 72.9
Stopped at this point. Not as good as the first sensor results but would this be considered stable enough to use? Or should I get my money back?

ajdelange · Apr 19, 2014

Yes, it's good enough to use but it does not meet Hach's own spec. They say in the instructions (or somewhere) exactly what their accuracy spec means and that is that readings are held for some period of time in one of the buffers at constant temperature (I don't remember the details).

What you decide to do depends on how stubborn you are. I remember going around with Corning on a meter many year ago. They sent me 5 electrodes before I got one that worked. Were I you I think I'd probably keep fiddling with this electrode to see if it 'wears in'. It is quite stable over about an hour. What is more concerning is the jump to 4.07 in the first half hour. That suggests that the reading was not stabilized at time of calibration. So I guess I'd recal at the end of the run and try again. If it doesn't settle down then it is clearly out of spec and you should demand either another electrode or your money back and your argument should be 'it doens't meet your spec.' How many cycles of that you are willing to go through is something you will have to decide.

ajdelange · Apr 19, 2014

I thought, after your experience, that I'd better go back and check my unit which I haven't used in a couple of months and which has been stored with only a couple of drops of water in the cap to keep the humidity up in there so the bulb won't dry completely. The results are in the graph. I did see drift in the first couple of minutes and from there on it was pretty stable over 2 hrs. The message from this picture is that I should have waited longer before accepting the cals. Had I done so the 'drift' would have been ±0.01 over 2 hrs. This may have relevance for you. Be absolutely sure you are doing the calibration correctly and wait a good long time (5 min isn't unreasonable) before accepting the cals. If you don't, after doing that, get a plot something like mine then you have a legitimate gripe with Hach. You can argue that not only does your unit not meet spec but that you have corresponded with others who are getting much better performance than you are. If replacing the electrode doesn't fix the problem that suggests that it is the meter, not the electrode that has failed. But be sure to get your new electrode thoroughly hydrated before coming to that conclusion.

monkeyman1000 · Apr 19, 2014

Thanks for the reply AJ. I set a timer for 7 minutes on each buffer before accepting the reading. I will redo the stability check again tonight to see how it goes. It definitely could be something I am doing wrong so I will recheck to be sure. Really appreciate the advice.

monkeyman1000 · Apr 19, 2014

Better results with the second stability check on the replacement sensor. When I calibrated I left the probe in each buffer for about 7 minutes before accepting the reading (the readings were stable after about 3-4 minutes). After I accepted the 4.01 reading I kept it in the 4.01 buffer. It dropped to 3.99 but stayed there for about an hour, then went to 4.00 for about 5 minutes then back to 3.99. I checked for about an hour and 10 minutes total with temps of the 4.01 buffer ranging from 69.9 to 75.9. I guess this means it is good for now? I am brewing tomorrow so will check a wort sample then.

ajdelange · Apr 20, 2014

Yes, much better. Glad to hear it's working out. There is a bit of an art to using a pH meter and it takes a while to get the hang of it.

monkeyman1000 · Apr 20, 2014

quick update from today. Recalibrated the meter just prior to taking a sample. I measured the sample at about 75 deg F. i took the sample about 47 minutes into the mash and got 5.34. Bru'water predicted 5.35 and Brewer's friend predicted 5.33. EZ water predicted 5.49. Hope this sensor lasts but gaining more confidence in the spreadsheets.