Calibrating Orion 128

[esse]

I've been wanting to get a better idea 'how much' is in solution when I'm dosing lime water via ATO. @Randy Holmes-Farley I understand from your articles that the best way to do that is by measuring conductivity.

I was thinking about converting a number from my TDS meter, but I wasn't sure that I was getting the right idea. Then, I saw there was an orion 128 meter selling on the boards. It arrived yesterday, but I'm lost about where to begin.

Initially, is there a calibration process that I'll want to do? I tried the probe in a cup of RODI water mixed with 1 tbsp BRS kalkwasser. It read 9.4 mS/cm at 20 degrees C. I let the solution sit overnight, and it's now reading 9.2 mS/cm at 18 degrees C. If I understand correctly, I should see something like 10.3 mS/cm?

 

[Randy Holmes-Farley]

I have and very much like the Irion 128, but I do not think the calibration is able to be adjusted. Maybe it can be internally somehow, but there's no externally available way to adjust it.

I used mine for many years, and the calibration never drifted. i used a commercial 30.1 mS/cm standard (among others) and it read a tiny bit low, but never, ever changed from the small offset.

The 10.3 mS/cm is at 25. Not sure what it should be at 20 deg C, but likely not much less than the 10.3 (maybe more).

I'd get or make a 53 mS/cm standard and see what the probe reads.

 

[esse]

I tried mixing 6.2 grams of table salt with 182ml rodi water +/- measurement error.
My refactometer read about 1.025 & the probe read 54.0 mS/cm at 25

I took a few more measurements using a couple of the standards that came with the device, and I tried the calibration fluid that I used for my TDS meter
12.9 mS/cm standard read 13.9 mS/cm at 18.3 degrees
1413 µS/cm standard read 1544 µS/cm at 18 degrees
342 ppm as NaCl standard read 734 µS/cm at 18.4 (the bottle also said 700 µS/cm)

Based on these measurements, I think the probe is reading just a touch high, which makes me confused about the low measurement for the saturated limewater solution.
I remeasured the kalk solution at 25 deg C and it's reading 8.6 mS/cm. My TDS meter is reading 5850 ppm

 

[Randy Holmes-Farley]

The first measurement using my DIY standard is very close, maybe as close as you can measure out 6.2 grams, since it is only off by 1.8%. I agree you might conclude it reads a little high. Mine read a tad low.

 

[esse]

If I assume the probe is reading fairly accurately, what might cause the low reading for the kalk solution. It seems fairly far off from the 10.3 mS/cm

 

[Randy Holmes-Farley]

If I assume the probe is reading fairly accurately, what might cause the low reading for the kalk solution. It seems fairly far off from the 10.3 mS/cm

I don't know, but if I wanted to use such a standard, I'd push the saturation even more by adding a few more tablespoons and mixing it a bit. Saturation can be hard to attain.

 

[esse]

Thanks so much for your help with this! I tried again and it seems to top out at 9.6 mS/cm despite adding another couple tbsp.
1 more question.. If I plan to dose vinegar with the ATO top off, how might that affect the reading?

 

[Randy Holmes-Farley]

Thanks so much for your help with this! I tried again and it seems to top out at 9.6 mS/cm despite adding another couple tbsp.
1 more question.. If I plan to dose vinegar with the ATO top off, how might that affect the reading?

The conductivity will be higher at saturation, but I'm not sure how much higher. Depends, too, on the amount of vinegar added.

If you just add vinegar (acetic acid) to a calcium hydroxide solution, the conductivity will likeyl decline if you do not add more calcium hydroxide to re-attain saturation, because acetate is a lot less conductive than is hydroxide:

CH3CO2H + OH- --> CH3CO2- + H2O