Conductivity Meters: pinpoint vs used lab grade units

Randy Holmes-Farley · Oct 6, 2017

I'm not sure how Oakton and Orion meters compare, but with the best modern meters you can set your own temperature coefficients:

from an Oakton Con 700:

http://www.4oakton.com/Assets/manual_pdfs/35411-00-01OaktonCON700IM.pdf

8. CALCULATING TEMPERATURE COEFFICIENTS To determine the temperature coefficient of your sample solution use this formula:

Where: tc = Temperature coefficient 25 = 25 ºC
CT1 = Conductivity at Temp 1
CT2 = Conductivity at Temp 2
T1 = Temp 1
T2 = Temp 2

NOTE: A controlled temperature water bath is ideal for this procedure.
1. Immerse the probe into a sample of your solution and adjust the temperature coefficient to 0% (that is, no compensation) by following instructions as described in Section 6.8.
2. Wait for 5 minutes. Note T1 and CT1 (conductivity at T1).
3. Condition the sample solution and probe to a temperature (T2) that is about 5 ºC to 10 ºC different from T1, and note the conductivity reading CT2. NOTE: Record your results for future reference. Ideally T1 and T2 should bracket your measurement temperature, and should not differ by more than 5 ºC.
4. Calculate the temperature coefficient of your solution according to the formula shown above. 5. Enter the calculated temperature coefficient into the meter.

The calculated temperature coefficient will now be applied to all the meter readings.

Tim Olson · Oct 7, 2017

Randy Holmes-Farley said:
I'm not sure how Oakton and Orion meters compare, but with the best modern meters you can set your own temperature coefficients:

from an Oakton Con 700:

http://www.4oakton.com/Assets/manual_pdfs/35411-00-01OaktonCON700IM.pdf

8. CALCULATING TEMPERATURE COEFFICIENTS To determine the temperature coefficient of your sample solution use this formula:

Where: tc = Temperature coefficient 25 = 25 ºC
CT1 = Conductivity at Temp 1
CT2 = Conductivity at Temp 2
T1 = Temp 1
T2 = Temp 2

NOTE: A controlled temperature water bath is ideal for this procedure.
1. Immerse the probe into a sample of your solution and adjust the temperature coefficient to 0% (that is, no compensation) by following instructions as described in Section 6.8.
2. Wait for 5 minutes. Note T1 and CT1 (conductivity at T1).
3. Condition the sample solution and probe to a temperature (T2) that is about 5 ºC to 10 ºC different from T1, and note the conductivity reading CT2. NOTE: Record your results for future reference. Ideally T1 and T2 should bracket your measurement temperature, and should not differ by more than 5 ºC.
4. Calculate the temperature coefficient of your solution according to the formula shown above. 5. Enter the calculated temperature coefficient into the meter.

The calculated temperature coefficient will now be applied to all the meter readings.

That's awesome!

BTW, the formula didn't show up, so here it is:

For fun I followed the procedure and calculated a Temperature Coefficient of -0.3375 for my DIY standard, although it may not mean much for the Pinpoint.

FYI my values were: T1 25, T2 22.2, C1 52.9, C2 53.4

Randy Holmes-Farley · Oct 7, 2017

I'm not sure what your standard was, but if it is a seawater mimic (as opposed to a conductivity mimic made from, say, KCl or NaCl), then you could use it as a correction on top of the Pinpoint's built in correction. Just using tank water to get a correction factor makes the most sense.

Tim Olson · Oct 7, 2017

Thanks again! For this test I used the DIY Morton's salt standard, which it sounds like is a conductivity mimic. I'll redo the test with tank water as you suggested and calculate the correction factor.

FYI, I use Aquavitro "salinity" salt mix (Seachem) , if that makes any difference.

Randy Holmes-Farley · Oct 7, 2017

Yes, the temp correction varies with the actual ions present, which is why it can vary and why someone might want to determine their own, and determining it for a tank water sample (or any seawater, really) is best if that is what you intend to measure.

Falcon53 · Oct 23, 2017

So I finally received the pinpoint conductivity standard yesterday, and got it to 25C using a calibrated thermometer. After calibrating, I measured my tank. It was showing 49mS, or 32ppt. I have been using the digital Milwaukee meter for years - calibrated per their instructions. It tells me my water is 1.026, or 35ppt.

I can't believe how far off the Milwaukee is. Before I do anything, is it possible I could have calibrated the Orion incorrectly? I'm assuming if it's reading 53mS at 25C, I must have done it correctly - assuming the Pinpoint solution is correct, which I trust more than the accuracy of my home scale!

Randy Holmes-Farley · Oct 23, 2017

Possible.

When you put the 53 mS/cm Pinpoint standard into the Milwaukee, what does it read?

Tim Olson · Oct 23, 2017

Falcon53 said:
So I finally received the pinpoint conductivity standard yesterday, and got it to 25C using a calibrated thermometer. After calibrating, I measured my tank. It was showing 49mS, or 32ppt. I have been using the digital Milwaukee meter for years - calibrated per their instructions. It tells me my water is 1.026, or 35ppt.

I can't believe how far off the Milwaukee is. Before I do anything, is it possible I could have calibrated the Orion incorrectly? I'm assuming if it's reading 53mS at 25C, I must have done it correctly - assuming the Pinpoint solution is correct, which I trust more than the accuracy of my home scale!

I've had a Milwaukee meter for a couple of years and was never able to get accurate results, even with the Pinpoint standard and calibrating it correctly. It was usually off by couple points, since the accuracy is +/- 0.002 (e.g. 1.026 would read 1,024 or 1,028). I finally quit using it, since I couldn't trust it and now only use my Pinpoint Salinity Monitor and a refractometer.

Hope this helps

Falcon53 · Oct 23, 2017

Randy Holmes-Farley said:
Possible.

When you put the 53 mS/cm Pinpoint standard into the Milwaukee, what does it read?

Sorry for the delay...I wanted to get better info.

So I warmed two separate bottles of pinpoint solution - one purchased this week and another a few years old - to 25c confirmed with a calibrated thermometer. I also used a third bottle of Aqua Craft calibration solution at current room temperature of 21.1c. I ran three tests of each solution (9 total) on the Milwaukee, cleaned with RO/DI between uses, and allowed to sit for 60 seconds before reading. Every single response was 1.028 - and slightly above that because I was getting 38ppt. Either all three calibration solutions were off (possible), and off by the same amount (fairly unlikely), or the Milwaukee reads too high, which I would say is most likely in this case.

I then used the Orion that I calibrated last night on the Pinpoint solutions only, as I didn't want to rip the top off of the Aqua Craft solution. The newer solution gave me 52.8mS, and the older solution gave me 52.6mS, which both round up to 35ppt.

I think it's highly likely at this point that the Milwaukee reads high, and my tank water is much lower (32ppt) than I previously thought (35ppt).

So Randy, would you disagree with that conclusion?

Second, what would you do from here if I want to get to 35ppt? I would assume I should just get my water changes at 35ppt and just keep going until I eventually get there. The weird part is that my alkalinity tests at 190ppm, Calcium at 450ppm, and mag at 1380ppm. I would not expect levels this high with my water at 25c and salinity at 32ppt. What are the implications for my big three if I raise the salinity from here?

Thanks for your input and my apologies to the OP for hijacking the thread!

Randy Holmes-Farley · Oct 24, 2017

I don't know how the Aquacraft is made, so I cannot comment on its suitability in a refractometer.

Some 53 mS/cm solutions or other conductivity calibration solutions (including my DIY)are not suitable for use in a refractometer (or more correctly, will not read exactly 35 ppt in them).

Lou Dell of American Marine (Pinpoint) claims their 53 mS/cm solution is also a seawater composition mimic, so it will be suitable for 35 ppt calibration or checking of a refractometer.

So while I cannot be sure your calibration solutions are not all "off", I think your conclusion that the refractometer is off is a more likely conclusion.

If you are at 32 ppt and raise salinity to 35 ppt, most ions will rise by about 35/32 = 1.094. That means that magnesium at 1300 ppm will become 1300 x 1.094 = 1422 ppm.

Falcon53 · Oct 24, 2017

Randy Holmes-Farley said:
I don't know how the Aquacraft is made, so I cannot comment on its suitability in a refractometer.

Some 53 mS/cm solutions or other conductivity calibration solutions (including my DIY)are not suitable for use in a refractometer (or more correctly, will not read exactly 35 ppt in them).

Lou Dell of American Marine (Pinpoint) claims their 53 mS/cm solution is also a seawater composition mimic, so it will be suitable for 35 ppt calibration or checking of a refractometer.

So while I cannot be sure your calibration solutions are not all "off", I think your conclusion that the refractometer is off is a more likely conclusion.

If you are at 32 ppt and raise salinity to 35 ppt, most ions will rise by about 35/32 = 1.094. That means that magnesium at 1300 ppm will become 1300 x 1.094 = 1422 ppm.

Thanks, Randy. I guess I'll let the salinity slowly increase and observe what happens. I'll probably also check the salinity with some other manual refractometers first.

Randy Holmes-Farley · Oct 24, 2017

Sounds good.

Happy reefing.

Velcro · Oct 24, 2017

My aquacraft did not read the same as my two DIY solutions I made. I made KCl standards based on Randy’s article using a lab grade balance and both of them read dead on 35.0 ppt with the factory cell constant. The aquacraft was brand new and was either 37ppt or 33ppt. I can’t remember exactly what but I remember it was two off.

I trust the diy solution.

Velcro · Nov 16, 2017

Still loving these meter... definitely the best thing I've used so far. I gave up on the commercial calibration solutions... made randy's KCl recipe using a new, sealed container of salt substitute (100% KCl) to prevent significant moisture content and weighed it with a lab grade electronic balance (1mg readability). This was the best balance that I had access to, but next time I make this I'll go back to my old employer and use their $20,000 Sartorius analytical balance that has 0.01mg readability (for fun).

As I said, Randy's DIY KCl solution read dead on 35.0ppt using the factory cell constant on what looks like a brand new conductivity probe. I lucked out and my used meter for $100 came with a probe that retails over $600... I don't think they knew what they were selling!!! I also used aquavitro salinity and weighed out 36.27 grams/L and it was again dead on 35.0ppt. This is proof enough for me to ignore the commercial calibration solutions unless I ever order one with NIST traceability.

Falcon53 · Nov 16, 2017

I just did another DIY batch and I am coming close to 35ppt...34.3 is what I am getting. But I did use old salt, so maybe I should try another method. What did you use for KCl?

Velcro · Nov 16, 2017

Falcon53 said:
I just did another DIY batch and I am coming close to 35ppt...34.3 is what I am getting. But I did use old salt, so maybe I should try another method. What did you use for KCl?

Falcon53 · Nov 16, 2017

Thank you

Falcon53 · Feb 2, 2019

Sorry to dredge this thread back up, but I was hoping @Randy Holmes-Farley or someone in the thread could answer...can you sanitize these probes with bleach without damaging them? If not, what can you use? I also need to ensure there is no cross contamination of copper. I mistakenly stuck the probe directly into my QT today instead of pulling a sample. I don't know if the fish in QT have disease, but I certainly don't want it in my display, nor the copper! Any help is appreciated!

Velcro · Feb 2, 2019

Falcon53 said:
Sorry to dredge this thread back up, but I was hoping @Randy Holmes-Farley or someone in the thread could answer...can you sanitize these probes with bleach without damaging them? If not, what can you use? I also need to ensure there is no cross contamination of copper. I mistakenly stuck the probe directly into my QT today instead of pulling a sample. I don't know if the fish in QT have disease, but I certainly don't want it in my display, nor the copper! Any help is appreciated!

Each probe will list what it is compatible with. That being said, it just needs to be dry for a couple days to be considered sanitized in regards to fish diseases. That’s how tank transfer method works... you just leave the tanks dry for a couple days.