Neptune Probe and Salifert test disagree on pH

Dkeller_nc

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A few comments about this that may be tangentially helpful for those that aren't scientists/engineers (those that are scientists will likely roll their eyes and say "yeah, dude, we know that!"):

While the definition of pH solely depends on solution chemistry and the concentration of the hydronium ion, actually assaying for pH has a lot more complexity than that. In particular, typical pH meters depend on the electrical potential setup between a reference electrode solution (often a reference electrode gel in the case of reef tank pH probes) and the test sample that is highly dependent on ion flow across a crushed-glass junction. Chemical pH assays (such as Salifert's) depend on the color change of a reference dye, which, while obviously related to pH, is a very different detection method than an electronic pH probe.

There are multiple conclusions from the above mechanistic description. The first is that pH probes are electrical devices that are subject to all of such device's potential pitfalls. The electrical potential I mentioned is quite small - in the millivolt range. That means that a LOT of details can have an effect on their accuracy. Some of those details are whether or not the crushed glass junction is plugged (which inhibits the ion flow), whether the probe is in a reasonable flow environment (in laboratories, one is often directed to stir the sample as the pH is determined), whether the reference solution/gel has been contaminated, and one of the bigger issues - whether the electrical connection between the probe's BNC connection and the actual meter is pristine, tight, and has little to no surface corrosion.

To obtain a reliable pH measurement by use of an electronic meter/pH probe, considerations of the condition of the pH probe, junction, electrical connection and pH measurement environment are important in addition to the pH calibration procedure itself. In particular, I'd advise a reefer using a continuous pH measurement device to examine the BNC junction connection on both the probe and the controller once every 3 months and ensure there's no visible corrosion. I'd also advise someone setting one of these devices up to pick a spot in their sump/tank with reasonably high flow rates past the probe - relatively stagnant locations may result in quite large pH swings.

With respect to the pH calibration procedure, there are several important factors. The first is one already discussed in this thread, which is temperature compensation. If one were to read the table of pH value versus temperature for a typical pH 7 and pH 10 standard solution, you will notice that the pH 7 standard's pH value changes very little with a few dozen degrees temperature difference, while the pH 10 standard's pH value changes quite a lot over that same temp range. This is one reason that equilibrating the pH standards to the temperature of the environment that you'll be calibrating the meter in, and turning the temperature compensation "on" in the device is important. Another factor associated with pH calibration and pH standard solutions is CO2 dissolution - this is likely why the Neptune folks advised not to allow the probe to set in the pH calibration solution for lengthy periods while waiting for the millivolt values to settle. Particularly when using small-volume, pouched pH standards, the pH value of the standard could conceivably change quite a bit from adsorption of CO2 from the air.
 

Bruce60

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A few comments about this that may be tangentially helpful for those that aren't scientists/engineers (those that are scientists will likely roll their eyes and say "yeah, dude, we know that!"):
Thanks for a well written description. This will help many hobbyists better understand pH probes.
 
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hllb

hllb

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Guys, this is getting interesting. I have an appointment on Tuesday evening with Neptune. I decided to recalibrate again today (I tried to clear the calibration yesterday with instructions found online but no clue if it worked or is a valid method).

recalibrated today and seemed ok during manual calibration. Put in the 10.0 solution and initially it read correctly. Then it started drifting downward and congrats imbued the longer it was in the solution. It got down to 9.95 after maybe 10 minutes. At that point, I mixed Fresh saltwater to 35 ppt (HW Marinemix Reefer). It measured 8.16 after drifting downward for a few minutes (I didn’t give it too long to drift just saw that it was drifting). Tested that water manually and got between 8.3 and 8.6 on salifert - I’d say probably 8.4 (so same as expected, another .25 low). Then I put the probe in the 7.0 solution. It drifted down again and now after maybe 20 minutes, is at 6.9, so at least .1 low.

I am using pinpoint calibration packets purchased in February. There isn’t an expiration date on them but the lot number starts with 2007. Hopefully that isn’t a year of manufacture? Now I’m wondering if the solution is super old or bad somehow. I may pick up new solution tomorrow (I open a new packet each time I calibrate)
 
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hllb

hllb

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Put the probe back in the fresh saltwater and let it sit and it went down to 8.1. Never tested ph for fresh mixed water before so not sure where it should be.

put the probe back in my tank and it’s at 7.49…good grief, it’s worse than before. Salifert says 8.0 and API agrees (maybe .05 lower but pretty close)
 

Randy Holmes-Farley

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Put the probe back in the fresh saltwater and let it sit and it went down to 8.1. Never tested ph for fresh mixed water before so not sure where it should be.

It need not be any particular value.
 
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Dkeller_nc

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One other troubleshooting process you may wish to try. It may be that you've stated in the thread that you've a new pH probe. Even if it is new, you might consider purchasing another. PH probes do degrade over time, and I've actually seen instances over the last few decades of working in labs that pH probes are "bad" right out of the box. I wouldn't say that's a common occurrence, but it does happen. It is also possible that you have issues with your actual controller, since there's a series of electronic components going all the way from the pH probe itself, the pH probe cable, the BNC connection of the probe to the controller, and within the controller itself that all influences pH drift and probe calibration ability.

Hopefully it's not the controller itself, but I'd think if you determine that's the case with Neptune, they'd warrant a replacement.
 
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hllb

hllb

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One other troubleshooting process you may wish to try. It may be that you've stated in the thread that you've a new pH probe. Even if it is new, you might consider purchasing another. PH probes do degrade over time, and I've actually seen instances over the last few decades of working in labs that pH probes are "bad" right out of the box. I wouldn't say that's a common occurrence, but it does happen. It is also possible that you have issues with your actual controller, since there's a series of electronic components going all the way from the pH probe itself, the pH probe cable, the BNC connection of the probe to the controller, and within the controller itself that all influences pH drift and probe calibration ability.

Hopefully it's not the controller itself, but I'd think if you determine that's the case with Neptune, they'd warrant a replacement.
I did meet with Neptune last night and everything seems to check out with the controller. I did order another probe - this BRS one just needs significantly more adjustment than the old Neptune one did, which does introduce more room for error (when calibrating the old Neptune probe, the numbers would be pretty close to expected, 703 and 1006; with the BRS probe, the numbers are 750-765 and 1050-1065). It just does seem like my tank runs low and perhaps my old probe has been wrong all this time. After resetting the probe port and recalibrating last night, it's reading my pH even lower now - about .1 lower than before the meeting LOL. I can't win. Ordering a CO2 scrubber now and it'll just have to sit on my couch until I get my new tank up this fall LOL.
 
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hllb

hllb

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Interesting and glad I ran across this thread. Seems no more accurate than seneye.
Well, I've slowly come to accept that 1) the liquid test kits are probably off, 2) my old probe was probably off even though it calibrated fine in February it was at least 2 years old.

I also just remembered yesterday that I have a seneye that I use for PAR readings...so I'm soaking a slide for that now LOL. I also ordered a CO2 scrubber this morning. Not sure exactly where I'll put it, but I got the cheap one from BRS so I'll give it a try, and then I'll have it for my new tank this fall. 10 to 12 weeks build time is sounding really far away right about now.
 

LRT

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Well, I've slowly come to accept that 1) the liquid test kits are probably off, 2) my old probe was probably off even though it calibrated fine in February it was at least 2 years old.

I also just remembered yesterday that I have a seneye that I use for PAR readings...so I'm soaking a slide for that now LOL. I also ordered a CO2 scrubber this morning. Not sure exactly where I'll put it, but I got the cheap one from BRS so I'll give it a try, and then I'll have it for my new tank this fall. 10 to 12 weeks build time is sounding really far away right about now.
Awesome im super interested to see comparison readings from apex to seneye so cant wait to see your findings on that.
 

BoxKing

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The true solution for these probes would be forced calibration, similar to the way the Temp Probe allows.

Neptune could easily program this option, yet they chose not to. This option would be at the risk of the user, and their trusted measures for PH/Salinity, but at least it would get rid of the headaches most of us have trying to get and maintain near accurate readings.
 

BoxKing

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The only probe I trust religiously is Temp... and go figure, I was able to force calibrate.

Now I have to use my Hanna Probes to check a few times a week to know what my current deviations are from the Apex probes. For a system as expensive as this, it should be more "plug n play". I find it tough to trust PH/Salinity figures, therefor Im reluctant to take full advantage of more detailed programming, or even the Trident system.

All in all, I would not buy this product again, nor suggest it to others. A simple Kasa power strip would have sufficed for scheduling, and all my other equipment runs off Mobious.

End Rant... and Neptune should get the ish together.
 
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hllb

hllb

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The true solution for these probes would be forced calibration, similar to the way the Temp Probe allows.

Neptune could easily program this option, yet they chose not to. This option would be at the risk of the user, and their trusted measures for PH/Salinity, but at least it would get rid of the headaches most of us have trying to get and maintain near accurate readings.
Yeah, I just can't keep questioning the calibration fluids, since I bought brand new ones that had expiration dates on them. So the chemical tests seem pretty clearly wrong....
 
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hllb

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Well, that kind of answers that question LOL. Our air conditioner died yesterday so we had all the windows open and fans on all night long. pH is currently 7.87 when yesterday at this time, it was 7.63. I should have everything I need for a CO2 scrubber by Monday I think.
 

Rob.bucek

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I’m starting to seriously doubt the Apex. I’ve seen a few posts where people are looking for help for something, posting their water params, and their ph is always low, and guess what… they’ve got an Apex! I’ve been having the exact same issue. Recalibrate… low ph… recalibrate… low ph… but EVERY other test (salifert, api, etc) has my ph above 8 while apex keeps telling me it’s under 7.5. This has been happening for 2 months straight… so if my ph was REALLY that low, I’m sure my tank would be dead… which is not the case. I’m following along to see what others have to say
I'm starting to wonder myself. My red sea test kit, my Hach pH probe, my alkatronic all test higher than my Apex. Coincidence? Sure makes a reefer wonder...
 
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hllb

hllb

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So, my original Neptune probe went haywire which likely contributed to this issue, and it was 3 years old so probably why it was reading a bit high. I installed a new BRS probe and it was reading pretty low but I calibrated multiple times to the same effect. It seemed harder to get a settled reading with it and after calibration would keep drifting downward. In frustration the other day, I found a Neptune probe in stock somewhere and bought it. Installed it today and it calibrated properly the first time, read the 10.0 solution spot on for 10 minutes and read the 7.0 solution as 6.97 after about 10 minutes. Seemed pretty good. Put it back in my tank and it seems better now. I’ve been routinely getting .25 to .3 lower than my Salifert test with the BRS probe. Now I’m only about .15 lower with the Neptune probe. So right now (post wc), my pH is sitting at 8.03. Hopefully it looks good over the next couple of days.
 

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