pH Calibration -- Land of Confusion!

[leebca]

I own the AC III Pro for two weeks. I've been trying to calibrate the pH probe. I've read some of the threads in this forum, but I still find it hard to just assume my tank water has been so low. This is the latest information:

Using buffer packets:
pH of 7.00 during calib: 7.77
pH of 10.00 during calib: 10.62
After calib:
Tank water: 7.63
pH 7.00 buffer: 7.02
pH 10.00 buffer: 9.98
pH of freshly prepared salt water: 7.67

pH test kits (3 diff brands) show 8.3 and 8.4 respectively.

So am I supposed to believe that my tank and new salt water is coming in at those figures?

Have you who own this equipment made adjustments to pH in your systems to get the 8.3 reading from the probe?

 

[Member No. 1]

Lee...
A few things...
Are you putting the temp probe along with the pH probe in the test packs when doing your calibrations?
Are you putting the temp probe along with the pH probe in the new saltwater you are checking?
Is your temp probe within 6" of the pH probe when in your sump/tank?
On the AC III Pro, the temp probe needs to be close to the pH and ORP probes in order to get correct readings. Taken from the manual:
"The temperature probe is the ground reference for the pH, ORP, and dissolved oxygen probes. It should be placed within 6 inches of these probes used."
I've calibrated mine several times, and they always come out correct.
I'm sure others will have a few ideas also.

 

[leebca]

Thanks (again) for your post, Member No 1

Temp probe always with/close to pH probe whenever (calib. & measurement).

Probes are within 3 inches of each other in their setup.

 

[EricJC]

Yep, know exactly what you mean!!! I had the same experience, did the same things, scratched my head the same way. Bought a Pinpoint and got "similar" results between the two, calibrating with different brands of 7&10 solution. Went with the Pinpoint calibration fluid based on articles by Dr. Randy Holmes-Farley; the different brands definitely calibrate differently. I've pumped my pH up to 8.1/8.2 on the meter, haven't seen any detrimental effects - then again, nothing really positive either. Corals seem to be able to withstand a fair range if acclimated gradually...

If others have had similar experiences- Please Post Them!

-E

 

[leebca]

E

How did you 'pump up the pH on the meter.?

You decided the AC III meter was reading too low?

Thanks.

 

[EricJC]

No, based on what Randy had to say, I'm trusting the meters over multiple brands of chemical test kits. I increased the pH in my tank to where it was reading 8.1-8.2 on the AC3Pro.

 

[leebca]

The plot sickens (thickens?). . .

I sent a water sample to a professional laboratory that studies marine aquarium water (they analyse water for public and private aquariums). They get:

pH of 8.33 at 24.2C

of the aquarium water the ACIII reads as 7.63 at 75.6F

It may be of some interest that, before calibrating the probe, it was reading 8.12 at 76F.

I repeted the calibration twice. Once with fresh packets; once with purchased bottled buffered standards. Essentially, the same results as in my first post.

I paid for and bought ($2K) the ACIII Pro and every probe, connector, calibration fluid, and accessory that Neptune offers and so far, all I'm sure about is that the temperature probe is working.

You don't want to know my opinion of the ACIII at this time!

 

[wfgworks]

Ok, I just got my ACIII PRO yesterday. I'm trying to calibrate the probes and have a few questions.

I'm placing the both the PH and Temp probe in the same solution. I started with the 7.0 solution and I'm getting a reading of 7.42.

When I place both in the 10.0 solution I'm getting a reading of 10.34.

Am I doing something wrong?

I'm use to using Pinpoint where you adjust screws to meet the solution of either 7 or 10.

 

[leebca]

The instructions to the ACIII Pro clearly state that it doesn't matter what the reading is when you calibrate, so long as the reading has stopped changing.

AFTER you calibrate, the pH reading should be 7 and 10 respectively of the buffers, if the calibration was correct.

 

[wfgworks]

leebaca

Thanks. I read that in the manual but wanted to be sure I was doing everything right.

Thanks again.

 

[leebca]

Welcome! :thumbsup:

 

[kattner]

Leebca:

I haven't had any problems with my calibration, except one time I had the probe wires near the cord for a powerhead and a cord for a chiller. This proximity caused improper readings due to RF interference. It was tough to notice because when I calibrated the probes I took them out of the tank and away from the other power cords when calibrating - so the calibration seemed ok, but when they got back in the tank they were off because of the RF. There are a few posts in this forum where Curt addresses this issue. Try and see if you have the probe cords near anything else electrical and try to separate them. Once I separtaed them, they work fine. Check and see if you have the same issue.

 

[EricJC]

After reading that someone else was experiencing what I was, I did some more research and additional testing. In one of Randy Holmes-Farley's articles, he mentioned an easy way to test:
1tsp Kalk in 1 cup H2O @ 25C = pH 12.45 with the pH changing at roughly .1 per degree Celsius.

Mixed the Kalk with RO/DI (got lucky and was right at 25C) and stuck my pinpoint in it after letting it settle a few minutes....guess what - 12.46!!! Took the solution to the AC3Pro and got a result of 12.25.

I haven't calibrated my meters for about 6 weeks now, I'll do it tommorow and retest. If .2 is the only difference, that's nowhere near what I would have expected based on chemical testing - chem read 8.3 and AC read like 7.8...

Lee, why don't you try this and post your results?

-Eric

 

[leebca]

Thanks Eric.

Mine is a FOWLR system and I don't make Kalk additions. Water changes keep me in the range. So, I don't have Kalk.

I purchased a field/hand-held pH meter and it is being shipped to me. I should have it in about one week.

I'd tolerate a 'true' reading that is steadily either below or above the actual. But, if the meter is read in three significant figures with a high reliability in the third figure, I would expect the reading to be within 0.1 pH units of reliability. My differential is running five times that.

 

[leebca]

Thanks Kattner.

The closest cord is a cord to a halide lamp. I will be sure they are kept separate.

I had thought at first that it would be a possible source of a problem. I turned off the halide lamp and retested. Nothing changed.

Unless you think that a cord, without electricity running through it, might be a source of intereference, I think this one cord issue was covered.

There are no other cords around, less than 3 feet. All equipment cords run in the front of the sump/refugium; all probe cords run in the back of the sump/refugium.

 

[kattner]

leebca:

Hmmm... sounds strange. Have you tried testing the pH of a cup of tank water using the probes away from all things electrical? It would be interesting to find out if the readings are ok if the AC and probes are not near the tank. For example, it seems like your calibration routine is ok in the sense that once you finish the pH 10 calibration the AC gets that correct. What reading do you get if you immediately put the probe in a cup of tank water right after calibrating? Also, scan this forum and search others. Curt has answered a number of questions on calibration. There may be something. I think I remember someone on one of these threads needing to ground their metal halides and someone else grounded their tank with one of those titanium whisker ground wires. You can test your ground by running a volt meter between a ground and the tank water to see if there is any current.

 

[leebca]

kattner,

I took my tank water pH from the refugium and took it from the sump. (In my system, the two are separately housed, connected by plumbing). I put the probes directly into each.

The pH of made up saltwater is made by cup and taken from the vat. That pH reading should not be affected by any stray voltage. Yet as noted in previous posts, the results of that reading, and the reading of my tank water is the same.

Before posting this thread, I scanned back to 6 months for discussions having to do with calibration of the pH probe. Anything I found, I had already addressed in my efforts.

I will see what I can do about getting a volt meter. I have that general curiosity about being grounded. I bought and have the whisker. Just haven't installed it (mostly from lack of knowledge).

Thanks for trying to help. Don't stop!

 

[kattner]

Did you figure anything out on this yet?? Did your readings get back to normal?

 

[leebca]

Nothing seems to change the reading. pH of calibration standards are 'accurate.' I still believe the reading of my aquarium water and the water freshly made is wrong. So, it measures salt water incorrectly.

Waiting for a pH meter to arrive for checking it.

 

[kattner]

Only thing I can think of is a problem with the probe It's a new probe? Black cap was taken off probe? If new probe, was it cleaned and rinsed before use (sometimes they have crud on them).