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! :D

[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).

[leebca]

New probe. It was capped. Liquid still inside cap. Cleaned and rinsed before use/calibration.

I just made up a fresh batch of new salt water mix. The pH the probe(s) are registering on a cup of that water is:

pH = 7.72 at 75.2F

Test kit says 8.4

Literature for Red Sea Salt (the salt I use) says 8.3 at 23C

If I put the probes in pH 10 buffer, I get a 10.02 reading at 76F. If I put the probes in pH 7 buffer, I get a 6.98 reading at 76F.

Maybe when Curt is back, maybe he has some ideas. But. . .don't give up Kattner. Thanks for your efforts. In the meantime, my system seems to be 'stuck.' Maybe it's a bad probe, but I don't understand why it reads the standards 'properly' but not salt water.

:D

[REEF-DADDY]

I am tagging along. My ph was reading in the range of 8.11 to 8.20 during the day. I was unhappy thinking it was too low. I recalibrated my meter and my ph now reads 7.9ish. Chemical tests are showing 8.2ish. Same exact thing

[leebca]

Thanks Steve.

I've checked mine against three outside pH meters. My ACIII lab quality probe pH reading is running about 0.50 units low compared to ALL other pH meter readings. There is no explanation as far as I've been able to determine.

[REEF-DADDY]

Originally posted by leebca
Thanks Steve.

I've checked mine against three outside pH meters. My ACIII lab quality probe pH reading is running about 0.50 units low compared to ALL other pH meter readings. There is no explanation as far as I've been able to determine.

This thread makes me feel better, I was stressed thinking my PH was 7.8! Theres not much more I can do I have all my windows open and I am dosing kalk!

[MSHUR]

i was going crazy last nite too:))) i calibrated 3 times and i have my ph at 7.95..but my pinpoint show 8.22...
guys, hows that possable?

mike

[kattner]

Leebca:

Have you had any success yet on the calibration issue? I have run some more tests on my unit and have tracked some things down I think. I am currently running an ACIIIPro with a lab grade probe. My ph has been running low as well. I had two problems.

First, I had some electrical interference. My calibration would work fine, but I had a weird anomoly early in the mornings. I have two wave pumps (left and right) that turn off at 4am and come back on at 7am. For some strange reason, when the right wave pump turned off, the ph would drop to 7.2 from about 8.0 at night. I replaced the pump (it was old) and installed a grounded titanium whisker as well. Everything seemed to work well, but the ph seemed chronically low, although I was no longer getting downward spikes. Recently, the spikes have reocurred, but I have traced it to some ac wiring near the ph probe wire. If I can keep them separate enough I think the problem goes away. Have to do more tests on this aspect of the issue. I am wondering if anyone knows of some type of additional shielding that can cover existing wires - like a shielded tube or something to run the probe wires through. I wrapped them with aluminum foil as a test which reduced the downward spike by about half but did not fully resolve the issue.

Now, the second issue was just plain low ph readings even though test kits have me higher and more normal readings. The probe is lab grade (about 6 months old) and I dose kalkwasser throughout the night. So, I am sure my ph is about 8.28 during the day and 8.1 at night. I had the same problem as you. The probe would calibrate correctly, give correct readings in the calibration fluid, but would be about .5 units too low in saltwater.

At any rate, I had an older non lab grade probe as a backup and so I installed this probe, cleaned it and calibrated. After about a day to stabilize, the readings came right up to where I expected them 8.29 during the day and 8.05 to 8.1 right before the lights come on in the morning.

Curious, I examined each probe carefully. The liquid inside the lab grade probe had a bubble in it almost an inch long. The bubble travels back and forth as you tilt the probe (just like a bubble in a level). The non lab grade probe barely has any bubble in it (slightly smaller than pea sized). I remember reading somewhere that the fluid inside these probes will dissipate over time, and when they do, the probe will no longer be accurate. Bottom line is that the probe that worked best had less dissipation of the internal fluid. Could this be the answer? Could the lab grade probe have simply been on the shelf longer before I bought it? I think I purchased it from one of the on-line vendors. I am wondering if you could check your probe and see if you have an air bubble in it and how large it is. Might be that your probe needs to be replaced. Maybe Curt can chime in on any of these observations and the shelf life of these probes.

One last thing to check - I think you mentioned you had a pinpoint ph monitor. Can you use the probe off the pinpoint on the ACIII unit and see if you get better readings with a probe you know to be working?

HTH

[collins]

I have a pinpoint probe connected to my AC3. It works fine

[leebca]

Kattner,

Thanks for your continued support on this. I think I've gotten a handle on it, just yesterday.

I received in a brand new lab probe and I swapped the old one out. Guess what? The new probe is on the mark (or reasonably close). I think it was a bad probe.

I've been dosing kalkwasser and I was confident of what the reading should have been (by various cross-checks), and your own observation -- what it should be.

I can only say that the probe I got was a dog. What made it so? I checked it after reading your post. The 'bubble' inside my probe seems to be split. One of the two bubbles is about the size of buck shot (large pea) and is on the outside of the core. Bubble two is twice that size and 'inside' with the core.

One bubble (the smaller of the two) seems to 'circulate' on the upper part/half of the probe, outside of the core. The second (larger) bubble seems to travel in the bottom half of the probe. Of the bottom half, it seems to take up about 25 to 30% of the length of that half and seems to be deeper inside, at the core of the probe. It's just about .5 inch long. The bubbles don't 'connect.'

My probe was 'new' and it never operated properly. I was so frustrated (and confronted by so much evidence that it was reading incorrectly) that I just went out and bought a new probe.

I'm in the process of returning the old probe and getting it replaced. Why should the liquid inside 'disappear?' I've known the old probes you had to fill with conductivity/water fluids, but aren't the current probes sealed?

Now. . .If it is true that they 'age' then why are we told they do and what to look for? This doesn't instill much confidence in me.:(

Do you remember where you read that about the probe & aging?

I now went to check the probe that is 'working.' The larger, inner bubble is inside the probe that is working. About the same size as the bad probe. BUT, the smaller (buckshot sized) bubble on the outside isn't there. Don't know if this makes a difference, or not. Maybe only a 'certain' bubble matters. Does yours have the inner and outer bubbles?

Lab grade? No wonder if labs can't get things straight.

I don't have a Pinpoint. Can't check that out. I have two field pH monitors (which now agree with the lab probe within .15 pH units --- good for the field monitors).

[leebca]

Thanks collins. You've got me thinking that Neptune has obtained a below-standard lab grade probe. I hope kattner and I were flukes for Neptune's sake, but if others who have experienced similar problems are because of the probe. . .:mad:

[kattner]

As further thought, I don't think it is a Neptune problem per se. I have purchased probes from them which have worked. I think the lab grade probe in question I purchased from Marine Depot - so it may have sat on their shelf for a while before being sold. It did seem to work ok for about 4 to 6 months. I think I would chalk these up to being flukes because if you think about it - lab grade probes are probably not sold very often because they are more expensive - so they may sit on the shelf longer before someone buys them. I'm just not sure what the shelf life of these probes should be, especially when they are capped and not used.

I will try and find the article on how these probes work. I think they are a gel based variety and the gel is supposed to leak out across a membrane over time - albeit in very small quantities without any harm to the water they are testing.

[collins]

I have an unused Neptune standard pH probe that came in the package I bought. Now I am thinking of testing it before I actually need it.

I used my Pinpoint probe because it was only 2 weeks old when I got my AC3. I thought I would save the Neptune one for when it craps out. I also knew what readings to expect from it and could use it to validate the AC3. (The Pinpoint one looks like it is made better also.)

Does anyone use the temperature compensation? I have it turned off, but temp changes still affect my pH quite drastically.

[kattner]

Curt recommends temperature comp be turned off since it has very little effect on adjusting pH. See thread below:

http://reefcentral.com/forums/showthread.php?s=&threadid=591763&highlight=temperature+compensation

[collins]

Thanks kattner. My probes are stuck to the side of my sump rather than floating in styro. When I do water changes, I sometimes forget to push the probes down Even with the compensation off, my pH reading goes haywire when the temp probe gets exposed to air and that trips my CO2 solenoid.

I wish the Neptune probes were water tight.

[leebca]

collins --- more potential for data!:bounce1:

If you do check your Neptune probe, I'd like to know the results compared to the pinpoint probe. Hopefully they are in tune.

[collins]

I'll let you know how I make out. I have to buy some calibration fluid foil packs. The bottled stuff that is readily available make poor references.

[kattner]

Collins,

I have my probes mounted in a probe holder I made, but I have the same issue when I do a water change. So what I do is just log into my ACIIIPro and manually turn off the heaters and chiller and any other affected device before I do the change - that way nothing fries or chills or over ozones.

I have also used a pinpoint probe on the ACIII and it seems to work fine and is very close to the Neptune one.

[collins]

Thanks again Kattner.

I usually turn off my skimmer and heaters when I do a water change. The pH probe is supposed to remain wet, so I lower the probes in the sump as the water level drops. I sometimes am not fast enough. I really need to get some styrofoam to save the extra step.

[REEF-DADDY]

For those whose probes are working correctly, where did you get them?

[willymec]

I too have a Aquacontroller III Pro

Lab grade probe.

All bought from Aqua Direct seller for neptune.

Have been trying to calibrate it for weeks

After calibrating with the 2 buffers it would read very low. 7.32-7.45

Made the 1 cup of kalk at 25 celcius temp and it read 12.56 and so I offset the ph by

-0.1 offset
0.9841 slope

and now it reads 12.45

put ph probe and temp back in tank and now reads

Temp 80.0
Ph 8.22

9:44 pm nothing but low 30 watt light on.

This seams more normal readings.

[willymec]

Without hijacking the thread I need help with trying to see the controller from another pc on the internet, like from work. Ive tried everything, nothing works. Changed my router settings every way possible. Can log onto it only from all pc's at home.

I have a ip sony cam that I can see and control from work and it is set up same way to the router.

[kattner]

Willy:

See this thread and the threads imbedded therein. You probably have dynamic ip addressing at home. You will have to do some extra work to point your at work browser to your aquacontroller through your isp's dynamic addressing scheme.

Let me know if you need more info - or start a new thread.

http://www.reefcentral.com/forums/showthread.php?s=&threadid=631158

[collins]

Originally posted by willymec
....Have been trying to calibrate it for weeks....


Are you using foil packs for references?

I have seen where the pH 10 reference that comes in a bottle reacts with CO2 and drops in pH.

I have also tested new pH7 and 10 product that comes in bottles and found them not accurate.

[willymec]

Thanks kattner,I looked thru it all and tried a bunch of stuff still confused and not working, I'm at work now so will have to wait tell I get home.
I cant understand why my ip camera works, I can see it and control it from work pc. So I must be getting thru my router to the cam.
I have the controller set up the same in the router except different ip address.

Collins

Yes using the bottles and there about to expire according to the label.
I will order the packets.
And recalibrate.

[kattner]

Collins

You might have a port conflict. Specifically, those web cameras typically use port 80 (for web access) to access the web camera. Even though you have different ip address for the aquacontroller and the ip camera, your router is probably set up to forward all port 80 inbound requests to the web camera's ip address. When you try and access the aquacontroller, the router may also be forwarding port 80 requests that should go to the aquacontoller to the web camera. Just a thought - without knowing more about the configuration.

[willymec]

Sorry about the last post looks like my name is collins, but I was replying to both Kattner and collins.

Kattner
I see what your talking about, but there are so many settings on my router I feel like I'm on a calf interstate interchange.

I started with default settings on the router and the I cant see the cam. Then I go into the forwarding and set up the cam ip address and it works.
Then I did the same with the controller setup its ip address on the forwarding and it dont work. So I will have to figure out all the other settings. I'm pretty computer minded been building my own and setting up local networks for along time. But the router has allways stumped me when it comes to internet in'sand out's.

Starting a new thread so I can stop hijacking this thread.
sorry.

http://reefcentral.com/forums/showthread.php?s=&threadid=642560
Thanks

[clp]

If anyone is having trouble with their lab grade pH probe reading low please send an email to support@neptunesys.com. Include:

Your name
Who you purchase the probe from.
When purchased.
The complete product code (found on the white band on the cord of the probe).
What type of calibration solution you using.
The raw readings for the 7.0 and 10.0 calibration solution (found at the bottom of the display during calibration).
The controller's reading of the 7.0 and 10.0 calibration solutions after calibration (use the run display).

Curt

[kattner]

Hi Curt,

Thanks for the info. I have looked around the web on these probes and have run across some info about the offset and slope. I'm wondering if the following is true for the aquacontroller probes:

Quote:
A new pH probe will have an offset of 0 mv at a pH of 7.0 and a response of 59mv per pH unit change (slope).

As a pH probe ages both the slope and offset of the probe will change. This is what dictates probe calibration frequency. Additionally the response time of the probe will change.

Generally speaking, when an offset of more than 30 mv (at 7.0 pH) develops or more than 2 minutes is required for a probe to stabilize in a buffer solution a probe has reached end of life or
needs reconditioning.

I got this info from the following link:

http://www.phadjustment.com/pH_Probe_Service.htm


So, I wonder if you can see if a probe is getting older by checking Aquanotes to see what the offset and slope numbers are after doing a calibration and comparing to the above standards.


Ken

[leebca]

Ken,

Or more to the point, can you tell an old probe by how long it takes to stabilize in the buffered standard?

I did notice this: The first malfunctioning probe took three to five minutes to stabilize. I thought nothing of this since I really didn't know how long it would take.

The second, functioning probe, took 2 minutes to stabilize in the buffered standard.

[leebca]

Thanks for post, Curt. And. . .Welcome back from vacation. Hope you refreshed yourself! :)

The e-mail and info you posted doesn't fix the problem. I returned my probe and got a new one. That fixed my problem. I don't know what my supplier is going to do with it.

[kattner]

leebca:

Apparently, based on the quote I found above, if the probe takes a long time to stabilize (more than 2 mins) then it must be older. I think my good probes take less than a minute to stabilize in the calibration fluid. I also found some information that says to look on the white tag on the probe cord and there is a code that references the year and month of manufacture - maybe another clue to its age. Curt can chime in and elucidate further. Kind of interesting.

[clp]

The white tag does have our internal part #, and date code of manufacture. The probes have a quite long shelf life (years) when the end cap is present with correct solution (KCl). KCl is the same solution as is present in internal to the probe. There are two main failure mechanism for pH probes:
- clogged junction - the more clogged the junction the slower the response time.
- diluted/incorrect reference solution: slowly the KCl diffuses across the junction and is replaced by salt water. The pH typically drifts up as the KCl is diluted by salt water.

You can gain some info from the calibration constants in the controller, however, I think the values referenced by Kattner are very conservative. You'll probably be throwing away good probes if you use those standards as to when to replace probes. I think a better way is to time the length of time for the probe to settle when calibrating. If less than 2 maybe 3 minutes then the probe is fine.

Curt

[kattner]

Curt,

Thanks for the insights. Someday you'll have to write a book about all this (when you retire). Welcome back and thanks for all the support.

[EricJC]

Kattner,

I find the length of time you are referring to when calibrating pH probes very interesting... Having had similar issues with mine, I went back to my original post about it (on 5/31) where I was asking Curt how long it took for the probe to "settle", as mine was fluctuating for > than 5 minutes! Just emailed Curt, hopefully he'll solve this issue once and for all.

-Eric

[collins]

leebca and kattner,

I cannot find anyone to supply KCl so that I will be able to properly store the Neptune pH probe after testing. It looks like the problem is being addressed regardless. I was hoping I could contribute to your solution and at this time - I cannot

good luck!

[leebca]

collins,

Sorry to hear that. I was looking forward to having your test results on the Neptune probe.

If you can't find the KCl, then how are we supposed to be able to ever store the Neptune pH probe?

[clp]

pH 4.0 calibration solution is a good alternative.

Curt

[collins]

ok I'll get a packet of it then Curt. I wasn't sure how the glass coating would hold up.

[Aquaduck]

Well I am having the same problem with my pH readings too. I just got a standard Pinpoint pH probe as an add-on to my Aquacontroller Jr. I did the calibrations using the 7.01 and 10.01 packets from Milwaukee. Settling time 1 to 3 minutes. Rinsed probe and placed in tank. pH reads 7.8 and my test kit says my pH is 8.1. Rinsed the probe in purified water and checked calibrations. Probe reads bang on at 7.01 and 10.01. Rinsed probe and placed back in tank and still reads 7.8. This morning the pH says 7.54, that's impossible, so I check with my test kit and it says 7.8.

Curt, the code on my probe is KK4. When I received it there was a white crust around the cap and the glass part of the probe was not sitting in any solution, however, the cotton ball in the end of the cap was saturated with solution. Is it possible my probe was dried out before I got it?

pH readings are stable and do not jump around they are just low by about 0.3 pH units when measuring saltwater.

Do I return the probe to you or where I bought it?

[clp]

Is this a Pinpoint probe? If so then this is not a probe we sell/officially support. The white residue on the probe around the rubber cap is normal and is just KCl (like salt creep).
A pH of 7.54 is not impossible - it is low but certainly not unheard of. Many color test kits read .2 or more units high. The manufacturers do this on purpose because most tanks do not have a pH of 8.3, but rather 8.0 to 8.1. Raise the readings .2 or .3 units, the customer gets the reading they want to see, and buys their test kits again.
What is the alkalinity? Are your running a Ca reactor? Most Ca reactor tanks I've seen have a pH that tops out at about 8.0-8.1, and drop to as low as 7.7 or 7.8 at night (assuming high alkalinity). Low akalinity can result in even lower pH readings at night.

Curt

[leebca]

It's best to test a pH probe against another pH probe/test unit, IMO. I had mine checked by three other probes, on three different units, standardized by different buffers/techniques.

[Aquaduck]

For me it turned out my calibration fluids were bad. These were one-time fluids that came in a tear open packet from Milwaukee. Another reef keeping buddy lent me his pH meter and calibration solutions. I placed his pH probe in my tank and was reading 8.1 vs mine at 7.9. So I calibrated my probe with his standards and now both are measuring within 0.02 pH of each other. The pH probe now agrees with my test kit as well.

[kattner]

Hey Aquaduck, was there any expiration on those calibration fluids?? I think I may have some of the same so I want to make sure mine are still ok.

[Aquaduck]

I ripped open the packets and used them without checking. (lesson learned) I suggest borrowing a known working pH meter, probe and calibration fluids from someone if you are having trouble with your readings for peace of mind.