Pinpoint vs. Digital Aquatics Ph accuracy

[imasleeper]

I have a Pinpoint ph monitor with a new electrode that I have calibrated using Hanna calibration solutions 7.01 and 10.01. Several days ago I purchased a Digital Aquatics Reefkeeper lite controller with a DA ph probe also calibrated with Hanna solutions 7.01 and 10.01. The issue is there is a .4 difference in ph readings between the two systems. They each read 7.00 (6.99-7.01 range) and 10.00 (9.97-10.01 range) respectively using the calibration solutions, which I have checked and rechecked. However the Pinpoint measures the tank ph at 8.41 and the Digital Aquatics measures 8.02. Clearly a .4 ph difference is alot and the question is which one should I believe. Anyone know of a 8.2-8.4 calibration buffer solution to check accuracy?

imasleeper

 

[dolfans1]

You can get a pH 8.00 buffer from some lab supply companies. Here is an example:

http://www.daigger.com/catalog/product/d-pH+Buffers/PH+Buffers/p-12136/Daigger%C2%AE+pH+Buffer+Solutions

There are no calibration buffers in the range of 8.2 to 8.4 that I'm aware of. The 8.00 buffer should answer your question.

Does either your Pinpoint monitor or Digital Aquatics have temperature compensation? If the tank water temperature is significantly different that the temperature of the buffers when you calibrated, this could account for the difference, as pH probes behave differently at different temperatures.

One other important point - make sure the pH has stabilized when calibrating. Some electrodes can take a while before they are stable.

 

[imasleeper]

The Hanna calibration solutions have a ph-temperature adjustment table on the package. I have a degree in chemistry and am very careful while making measurements including waiting an appropriate time for the ph measurement to settle. The Digital Aquatics calibration process shows the actual raw data measurement which allows one the see when the measurement has settled. Thanks for the information on a 8.0 buffer solution.

imasleeper

 

[TheH]

Is the pinpoint pH probe clean? This is the older of the two, right?

 

[braddo]

Its intersting that your DA pH probe reads low. I've always felt my DA probe read low even after calibrating too but I've never had a known 8.0 - 8.3 test solution to verify with. I bought a few different pH test kits and they always show my pH as being ~8.0 -8.3 however my probe reads 7.8 - 8.0. I think I will have to pick up some certified pH 8.0 buffer to test against myself.

 

[Randy Holmes-Farley]

I do not think getting a new calibration solution is needed. Some meters can suffer from interference from aquarium lights (the ballasts emit interference). The Pinpoint is one.

Try calibrating both in the same calibration solutions away from the tank (waiting 10-15 minutes in each solution before allowing it to decide it is stabilized, if possible), double check that they read those two solutions correctly, then try measuring each in the same tank water in a cup away from the tank.

I expect under those circumstances, they will likely read the same.

 

[CJO]

I do not think getting a new calibration solution is needed. Some meters can suffer from interference from aquarium lights (the ballasts emit interference). The Pinpoint is one.

Wow- that's a bit scary! I hope that they reference this in their manual!!

CJ

 

[imasleeper]

TheH---Yes the probes are clean. The pinpoint probe is 3 months old. I have ordered two new probes last pm (both pinpoint) just to see if it is a DA vs. pinpoint probe issue. I will try the "away from the tank" measurement test once the new probes arrive to determine if it is a ballast interference issue. If the difference remains than it means the gamma values of the respective curves are set differently within the two devices.

I'll report back once I have finished further testing.

imasleeper

 

[Randy Holmes-Farley]

:thumbsup:

Good luck.

 

[TheH]

Regarding the pinpoint getting interference from other electrical equipment, they have released a new ac adapter that does not get this interference. The information is on their website.

 

[Randy Holmes-Farley]

Good to know, thanks

 

[imasleeper]

I have finally got around to replacing both the DA and Pinpoint ph probes with 2 new Pinpoint probes. I calibrated both systems with Pinpoint calibration solutions and now both systems read within 0.02. Using a cup of aquarium water away from the tank, I found out the original DA probe was reading about .2 low and the old (3 months old :-(! ) Pinpoint probe was reading about .2 high compared to the new probes. BTW there does not seem to be any interference with the MH ballast and the Pinpoint unit in my system.

imasleeper

 

[Randy Holmes-Farley]

:thumbsup:

Sounds good!

Happy Reefing.