Ph Calibration Accuracy

Meitzler

New member
I have an AC3 and P1000. I am using 3 pH probes in the same tank. How close should they calibrate. They are brand new Neptune probes and vary by .09. Example 7.91, 7.82, 7.86. My first impression is that they should be closer +- .02 My pHA2 always has a higher reading even though they have all been calibrate with the reference solution.
 

jstlsn

New member
I would think that if they were all calibrated with the same solution that they should all read the same. How long do you leave the probe in each solution when calibrating?

Brad
 

Tech Diver

New member
First off, you need to know what is the accuracy rating of the probe. There is quite a range of quality in probes on the market. Generally, the higher the accuracy, the higher the price. Some probes read consistenly to within 0.05 pH while really good ones will be accurate to within 0.005 (these probes cost several hundred dollars). Most true "laboratory quality" probes are accurate to within 0.01 pH, but the use of the term "lab quality" is up to the merchant and there is no regulation to control this.

Second, there is a great range of how rapidly probes read the pH values, which is something you need to be keenly aware of when you calibrate. The function is an asymptotic one. So in theory, you never actually reach the final value. Practically speaking though, a high quality fast probe will pretty much settle after two minutes, while a slow one will take at least five. The age of the probe has a great deal of influence on this, and shelf storage is part of the aging process. That's why it is important to know how "fresh" is the probe that you purchase. Nearly all high quality probes have an indicated date of manufacture.
 

Meitzler

New member
I leave the probes in the solution until the reading no longer varies for 30 seconds. I use the 7.0 and 10.0 calibration solution in the small sealed bags. I open a new one each time I calibrate all three probes. I'll need to check the manufacturer date on the new ones. Thanks.
 

Tech Diver

New member
Check the date of the calibration fluid. They typically expire after one year. Also, they can vary significantly in consistency depending on the manufacturer and you are using invidual packets which could be the problem. I only buy my calibration fluid in pints.

One way to get around the inconsistency with three individual packets is to open all three at once, combine the same pH value fluids in a clean glass vessel, then split them up in three parts before you calibrate.
 
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