Reading discrepancy in pH monitors

[debdp]

After receiving the bus cable yesterday for the new RKL set up (it was missing from the package) I calibrated the pH probe and set it up. Since I had the solution out I went ahead and calibrated all of the pH units I have... two other RKL's, two Pinpoint pH monitors and a Pinpoint pH controller. I was working in the room with the freshwater and frag tank and after calibrating one Pinpoint monitor and the controller put both probes in the SW tank. The monitor gave a reading of 7.9 but the controller gave a reading of 7.6. I recalibrated both of them and ended up with the same discrepancy. I recalibrated them one more time, same readings. I switched probes and had to recalibrate again because the readings were completely off. But once calibrated had the same .3 +/- difference in readings. I took some water from the tank and checked it with two of the RKL probes. Both gave the exact reading of 7.77. The pH monitor I tested is on battery but the battery was just replaced a few days ago.

Is there anything else I can do to check these two pieces of equipment to get a more accurate reading?

 

[bimmerzs]

First....make sure all probes are absolutely clean before starting as it is possible to contaminate the cal solution's. You can then try floating the calibration solution in the sump/tank(make sure to rinse off the solution containers in vinegar solution first) and allow them to come to the same temp as the tank and then retry the calibration. Also keep in mind that these are electronic devices, and no two are exactly the same, they all have allowable ranges, usually +/- 1% for accuracy. Also... most controllers(don't remember if the rkl has this) have an offset adjustment for ph, temp etc so when using multiple controller, this allows you to choose one as the master unit so you calibrate the master then each slave unit, then use the offset adjustment to match the slave to the master unit. This is one downfall to having multiple controllers but the bottom line is still stability. [smilie=smile.gif]

Cheers,

 

[debdp]

Okay, thanks. I didn't think about being able to adjust them to match. I was more concerned about the Pinpoint controller's accuracy since it will be used on the calcium reactor and the RKL probe as the master for the same tank.

 

[SumThinhWong]

you have to also remember that running on battery can give you a slightly different reading then if your running on a constant power source. A pH meter is a high-impedance voltmeter that measures the very small, direct current potential generated between a glass pH electrode and a pH reference electrode. Changes in voltage of the battery even if slight can change the reference voltage of the meter. Also as a pH probe ages the electrodes use to take the readings a degrade over time and offset the measurements

 

[washingtond]

I am using both a Apex and Pinpoint on the same tank and have never had the exact same readings after calibration. But what is important to me is that they both track each other up and down the pH scale. One will read 8.20 while the other 8.22.

Sent from my Sony Tablet S using Tapatalk

 

[debdp]

I checked my email late last night and saw a reply from American Pinpoint...not expecting one until Monday. He said that maybe one of the probes is not "holding" the calibration. So I retested all three Pinpoint probes this morning. I swapped the battery with the AC converter and moved the battery to the FW probe set up. There was no change in either readings. After two hours of tests:

Calibrated to 7.0
1) Monitor used on SW tank: in 7.0 solution after 60 minutes dropped to 6.83
2) Monitor used on FW tank: in 7.0 solution after 60 minutes dropped to 6.97
3) Controller: in 7.0 solution after 60 minutes dropped to 6.89

Calibrated to 10.0
1) Monitor used on SW tank: in 10. solution after 15 minutes went to 10.08
2) Monitor used on FW tank: in 10. solution after 15 minutes went to 10.01
3) Controller: could not calibrate it to 10.0, it would only go to 9.93 s the screw would not turn any more... after 15 minutes went to 9.95

Recalibrated all three in the 7.0 solution before placing them in the SW tank for readings. The FW probe appeared to be the most accurate in previous tests.

1) Monitor used on SW tank started at 8.00 and after 20 minutes jumped to 8.26
2) Monitor used on FW tank started at 8.08 and after 20 minutes went to 8.10
3) Controller started at 7.65 and after 20 minutes reads 7.77.

Readings on 1 and 3 continue to go up while readings on #2 (FW) stays within .01/.02 While I'll still forward this info to American Pinpoint to verify, I'm pretty sure I'll be needing to order two new probes. I just want to be sure he doesn't say there's something wrong with the equipment.

I also switched the battery back to the SW monitor and the AC cord to the FW monitor and turned them on. There was no change in either readings. So after writing this and checking them one more time the readings for #1 are now up to 8.30, #2 is still at 8.10.

 

[bimmerzs]

Do you have any of the PH cables close to power cables.....possible interference. Also make sure there are no ballast real close as some can also cause interference. I'm assuming you are letting the readings settle in each solution for a few minutes, and are no longer changing, before calibrating. [smilie=wink.gif] You can test the "holding" aspect by calibrating it in one solution and just let it sit in the solution and see if it changes over time or hold the value.

Cheers,

 

[debdp]

@bimmerzs wrote:

None of them are by any cables or ballasts. And as far as sitting in the solution that's what the tests I noted above did. They sat in the solution for 60 minutes in the 7.0 but only 15 in the 10.0. I'm pretty sure there's something wrong with the monitor I've been using over the frag tank. It's now giving me a reading of 8.5, while the controller in the same tank is reading 8.23... about what I would expect for that tank this time of day.

 

[bimmerzs]

Ahhh....I see, must have read over that too fast. Well if you have also interchanged the probe and it was ok then sounds like the meter could be the issue.

Cheers,