pH probe variance? How much is normal?

[Wrasse-cal]

I have two Neptune double junction pH probes. Both were calibrated at the same time. One probe will go in my calcium reactor, the other in my sump.

Right now though, the probes are in the same compartment in my sump, but read different values: 8.18 and 8.30. When I noticed this, I re-calibrated both probes. No change.

What is the acceptable variance for pH probe accuracy and precision?

 

[Brew12]

I have two Neptune double junction pH probes. Both were calibrated at the same time. One probe will go in my calcium reactor, the other in my sump.

Right now though, the probes are in the same compartment in my sump, but read different values: 8.18 and 8.30. When I noticed this, I re-calibrated both probes. No change.

What is the acceptable variance for pH probe accuracy and precision?

Not that much. Are you sure you have temperature compensation set the same for both probes?

 

[Wrasse-cal]

Thanks. I’ll contact Neptune.

 

[Brew12]

Thanks. I’ll contact Neptune.

I would swap which modules the probes are connected to and see if the problem follows the probe.

 

[Wrasse-cal]

I would swap which modules the probes are connected to and see if the problem follows the probe.

Will do. They’re both connected to my 2016 Apex...it’s only been up and running for a couple months.

 

[Brew12]

Will do. They’re both connected to my 2016 Apex...it’s only been up and running for a couple months.

I don't use Apex anymore but swapping where they are connected will let you know if it is a probe problem or it is something in the Apex. If it is in the Apex it could be a setup issue (like temperature compensation being turned on for one and not the other) or a hardware issue.

 

[Wrasse-cal]

So I swapped the probe connections. The probes went from 8.25 and 8.32 to 8.27 and 8.29.

I then checked the probes and one, the one reading 8.32 registered 7.11 in a 7.0 pH solution. I re-calibrated that probe and now the probes are reading within .02 of each other.

Odd.

 

[Randy Holmes-Farley Verified Community Expert]

I would swap which modules the probes are connected to and see if the problem follows the probe.

Presumably the two probes were calibrated separately (if not, the whole procedure is faulty), and the calibration is presumably by the plug in port, so you are also swapping the calibration, which is likely to lead to errors even if both read perfectly before.

 

[Randy Holmes-Farley Verified Community Expert]

So I swapped the probe connections. The probes went from 8.25 and 8.32 to 8.27 and 8.29.

I then checked the probes and one, the one reading 8.32 registered 7.11 in a 7.0 pH solution. I re-calibrated that probe and now the probes are reading within .02 of each other.

Odd.

Just calibration drift, probably.

 

[Wrasse-cal]

Just calibration drift, probably.

Maybe, but the big variance first noticed was immediately after both probes were calibrated for the first time, and until today, the variance stayed after re-calibration.

 

[Brew12]

Presumably the two probes were calibrated separately (if not, the whole procedure is faulty), and the calibration is presumably by the plug in port, so you are also swapping the calibration, which is likely to lead to errors even if both read perfectly before.

Yup, didn't think of that.. would have to swap the calibration numbers, also.

 

[Wrasse-cal]

Presumably the two probes were calibrated separately (if not, the whole procedure is faulty), and the calibration is presumably by the plug in port, so you are also swapping the calibration, which is likely to lead to errors even if both read perfectly before.

I did calibrate both separately. Probes are now within .01 of each other, so problem solved? Still weird IMO.