pH probe variance? What is acceptable?

[wrasse-cal]

I have two pH probes. Both have been 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: 8.16 and 8.28. When I noticed this, I re-calibrated both probes. No change.


What is an acceptable variance for pH probe accuracy?

[zombie]

The maximum error should be the RMS of junction error (0.02 pH), linearity error (0.04 pH), measurement error (0.01), and buffer error (0.01). This results in an error of 0.047 for each probe.

Under absolute worst case conditions where each probe is the polar opposite of the other, The maximum permissible difference would be about 0.1 pH. Considering the likelihood that all errors would compound oppositely in two probes, one should expect a difference of less than 0.065.

You might be an engineer if...You have no life and can prove it mathematically.