Thanks for the quick response, DR.
Both meters have identical probes, purchased at the same time, and appropriate for the solution (phosphate buffer) according to the manufacturer. Both meters are being calibrated immediately before use with the same NIST traceable pH meter standards, and both meters seem to have no trouble reaching their endpoint. We do not have temperature correction on either unit, but the meters are in labs that have a stable enough temperature for measuring phosphate buffer at neutral pH (according to literature).
On the new meters, the pH will read, say, 7.0. On the 30-year old meter, the same solution will read 7.2.
Someone who has been here a long time said they remember a Corning service guy saying that the machine would eventually reach a point at which it wouldn't be able to update the calibration slope (even though it would calculate and display a new one with each calibration) and it would continue to use the final slope it could "remember." He thought the guy said it was a RAM/ROM issue, or something.
If we've had an incorrect slope being repeatedly used for the past several years it could explain what is happening, but what I just typed above isn't very detailed for a CAPA justification ...
Corning doesn't support this instrument anymore, and in fact has sold the pH meter business to someone else.
Thanks again, DR!
H_H