The pot adjustment was probably set at a threshold volt level. Was there any paint or glue on the pot? Can you examine it closely with a magnifier?
You'll have to go through some effort to restore any semblance of calibration.
First see if the manufacturer's website tells you anything about re-calibration. Call on the phone maybe. They may quote you certain volt readings that you should see at certain test points.
If you can't go that route...
Try dipping the probe in something acid that you can expect a certain pH (such as lemon juice).
Adjust the pots so you obtain a sensible reading.
Avoid turning the pots to either extreme, just in case it may admit overmuch current and they omitted safety resistors.
Make drawings of the pot positions as you go.
Watch for a point where the reading changes, in case it only occurs within a narrow window of adjustment.
See if dipping it in water brings it back to 7. Alternate as you need to.
Then do the same process with something alkaline (ammonia?).
Make drawings whenever you make progress.