I am not sure if this will help anyone or not, but figured I would throw it out there.
Two years ago we had our pool remodeled with skimmer replaced due to impending leak. The lid was replaced with one of the Pentair lids that had the thermometer in it. Quickly realized the temp was WAY off which is probably why the guy gave it to us. I contacted Pentair and they were willing to replace it, but I could not find our receipt for the skimmer repair so I did not pursue.
I was about to order a new thermometer online and decided to first make sure I could remove it from the lid. With a little work one night I got it out so I figured I would order a new one and headed to bed. Before I fell asleep I had a thought and had to get up to try it.
Sure enough the nut on the back of the gauge that I thought was just holding the probe on can be turned to calibrate the temp reading ... wish I would have found this a long time ago
Hope this may help someone in the future.