Checking temp gauges, changing spark plugs

[ccsmith51]

I had the cowling off this evening to replace my plugs, and I decided to put my OT and CHT sensors in boiling water to see what the gauges read.

The OT read as close to 212° as I could tell. So, that means my OT's really are as low as they read, barely getting to 175° most flights. The back of the cooler is blocked off about 60%. Do I need to block more off to get higher temps?

The CHT read about 190°, as close as I could discern, so say 20° low. It is a ring sensor under the #3 bottom plug. So, my question is will it read 20° low over the operating range? Or will the error change as the temp changes?

I replaced all eight spark plugs. I checked the resistance of all the old ones, and all were between 1.3 and 2.6 kohms, except #1 top. That one measured about 10.6 kohms. On my last flight I had a left mag issue that cleared with leaning and higher RPM, so that bad plug probably was the cause.