I have an Auber temp control probe mounted a couple of inches above yours and the probe is about +30f then the dome lid thermo on my 18.5 WSM.
Old user name was JP7794
Jim - KCBS CBJ
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18.5" WSM (1983 model) | 18.5" WSM (2010 model) | Packer Green Thermapen
| Auber PID Temp Controller | QMaster Senior Temp Controller | Mini WSM | Green Bay Packer UDS