Mine varies, In morning it is around 26-27 then nearer the late night when the sump light have been on for the last hour main lights are one its just peaking at aroun 27-28
But I have a digital laser temp reader and it is accurate and the highest I have ever got is 27.2 deg
This is what I use
Technical Info
Temperature Range 0-500°F (-18 to 260°C)
Temperature Display 1°F or .5°C
Ambient Operating Range 32 to 120°F (0-50°C)
Repeatability +/- 2% of reading or +/- 3°F (+/- 2°C)
Response Time 500 mSec, 95% response
Emissivity Pre-set .95
Power 9 VDC Alkaline or NiCad battery
Accuracy ±2% of reading or ±2°C (±3.5°F)
Distance to Spot Ratio 6:1
I think it cost about £60 but Ive had it for around 3 years now