Masterbuilt 30 temperature vs Probe

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Hi I'm very new to smoking. I got Masterbuilt 30 smoker ant Therm Pro 08 thermometer. This thermometer has a separate prob to measure temperature of the grill/smoker.

I see pretty big differences (15F) between what built-in and my thermometer readings.

Who should I trust? Is it normal for such discrepancy?

thank you,

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