Inaccurate temp reading

[scratchchance]

I recently picked up a 15 brew kegel for mashing after having to split my kit with my friend. It has a Blichmann thermometer. On the keg the previous owner had written that the temp reading was inaccurate which I found true on my first brew. I pulled the thermometer out and tested it in boiling water and it is accurate.
Any ideas why during mash it does not accurately read the mash temp? I had to use a hand digital thermometer. It is positioned near the bottom of the keg where you would want it and my previous mash keg had the thermometer.

 

[Indian_villager]

Could just be broken, or gotten water on the internals. I typically use digital gauges for mine, but my vessels aren't put over a flame.

 

[MaxStout]

Accuracy may not track throughout the thermometer's full range. It could be accurate at one temp but diverge at other temps.

If you have another thermometer you know is accurate, you should check it at a few other points. Heat some water in a saucepan to around mash temps. See how they compare.

 

[hotbeer]

Mash temps may not be the same throughout. Especially if the heat source is on and not being stirred. So if you are going to compare temps to tell whether one is accurate, you need to compare the temperature at the same spot.

Having a temp probe that is in a fixed position never seems ideal to me. I'm always checking everywhere and stirring if the heat source is on.