Question: I made caramel on my induction cooktop. The temperature was correct, but the caramel texture is grainy, like all of the sugar didn't dissolve properly. Do I need a different thermometer? I used a Wilton clip-on stainless steel mercury thermometer. Or does the cooking time need to be adjusted?

The recipe I am using is: 1cup butter,2 1/4cup brown sugar, dash salt,1cup light corn syrup,1can sweetened condensed milk,1teaspoon vanilla. Melt butter in heavy 3 qt. saucepan. Add brown sugar and salt. Stir until thoroughly combined. Stir in corn syrup and mix well. Gradually add milk stirring constantly. Cook and stir over med. heat until candy reaches 245F remove from heat and stir in vanilla. Pour into 9x9 pan. Let set 24 hours. Cut into squares, wrap in wax paper. 

I am close to sea-level.

Answer: 
Your thermometer should be fine, the graininess probably has more to do with technique than temperature. There's quite a bit of controversy as to when and how to stir and what to do about sugar crystals that form along the sides of the pan.

Just to be sure about your thermometer, test it with boiling water. If you're at or near sea-level the thermometer should read 100C or 212F. If your thermometer is fine, then we need to take a harder look at technique.

Edit: Looking at your recipe and considering your change in cooktop, I can only see a possible problem if the candy reaches 245F more quickly than you are used to. Try lowering the temp a bit at that stage so that it takes longer for the candy to reach 245F. It may take some getting used to, but induction should perfect for candy making.

It pretty much goes without saying, so I am assuming no considerable change in your ingredients.
I solved the problem of grainy caramel made with condensed milk by: taking the grainy caramel off the cooktop and decanting the entire batch into my high power blender (2200w) for 3 minutes on the highest setting. Not only did the graininess disappear completely but the heat generated by my amazing blender continued to cook my caramel. Now the caramel filling for my banoffee pie is PERFECT and ready for the bananas and whipped cream! I've never before made caramel with condensed milk. Next time I'm just going to use my blender from the beginning.