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: 
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.
I own a bakery and made Caramel weekly. I finally purchased an induction burner for the back of the bakery so as to not have to carry boiling sugar down a long corridor. First my staff and then myself experienced repeated failures, i.e. graininess every time we used the induction burner. We finally went back to the gas range and everything went perfectly well. Not once was it possible to reproduce the smooth dark, rich caramel we used at the bakery when using the induction burner. I wish I knew why, I used the same pot, the same technique, same temperature, and ingredients. A mystery till this day but also a very dependable result- unusable grainy mess. I hope you find your answer. If you had previous success making caramel, and changed nothing other than the induction burner it may well be something with the way an induction burning heats the sugar crystals not anything you did incorrectly. I can only speak to my own experience but I will never attempt caramel with an induction burner again. I hope this helps.