Small square Bales anyone?

I have a baler that I need to know the rpms for. Just starting out so. The arm that pushes the hay together is what I am trying to figure out the revolutions per minute. What I have thought about was a hall sensor near the arm with a processor to calculate the revolutions and blue tooth it to a smart phone to give me the count while baling hay. Have more to get done too but need this one done so my gf doesn't burn it up. Thanks

The concept seems fine. How far would the data have to be transmitted? (Is bluetooth capable of the distance)

Have you any Arduino experience?

If not I suggest you start with two separate explorations on you table-top. First, using a hall effect sensor to detect and calculate RPM. Second, communicating between Arduino and phone. Only combine the ideas when you can do them separately. Keeping them separate will mean your sketches are short and as simple as possible and it will be much easier to get advice here.

A hay-baler will involve a lot of dust and vibration. And maybe wet if the baler is stored out in the open. You will need to plan to protect your electronics.