  Accessing and characterizing a flow impose a number of constraints on the
employed measurement techniques; in particular optical methods require
transparent fluids and windows in the vessel. Whereas one can adapt apparatus,
fluid and methods in the lab to these constraints, this is hardly possible for
industrial mixers. We present in this article a novel measurement technique
which is suitable for opaque or granular flows: an instrumented particle, which
continuously transmits the force/acceleration acting on it as it is advected in
a flow. Its density is adjustable for a wide range of fluids and because of its
small size and its wireless data transmission, the system can be used both in
industrial and scientific mixers allowing a better understanding of the flow
within. We demonstrate the capabilities and precision of the particle by
comparing its transmitted acceleration to alternative measurements, in
particular in the case of a turbulent von K\'arm\'an flow. Our technique shows
to be an efficient and fast tool to characterize flows.
