I want to measure the current or voltage needed to keep a motor spinning at a set rpm. The reason is so the motor can be attached to a paddle that is submerged in various liquids to indicate their relative viscosity. Any ideas very appreciated.
What voltage/current are you attempting to measure?
what microcontroller are you using?
All you need are two resistors and an Uno
If your motor has brushes the current will always vary with the turning of the commutator. Sometimes the brushes will only contact a single motor winding and some times they will make contact with two segments, so the windings will be in parallel and the current will pulse to twice the current.
Would not a better way to measure your fluid be to just monitor the motor RPM?
With more viscosity the motor will spin slower, or you will need to increase the power to spin at the same speed.
So, I suppose that you need to measure both; the RMP's and the power consumed.