Loop calibrators are capable of measuring (sinking) and simulating (sourcing) DC mA currents for calibration of process devices. A good mA calibrator will have enough power to drive for sustained periods into a process loop with a reasonable load.

Higher end units feature DC Volt measurement and sourcing as well. Look for automated features that will help with 'stroking' valves or programmable operation to allow you to remotely monitor the signal changes from the control room.