Martindale tek300 20ma loop calibrator martindale martek300 p the martindale tek300 process control calibrator features sink source and monitor modes for 0 20ma and is ideal for most process control applications as well as general purpose signal injection applications p p the handheld unit is fully portable and has a large 3 frac12 digit lcd display with overload protection for extra safety p martindale martek300 20ma loop calibrator