Medical Equipment Expert Testifies - Day 4 -

The guy who manufactures the medical device that Murray allegedly snapped onto Michael Jackson's finger -- after Alberto Alvarez called 911 -- claims it was the wrong device to monitor Michael Jackson.

It's called a pulse oximeter -- and Bob Johnson of Nonin Medical Equipment testified today Murray was using the wrong model for the job.

Bob claims the model Murray used to monitor Michael Jackson had no audio alarm, so if anything went wrong, Murray wouldn't know. The device Murray used to monitor Jackson's pulse and blood oxygen level while he used propofol to put him to sleep was the focus of testimony by the first witness Friday.

Johnson said the Nonin 9500 pulse oxymeter was "designed for spot checking of vital signs" and was "specifically labeled against continuous monitoring."

Since the $275 device did not have an audio alarm, it required someone to constantly keep an eye on the tiny screen, Robert Johnson testified. Murray would have been better equipped with his company's table top version that would cost $1,200, Johnson said.

MJFS - sources: tmz / cnn