Persons who use multimeter to work on detectors ( meter can lie to you)

The link below,,,,notice it says it's set to check continuity,,,,but is it really???

A little story,,,an experience I had some time ago.

We had this C130 aircraft that had a recurring RPM indicator problem,,,,it would fluctuate intermittently.

Troopers worked on for 2 days,,,never found anything wrong physically or electrically,,,,but on engine run up test-- sure enough rpm gauge fluxed.

Well, I got involved and was supervising the crew,,,,sure enough,,didn't seem like nothing was wrong electrically.

But I noticed the technicians were checking wiring,,,,with detector in what some call the beep mode,,,,meaning supposingly the detector beeps when you are testing,,,you have continuity.

Well,,,I was very familiar with the test meter being used,,so I personally checked the system,,,except I put the meter in true ohms position.

What changes were noticed,,,,when the wiring on the aircraft engine connected the tachometer generator was shook,,,,meter readings of small ohms changes were noted,,,2-5 ohms of resistance were noted.
Problem was actually caused by a cold solde joint on connector.

Anyway,,,I can't vouch for all,,but at least some multimeters,,,,this diode position,,,while it is true meter will beep when there is continuity,,,,this is not absolute test.
Some meters,,,have a threshold built in,,,like if actual resistance is 15 ohms or less,,,,multimeter beeps.

Now when lighting is bad,,,or if you can't see the meter due to position of op,,vs meter position,,,,this is when generally folks really like the so called "beep mode"

But be careful by doing and assuming all is well continuity wise.

I know some here already work on their detectors,,,and some may be thinking of starting.

[learn.sparkfun.com]



Edited 4 time(s). Last edit at 01/12/2017 05:40AM by tnsharpshooter.

This is called a 'dynamic short'. And...... a (very high) resistance DIGITAL volt meter probably will not detect. But...... an old analog (Like Simpson 260) multi-meter is more apt to detect. . . . . due to higher current testing requirements.

Heh,
Many times I have seen a malfunctioning powered circuit begin working correctly while making a measurement across a component.



Edited 1 time(s). Last edit at 01/12/2017 07:45PM by Yeasty.