Taking OHM readings

[Lovison]

I learned a valuable lesson the other day. I'm not electrical but work with many issues related to it.

We have a submersible motor we use and it requires
that we take ohm readings to checkt the integrity of
the moisture sensors. I'm suppose to get 330k ohm
with a +/- 10% variance. Within that range the
sensors are working within acceptable parameters.

But periodically I get in a rush and can't find my
clamps so I will hold the red and black leads in my
hands (acting like a clamp) against the two leads on
the sensors. Well I got a low reading of about 279k.
So I called the motor engineering center to ask about
this reading.

He said not to hold the leads with my fingers as this
would give me a low reading since it would be going
through my body. Do you guys agree? Not that I'm
second guessing this guy but I just thought I toss
this out to see if other's have experienced this as
well.

It may make a good Helpful Tip to put out. It's so
simple that anyone could make this mistake.






Wayne E. Lovison
service-parts@naglepumps.com

 

[jlhmaint]

he is right, your body will change the reading, take your ohm meter leads on your voltmeter with your meter set on OHMs squeze the leads with your fingers and see what you get it will change depending on if hands are wet or dry. Your body has resistance and that is what is affecting your reading same with testing a fuse if its laying in your hand you will get a close to good reading because of the path through your hand.

 

[busbar]

Experiment a bit and prove or disprove to yourself the accuracy of the support guy's statement. It will make a lot more sense if you do.

 

[Lovison]

Hey Guys;

Spend $45 and got the correct leads and clamps. What a diffrence. The readings went right where they were suppose to be.

I needed 330K Ohm and holding by my hands I only got 279K.
Put the clamps on and I got 356K right in the ball park or spread of the tolerance.

It was well worth it and like I said someone should write a Tip for others to pick up this little tid bit of importance.

Thanks Again Guys






Wayne E. Lovison
service-parts@naglepumps.com

 

[jbartos]

Suggestion: Strictly speaking, the leads are supposed to be insulated and held properly where they are insulated, not where the leads tend to be bare metal. In such case, the readings would be very close to what they are expected to be.

 

[electricpete]

Wayne - thanks for offering a tip. But to be honest I really don't think you'll find any electrical guys who are inclined to hold those leads with their fingers. For one thing we avoing touching energized circuits. I don't know if you were using a megger with relatively high tes voltage or simply an ohmmeter with relatively low test voltage... but either way it goes against all training and instinct to hold energized leads in your bare hands.

 

[IRstuff]

Speaking from personal experience, even "unenergized" circuits can be problematic if there are safeguard failures. I've gotten zapped from a high voltage ionization chamber where the grounding bar failed to engage. It was highly unpleasant.

As for the ohm-meter, it's something almost every electrical student has done at home or in lab. Then only times I've used finger pressure is on items where there was mechanical difficulties, like light bulbs, where I either cared only about open/short conditions or knew that the parallel resistance was too low to be impacted by my body resistance.

TTFN

 

[electricpete]

IRstuff's first paragraph points out another very compelling argument for never using your fingers. I don't care what "almost every electrical engineering student has done", it's not a good habit to get into.

 

[jbartos]

Suggestion: If for some reason the leads are old, then you may wear insulated gloves. These are highly recommended for people who do not have electrical background and safety training, and who have to work around electrical equipment, to do some routing tasks. I just puchased insulated shoes up to 600V for 99$ for my field trips.