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TherapyChick

join:2003-09-19
Fayetteville, NC

4 edits

[Electrical] Multimeter question

Click for full size
Craftsman 82141
So my 5th grader is doing a science project to measure resistance in various objects such as an apple, potato, water, metal spoon, etc.

I've read up on resistance and multimeter, but want to make sure I understand it better (so I can explain it to her...)

First I plugged the red lead into the plug on the Craftsman 82141 multimeter labeled V mA and the black lead into the plug on the multimeter labeled COM. (that's right?)

So we set the meter to 2000k Ohms (which is 2 million Ohms right?) and stuck the probes into each end of the following and got the readings below:
Potato 308 Ohm
Apple 115 Ohm
Water 252 Ohm

So since it's set on 2000k Ohms, do you multiply the readings above by 2 million?

I'm thinking maybe this project might not be the best idea now but hoping I can help her figure it out and she really wanted to do it even though neither of us are electrical geniuses!
--
Therapy Chicks


SparkChaser
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Downingtown, PA
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Nope

308,000
115,000
252,000

Also best to keep the probes the same distance apart for all samples.

Nice to see another science project besides me.

Edit: another thing to try is distilled water.



LazMan
Premium
join:2003-03-26
canada

1 recommendation

reply to TherapyChick

Without knowing what make/model meter you have, impossible to know if you have the test leads in the right terminals...

The "COM" is almost certainly right; however, on many meters, current (mA) is on a different terminal then resistance (usually labeled with an "Ohm" sign - Kinda like an O, with a broken line across the bottom)...



SparkChaser
Premium
join:2000-06-06
Downingtown, PA
kudos:3

Good point LazMan See Profile but the numbers are about right. Tap water 250K at about 2-3 inches


robbin
Premium,MVM
join:2000-09-21
Leander, TX
kudos:1

1 recommendation

reply to SparkChaser

said by SparkChaser:

Edit: another thing to try is distilled water.

Then it would be interesting to add salt in various quantities.

TherapyChick

join:2003-09-19
Fayetteville, NC
reply to LazMan

I updated the OP with a pic and model of the multimeter (Craftsman 82141)
--
Therapy Chicks


TherapyChick

join:2003-09-19
Fayetteville, NC
reply to SparkChaser

said by SparkChaser:

Nope

308,000
115,000
252,000

OK thanks, so another probably stupid question.

So I know that 0 (zero) means no resistance, like what you get if you simply touch the leads together or test metal or something with pretty much "perfect" conductivity property. And a 1 (one) means that it's full resistance like what you get with the leads touching nothing or putting them on plastic or wood for example.

So I would have thought that everything else would be a number BETWEEN 0 and 1, so I don't understand what the big numbers mean really.

Or am I wrong about the 0 and 1 understanding?
--
Therapy Chicks


SparkChaser
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reply to TherapyChick

You're right. That's what the connection are.

Edit: the ranges are to put you in an area where it can give some reasonably accurate measurements. Trying to go from 0 to infinity gets hard to display.


robbin
Premium,MVM
join:2000-09-21
Leander, TX
kudos:1
reply to TherapyChick

said by TherapyChick:

So I know that 0 (zero) means no resistance

Actually I believe the scale is 0 ohms to infinity. The more ohms, the more resistance to conductivity.


tschmidt
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Milford, NH
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reply to TherapyChick

Not exactly resistance goes from 0 (nothing) the infinity. Not sure about that meter but most have some way to indicate resistance higher then they can read (typical the word Over, OL or perhaps flashing display). There is no such thing as "full resistance."

It would be fun to get a variable resistor (perhaps Radio Shack or salvage it out of an old piece of equipment) and watch the display change are you turn the shaft.

/tom



pike
Premium,MVM
join:2001-02-01
Washington, DC
kudos:3
reply to TherapyChick

The 1 by itself is the meter's way of telling you the reading is out of range. Select a larger range or, if already set at the highest, you probably have an open circuit.



Booost

@optonline.net

2 recommendations

reply to TherapyChick

That's an ohm (Omega) symbol between the "V" and "mA", so you have it connected correctly.

When you set it to 2000k ohms, the display should have shown a "k" symbol somewhere, to indicate the reading is in thousands of ohms. (The 2000k is the maximum range at that setting.)

The main thing to be careful with when using multimeters is if you have it set to measure current (shown as "Aac" or "Adc" on your meter.) At the current setting, you can blow the fuse in the meter if you put the leads across a voltage source. So be careful when you have it on the current settings. Never leave it on that setting, because someone might hook it up to a voltage source before checking the setting.


TheMG
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Canada
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said by Booost :

The main thing to be careful with when using multimeters is if you have it set to measure current (shown as "Aac" or "Adc" on your meter.) At the current setting, you can blow the fuse in the meter if you put the leads across a voltage source. So be careful when you have it on the current settings. Never leave it on that setting, because someone might hook it up to a voltage source before checking the setting.

Been there done that. Ruined a set of probes as well as the receptacle that I was testing (cheap Radio Shack meter did not have a fuse on the 10A current setting). Essentially the probes of the meter turned into a mini arc welder during the time it took for the breaker to trip. At least I was able to confirm that there was in fact power at the receptacle!

robbin
Premium,MVM
join:2000-09-21
Leander, TX
kudos:1

Hopefully the OP is aware that a 5th grader should not be testing line voltage even for a school project.


Mr Matt

join:2008-01-29
Eustis, FL
kudos:1
reply to TheMG

Meter is a poor design. The multimeters I use have one +jack for AC/DC Volts and the other +jack for Amps and Ohms.


robbin
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join:2000-09-21
Leander, TX
kudos:1

2 recommendations

I've never ever seen a meter like you describe. All of the meters I have used, including Fluke, have volts and Ohms on one terminal and Amps on another.



Raphion

join:2000-10-14
Samsara
Reviews:
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reply to Mr Matt

said by Mr Matt:

Meter is a poor design. The multimeters I use have one +jack for AC/DC Volts and the other +jack for Amps and Ohms.

It doesn't; the ohms you see on the left refers to the selector legends, not to the jack below.

lutful
... of ideas
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Ottawa, ON
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reply to robbin

said by robbin:

Hopefully the OP is aware that a 5th grader should not be testing line voltage even for a school project.

My son inserted the probes into an AC socket (while I was in the bathroom) right after I warned him about that. He had originally wanted to measure DC voltage across his batteries to see which ones are still good.

For younger kids, there is an excellent electronics learning kit called snapcircuits:
»www.snapcircuits.net/


Hall
Premium,MVM
join:2000-04-28
Germantown, OH
kudos:2
reply to TherapyChick

said by TherapyChick:

So since it's set on 2000k Ohms, do you multiply the readings above by 2 million?

I thought the setting you choose with the dial was the maximum it would read up to and that it also scaled the readout accordingly. For example, look at the VAC section - it has 200 and 600. Who has 200v ? Key is it covers a common range (in the US).

On the 200v setting, testing a ~120v outlet, I get 126.1. Same outlet on the 600v setting, I get 126.

I have the exact same meter, by the way.


Msradell
P.E.
Premium
join:2008-12-25
Louisville, KY

1 recommendation

When you move to a higher voltage scale (or resistance scale) you reduce the accuracy of the unit. That's why only 200 V scale you got 126.1 in on the 600 V scale you got 126. The accuracy went from 000.0 to 000.


TheMG
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Canada
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reply to Mr Matt

said by Mr Matt:

Meter is a poor design. The multimeters I use have one +jack for AC/DC Volts and the other +jack for Amps and Ohms.

The meter in question did have separate jacks. It had one jack for V/ohms, one for 300mA range (fused) and one for 10A range (not fused).

I stupidly left the probe plugged into the 10A jack when attempting to measure voltage. This was over a decade ago when I was young and careless.

Nevertheless, it's still very poor design to have a non-fused current jack on a multimeter.


Raphion

join:2000-10-14
Samsara
Reviews:
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reply to Raphion

said by Raphion:

said by Mr Matt:

Meter is a poor design. The multimeters I use have one +jack for AC/DC Volts and the other +jack for Amps and Ohms.

It doesn't; the ohms you see on the left refers to the selector legends, not to the jack below.

Now that I actually read this, i see Matt was talking about his own meters, not the OP's meter. I thought he said the OPs meter had the high current and resistance jacks combined in one. Sleep deprivation is bad.


Booost

@69.157.170.x
reply to TheMG

said by TheMG:

said by Booost :

The main thing to be careful with when using multimeters is if you have it set to measure current (shown as "Aac" or "Adc" on your meter.) At the current setting, you can blow the fuse in the meter if you put the leads across a voltage source. So be careful when you have it on the current settings. Never leave it on that setting, because someone might hook it up to a voltage source before checking the setting.

Been there done that. Ruined a set of probes as well as the receptacle that I was testing (cheap Radio Shack meter did not have a fuse on the 10A current setting). Essentially the probes of the meter turned into a mini arc welder during the time it took for the breaker to trip. At least I was able to confirm that there was in fact power at the receptacle!

We had techs who would connect up a device to be tested along with the meter, turn on 440 V (IIRC), then check the jacks the leads were connected to and select the meter settings. They blew-up several meters this way, including a very expensive Keithley meter (which Keithley repaired at no charge - twice!).

We also had a tech nearly blow himself up when he failed to disconnect the equipment previously being used at the station and putting a large voltage on a very large electrolytic capacitor. I was in the next room and thought someone set off a pack of firecrackers.

We solved the problem by:
1. Getting Fluke meters with 600 Volt fuses.
2. Requiring techs to have an engineer check their setup before turning on power.


Thespis
I'm not an actor, but I play one on TV.
Premium
join:2004-08-03
Keller, TX
reply to robbin

said by robbin:

said by SparkChaser:

Edit: another thing to try is distilled water.

Then it would be interesting to add salt in various quantities.

My tech theatre classes have built small, battery powered salt water dimmers in the past. Fun project!
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Pick two...


SparkChaser
Premium
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Downingtown, PA
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Yes, it is fun. People don't believe that ultra pure water is about 18Megs.cm resistance. That's a way to measure purity. It also fluoresces.



John Galt
Forward, March
Premium
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Happy Camp
kudos:5

1 recommendation

said by SparkChaser:

Yes, it is fun. People don't believe that ultra pure water is about 18Megs.cm resistance. That's a way to measure purity. It also fluoresces.

I used to run and maintain the DI water plant at SLAC. One of the things they told you was to not drink the water on a regular basis.

This DI water plant supplies the water that cools the INSIDE of the power conductors feeding the magnets and the klystrons. You had to make sure that you had your plumbing fittings connected to your conductors before you landed the terminations, and your differential pressure switch monitoring the water in your wires before energizing the load.
--
Many believe, but few will question...I decline to be blind.


robbin
Premium,MVM
join:2000-09-21
Leander, TX
kudos:1

said by John Galt:

your differential pressure switch monitoring the water in your wires

If anything should be called Hydro, that would be it!


cowboyro
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Shelton, CT
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reply to TherapyChick

Keep in mind that sticking two electrodes in a potato creates a mini-battery if the composition of the metals is different. Before taking a measurement make sure that the voltage measured after sticking in the electrodes is 0, otherwise your results will be seriously skewed. If the voltage is not 0, try using something else as electrodes - such as two identical stainless steel forks.



cowboyro
Premium
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Shelton, CT
Reviews:
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1 edit
reply to Msradell

said by Msradell:

When you move to a higher voltage scale (or resistance scale) you reduce the accuracy of the unit

No, you don't. You reduce the resolution. Totally different things.
And for an example you measure the AC voltage on the 200 and 2000V ranges.
Your readings are 116.6 and 116V respectively. The more accurate one is the one closer to the actual voltage, which can very well be 114.17V or 117.03V


SparkChaser
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reply to cowboyro

said by cowboyro:

Keep in mind that sticking two electrodes in a potato creates a mini-battery if the composition of the metals is different. Before taking a measurement make sure that the voltage measured after sticking in the electrodes is 0, otherwise your results will be seriously skewed. If the voltage is not 0, try using something else as electrodes - such as two identical stainless steel forks.

I thought about that but wasn't sure the OP would understand. It would certainly add to the science project.

EDIT: example




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