Digital Multimeter for resistance
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LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
The numbers you posted are full-scale readings for each range. For example, if you get a reading of 3.87 with the scale set to 20K, then the resistance is 3.87K ohms. If you had the same reading with the scale set to, say, 2M, then the resistance is 387K ohms.
The reason they use a "2" is to use what is known as a half of a digit. In other words, a typical 8-segment display can show all 10 digits, whereas a 1 can be displayed with a vertical line (the 'half digit'). So the max readout is actually 199. (with a 2-1/2 digit readout)
The reason they use a "2" is to use what is known as a half of a digit. In other words, a typical 8-segment display can show all 10 digits, whereas a 1 can be displayed with a vertical line (the 'half digit'). So the max readout is actually 199. (with a 2-1/2 digit readout)
On the 200 scale Iam getting 44.9 Is this the ohm reading because the resistance did not go over 200?
On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?
On the 20K scale .04 Do we multiply this by 1000 too?
on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?
Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
Measuring the resistance of that bulb while off (cold) = 44.9 ohms @ 200 ohm scale
Question: Does the resistance change that much?
On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?
On the 20K scale .04 Do we multiply this by 1000 too?
on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?
Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
Measuring the resistance of that bulb while off (cold) = 44.9 ohms @ 200 ohm scale
Question: Does the resistance change that much?
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On the 200 scale Iam getting 44.9 Is this the ohm reading because the resistance did not go over 200?
Yes.
On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?
It's .045 K = 45 ohms, so, yes.
On the 20K scale .04 Do we multiply this by 1000 too?
It's .04 K = 4[0] ohms, so, yes.
on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?
Full scale is 200,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
Such a meter displaying this many digits would read
200,000,045 ohms
so you would see a zero in the fourth place of your meter.
If you really want a challenge, try to decode the accuracy specs on your meter and check if these readings indicate your meter is within tolerance, assuming the 44.9 ohms is exactly correct.
You might also Google
"significant figures"
and
"scientific notation"
Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
Measuring the resistance of that bulb while off (cold) = 44.9 ohms
Question: Does the resistance change that much?
Wiki says the cold resistance is 10x to 15x less than the hot resistance.
Since the resistance drops as the current drops an incand. bulb in series with an outlet [a "voltage source"] acts somewhat like a "current source."
Yes.
On the 2K scale .045 Do you multiply this by 1000 unless you go have a whole number?
It's .045 K = 45 ohms, so, yes.
On the 20K scale .04 Do we multiply this by 1000 too?
It's .04 K = 4[0] ohms, so, yes.
on the 2M scale .000 If there was a digit to the right of the decimal point, what would that be multiplied by?
Full scale is 200,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
Such a meter displaying this many digits would read
200,000,045 ohms
so you would see a zero in the fourth place of your meter.
If you really want a challenge, try to decode the accuracy specs on your meter and check if these readings indicate your meter is within tolerance, assuming the 44.9 ohms is exactly correct.
You might also Google
"significant figures"
and
"scientific notation"
Also doing the math to find resistance of 25W, 120 volt bulb = 576 ohms
Measuring the resistance of that bulb while off (cold) = 44.9 ohms
Question: Does the resistance change that much?
Wiki says the cold resistance is 10x to 15x less than the hot resistance.
Since the resistance drops as the current drops an incand. bulb in series with an outlet [a "voltage source"] acts somewhat like a "current source."
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Yes. 44.9 ohms.
No. That means .045 thousands of ohms, a.k.a. .45 hundreds, or 4.5 tens, or 45 ohms (rounded).
On that scale you should read .004 or something useless.
Nothing. Switch scales. Stop thinking of multipliers.
The closest scale that's higher than your measurement gives you the most accurate reading.
Absolutely.
Correction
Full scale is 200,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
Such a meter displaying this many digits would read
200,000,045 ohms
so you would see a zero in the fourth place of your meter.
should be
Full scale is 2,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
Such a meter displaying this many digits would read
2,000,045 ohms
so you would see a zero in the fourth place of your meter.
Sorry.
Full scale is 200,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
Such a meter displaying this many digits would read
200,000,045 ohms
so you would see a zero in the fourth place of your meter.
should be
Full scale is 2,000,000 ohms but not all the digits are displayed, though there are probably laboratory meters that can do this.
Such a meter displaying this many digits would read
2,000,045 ohms
so you would see a zero in the fourth place of your meter.
Sorry.
Now I am really confused
Now I am really confused
The ohmmeter reading are actual readings. Take my 20K reading of .04. Larry says "For example, if you get a reading of 3.87 with the scale set to 20K, then the resistance is 3.87K ohms.
So that means my resistance would be .04 ohms, but my reading at the 200 ohm scale was 44.9 ohms.
Now I am really confused
The ohmmeter reading are actual readings. Take my 20K reading of .04. Larry says "For example, if you get a reading of 3.87 with the scale set to 20K, then the resistance is 3.87K ohms.
So that means my resistance would be .04 ohms, but my reading at the 200 ohm scale was 44.9 ohms.
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
This a simple math explanation that I came up with.
V=IxR
P=VxI
Line voltage=120V
25W=120VxI
25W/120V=.208A
120V/.208A=576 ohms
Meter=Approx 9V
9V/.208A=43.2 ohms
9Vx.208A=1.872W
25W/1.872W=13%
120V/9V=13%
576 ohms/43.2 ohms=13%
V=IxR
P=VxI
Line voltage=120V
25W=120VxI
25W/120V=.208A
120V/.208A=576 ohms
Meter=Approx 9V
9V/.208A=43.2 ohms
9Vx.208A=1.872W
25W/1.872W=13%
120V/9V=13%
576 ohms/43.2 ohms=13%
PetrosA
Senior Member
- Location
- Lancaster, Pennsylvania
I'm not sure that your 9V calculations are applicable since most DMMs aren't using the full battery voltage for Ohms readings but are calibrated to show a standard value. If they weren't, my 6V meter would show something different than my 9V meter and neither would show the same as a 3V meter. It doesn't work that way. Aside from that, the reading of a cold filament will be very different than that of a hot one which is why you can't use the wattage of a bulb to figure out its resistance when not lit. The same temperature issue comes into play when measuring resistance of heating elements (ex. floor heating cable or water heater elements) as well - the specs will give you a specific resistance but should also state at what temperature the measurement was taken.
No, it means .04 K ohms. Some meters put a small K or M at the far right as you change scales so it isn't so confusing. All you're losing as you change scales is precision, so try to keep leading zeros off the meter. 0.04 K ohms is the same as 40 ohms. Close to your first measurement of 44.9.
zog
Senior Member
- Location
- Charlotte, NC
k Ohms, not K Ohms.
My last post still has an error in it. I hope I do better on this post.
You could post a link to your meter.
In general, 45 ohms measured on a DVM with 3 displayed digits will read
45 on 200 scale, and
for the 2000 full scale you'd see 4, and
for the 20,000 ohm full scale so you'd see 0, and
zero for the rest.
see
http://en.wikipedia.org/wiki/Multimeter
There are also "counts" to be considered.
The applied voltage or current to the Device Under Test on the different ohm scales should be shown in the DVM manual.
This is only important when the resistance of the device you are measuring depends on the current through the device, e.g., semiconductors.
I use B+K Precision 391. Sometimes those extra displayed digits contain valuable info.
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Generally on the 200 ohm range, the reading is taken directly in ohms. 5 means 5 ohms.
In the 2K, 20K, and 200K ranges, the reading is generally in Kohms. 5 means 5 Kohms. If you want to convert this to ohms, multiply by 1000. So a reading of .05 would be .05 Kohms, or 50 ohms.
On the 2M and 20M ranges, the reading should be in Megaohms. 5 means 5 megaohms. TO convert this to Kohms, multiply by 1000. To convert to ohms, multiply to 1,000,000.
Steve
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