Understanding how to interpret multimeter readings

[wire55]

Hi was wondering if any one can help a newbie out .

I am learning how to understand and to interpret multimeter readings at school .

Our teacher set us tasks where we have to go online and find out the answers ourselves..

Now i am using a digital auto ranging multimeter on the ohms setting ....

I was testing for a low resistance reading which came up on meter as 00.1 ohm

Would this reading indicate a reading of 1 ohm or is it a reading in milli ohms .. Sorry for sounding dumb but the two zero's before the dot throw me .

Thanks in advance.

 

[Dean Huster]

For an autoranging DMM, the key is to look at the display for an annunciator indicating "M", "K" or [nothing]. If the "M" is showing, the resistance reading would be what you see on the numerical display in K ohms; if "K", the reading would be in K ohms; if none of the annunciators are showing, the reading is in ohms. So a display of "00.1" with the "M" showing would be 00.1M ohms which is the same as 100K ohms; if the "K" was showing, a display of "00.1" would be interpreted as 00.1K ohms which is the same as 100 ohms; and finally, with a display of "00.1" and no annunciator showing, it would be interpreted as 00.1 ohms which is the same as, well, a tenth of an ohm. In this case, the meter has autoranged down to the lowest resistance (ohms) range and you're seeing the resistance of the leads and any contact resistance in the probes -- for all practical purposes, zero ohms.

The two zeros before the decimal point are there simply because this particular meter model does not blank out all the leading non-significant zeros, and few meters will actually do that. A meter with a larger display (more digits) might indicate "0000.1" in a similar situation.

 

[Reloadron]

To add a little to Dean's post. Many times what the meter display shows is a function of the meter's quality. Attached are two examples. The Omegaette is a lower end digital meter, it does not even have an auto range function. It is up to the user to know the range chosen and interpret the data. The Fluke however clearly shows the small Ω symbol telling the user the display is indicating Ohms and not Kilo Ohms or Mega Ohms.

In your case since we can't see the meter if it displays 00.1 we assume it is on an Ohms range in which case the value would be .1Ω since the leading digits are zeros.

Ron

 

[squishy36]

Also realize that the typical multimeter can't reliably measure resistances on the order of an ohm or less because it doesn't deal with lead resistance or contact resistances. You can make low resistance measurements by using a current source to provide a constant current through the unknown resistance, then measuring the voltage drop across the resistance with your meter (this is called a 4-wire or Kelvin measurement).

 

[Reloadron]

Also realize that the typical multimeter can't reliably measure resistances on the order of an ohm or less because it doesn't deal with lead resistance or contact resistances. You can make low resistance measurements by using a current source to provide a constant current through the unknown resistance, then measuring the voltage drop across the resistance with your meter (this is called a 4-wire or Kelvin measurement).

Hi Squishy

That isn't quite true. If you look closely at the image of the Fluke DMM you will see a REL Δ button which is used to zero out the meter. The .1 ohm shown in the image was the lead resistance in the measurement plane. However, pressing the relative button subtracts the lead resistance from the actual reading. So in the case of higher end DMMs it is possible to read low resistances.

The resistance in the test leads can diminish accuracy on
the lowest (400-ohm) range. The error is usually 0.1 to 0.2
ohms for a standard pair of test leads. To determine the
error, short the test leads together and read the resistance
of the leads. Use the Relative (REL) mode to
automatically subtract the lead resistance from resistance
measurements.

Another method would be to short the test leads together and note the reading, then subtract that reading from the actual measured value later.

While the preferred method would be to use a meter actually designed for measuring low resistance and a kelvin configuration as you mention, it is possible to get fairly accurate measurements depending on the DMM used.

Ron

 

[wire55]

To add a little to Dean's post. Many times what the meter display shows is a function of the meter's quality. Attached are two examples. The Omegaette is a lower end digital meter, it does not even have an auto range function. It is up to the user to know the range chosen and interpret the data. The Fluke however clearly shows the small Ω symbol telling the user the display is indicating Ohms and not Kilo Ohms or Mega Ohms.

In your case since we can't see the meter if it displays 00.1 we assume it is on an Ohms range in which case the value would be .1Ω since the leading digits are zeros.

Ron

Thank you both for replying ..My meter does show kilo ohms and mega ohms if the resistance is in that range that i am testing.But the test i did was to find the lowest earth resistance on a tumble dryer which should be no more than 1ohm resistance ..I placed the black probe on the earth pin of plug and the red probe to the metal shell of the dryer ..The reading was 00.1 (no symbols showing) so am i right in saying that the 00.1 reading means 1 tenth of an ohm ??

Also the picture of the Omegaette meter you shown, shows a reading of 00.3 on the 200 ohms range ..would that mean say i had that same reading when doing the test i mentioned..Would that reading of 00.3 be 3 tenths of an ohm??

Sorry for all the questions but if i don't ask then i won't learn anything lol.Also i am from the uk.

Thanks again. p.s quote Ron wrote - Another method would be to short the test leads together and note the reading, then subtract that reading from the actual measured value later. Ron.

Wire55 wrote - I did short out the leads as above when i did my test but it was 00.0 when leads were together so i did not need to subtract anything..

 

[Reloadron]

Hi Wire 55

Based on what you cover then yes, the readings you got would be a valid 00.1 Ohm or simply stated .1 Ohm.

The Omegaette was an example of a very inexpensive DMM, actually that one is about a $10.00 USD meter that was given to me as part of a promotion. It is suitable for basic task but nothing requiring higher accuracy or resolution.

Feel free to visit these forums anytime and ask questions because as far as I know that is what this place is all about.

Ron

 

[wire55]

Hi Ron,

Thank you for taking the time out as it has answered that particular question for me ..Had a feeling that it was 1 tenth of an ohm with the 00.1 but was not 100% sure so i can carry on with my homework and will certainly come on here again.

Regards

sam