Reading a multimeter

jimbo1421

Hello

I bought a $30 digital Radio Shack multimeter recently and when I went to check the resistance on old and new replacement resistors I began to wonder about how accurate the MM really is. I hook up the alligator clips to the resistor, set the whole assembly down on a table and wait for the readout to stabilize. On the old, nominal 2.7Ω 5% resistor the readout fluctuates between 2.6-2.8Ω; on the new 2.7Ω (2%) resistor the readout fluctuates between 2.7-2.8Ω. Checking the old 4.7Ω (10%) resistor I get 4.4-4.6Ω; the new 4.7Ω (2%) resistor reads 4.7-5.1Ω.

So for my purpose of testing the resistors what value should I choose from the measured range, high, low or in between? (I assume that the percentage figure printed on the resistor is +/-. That is, 5% of 2.7 is 0.054, the resistor could vary between 2.65Ω and 2.75Ω) It appears then that all the resistors tested meet the old percentage specification, but the new 4.7Ω resistor does not meet the new 2% spec.

I also looked at the multimeter specifications which read, "+/- 1.2% of Reading, +/- 4 in last digit". Well, 0.4 is 15% of 2.7 and 9% of 4.7! So is this multimeter any good at all for testing these resistors?

Or am I overthinking again?

Regards,
Jim

F1nut

It's a $30 Radio Shack multimeter. If you want dead accurate, get a Fluke.

jimbo1421

Well, you're right. Quality usually costs. I checked at Fluke and Grainger. The Fluke 114 ($145) has DC accuracy of 0.5% vs. 1.5% for the $30 Radio Shack MM.

Jim

AudioGenics

I am on my 2nd FLUKE....25 years old...
(the first one was stolen)

WilliamM2

It's odd that it fluctuates like that. I have a $30 Craftsman mutimeter, and a Fluke. The Fluke is more accurate, but neither meter fluctuates while reading a resistor, or a cap.

I'd take it back. As far as the resistors, rather than worry about the perfect value, I try would try to match them in pairs for each channel.

jimbo1421

WilliamM2 wrote: »

It's odd that it fluctuates like that. I have a $30 Craftsman mutimeter, and a Fluke. The Fluke is more accurate, but neither meter fluctuates while reading a resistor, or a cap.

Hi William

Are your MMs digital readout or analog? Does it matter?

Jim

TECHNOKID

F1nut wrote: »

It's a $30 Radio Shack multimeter. If you want dead accurate, get a Fluke.

I agree with F1 since:

* Your components has 1 - 2% accuracy (usually military specs) and 5 - 20% accuracy (range of consumer goods)

* Your multimeter will also have range between let say 1 - 20%: again 5 - 20% will be related to pricing and the care (calibration) to keep/increase that accuracy. For example, a fluke multimeter used in a factory that produces high quality component or the military will have their multimeter recalibrated on a specific schedule ( 3 - 6 months).

* When doing your measurement, you must account for both the tolerance of the component(s) and of the multimeter so for example if you have 10% on the component and 10% on the multimeter that would give you a tolerance of 20% both ways for example (your 4.7 ohms could fluctuate between 3. 76 - 5.66 ohms).

So basically, your multimeter is reflecting what you paid for and the calibration care. If you are designing or creating new circuits, you then would want as accurate reading as possible. However, if you are only troubleshooting once in a blue moon, all you care is "short, close to value or open readings". However, your frequency and the quality of tools you normally use will determine what you pay for your multimeter. Personally, I don't care for radio shack as measurement tools but I am from a technical background and always was used to the best for accuracy however, a hobbler doesn't have the same requirement.

Cheers!
TK

jimbo1421

Hi TK,

Thanks for your explanation. +/- 1.5% I can live with. It was the "+/- 4 in Last Digit" that makes this MM kinda useless since 0.4 is 15% of the nominal value of the 2.7Ω resistor that I was testing. You'd think the Chinese could do better than that.

Jim

lightman1

Hot's on the left. Cold's on the right.

dcmeigs

Of course Fluke is the last word in multimeters and rightly so. I've had mine for well over 20 years now, used out in the rain and snow, dropped and abused, the LCD is so dim now that it's becoming difficult to read but it's still dead on.

Now, having said that, please allow me to recommend a good cheap VOM: here's the LINK. This Amprobe AM-240 VOM costs just $40 full retail, has autoranging resistance scales and has +/-1% + 2 resistance accuracy and half that for DC voltage. And they have high quality leads. The Amprobe and the Fluke agree closely on resistane values however the Fluke settles into the value much faster than the amprobe.

I use these when I need to monitor DC power supply voltages at multiple points since I'm too cheap to buy 4 Flukes. It's much safer to clip a few of these on the circuit than to be probing around around unknown high voltages (tube amps can put the hurt on you). Just clip a few of these on, power up, stand back and observe.

If you really want cheap, go get the buck 99 specials at harbor freight. They are probably just as good as your rat shack meter BUT I wouldn't trust the insulation on the probes at voltages above 120. I have a bunch of these too for the above stated purpose (with modified probes) and they work quite well. Accuracy is not their strong suit but their error is consistant throughout a given day.

Just my +/-2 cents.

TECHNOKID

jimbo1421 wrote: »

Hi TK,

Thanks for your explanation. +/- 1.5% I can live with. It was the "+/- 4 in Last Digit" that makes this MM kinda useless since 0.4 is 15% of the nominal value of the 2.7Ω resistor that I was testing. You'd think the Chinese could do better than that.

Jim

Well, the old saying is definitely working here, "you get what you pay for!" Personnally, I would never buy any of my electronic measuring tolls from radio shack, I prefer paying a little more and buy reliability. It doesn't have to be top of the line or the top of proffesional choice. I agree with dcmeigs, amprobe makes good reliable products. When I was operating my repair shop, fluke was the aim but I wouldn't necessarily go that route now. Buying good tools is important but depending on the tasks and the frequency of use someone may simply go somewhere in the middle.