Ohmmeter accuracy?

[Geo-TH,In]

When in college I worked at Crane Naval (USNAD Crane, In.) calibration lab where every piece of electronic equipment had to be calibrated on a regular basis.

I will admit I have a cheapie HF because I've damaged my simpson analog meter. Even if I had a good analog meter, I couldn't use an analog to troubleshoot the EFI in my Kawasaki mule.


I was thinking of putting 100 one ohm precision 1% resistors in series to calibrate ohmmeters. I may have a Radio Shack analog meter, if I can find it.

Amazon,
EDGELEC 100pcs 1 ohm Resistor 1W, 1 Watt 1% Tolerance Metal Film Fixed Resistor, Multiple Values of Resistance Optional

$6.99

For $7 I can buy 100 1% 1 ohm resistors and calibrate my old RS meter.

Is there a digital Voltmeter that you can zero the ohm scale like the antique Simpson 260's?

How do you know if your ohmmeter is accurate?

 

[riverbend]

I know they are not. Touch the leads together and see what 0 ohms look like.

For most of what I have to do +/-5% is close enough. It is an interesting thought to have either a set or a series of resisters to see how far off each range is. Not that it can be adjusted...

 

[Geo-TH,In]

+/-5% of WHAT?
If my memory is correct it's +/-5% of full scale for voltmeters.
+/-5% of a certain value when measuring ohms.
I'm guessing because I don't remember when measuring ohms, if the +/-5% was +/-5% the mid range value or something like that..

Some people want to measure the ohm of a 6 volt coil and ballast resistors.
We are talking about 1.5 ohms or something close to that value.

I want to calibrate an old analog meter. Not a biggie if an analog meter reads higher than it should when it's actually 2 ohms.
I'll know the higher reading is really 2 ohms.

 

[sotxbill]

When I touch the leads on my digital' meters, I have to wiggle the plugs back and forth to clean it a bit to get it to zero or at least drop to 1 ohm to zero. So even the lead resistance and connectors come into play. A analog at least has a adjustment for full scale/zero to calibrate it and allow for battery voltage sage and lead resistance!!!!! But I keep my analogs up on a shelf as its too easy to get the leads backwards, or wrong setting or range. They also have a lower input impedance, so that they can load the circuits more. I dont know of any digital's that have a zero button to comp out lead resistance,etc. At least you would have a standard reference to check any meter against. I've moved to a dc amperage clamp meter now and can quickly check charging systems and current draw on any wire now. Life is good.

 

[Geo-TH,In]

Analog meters hate it when you measure 240vac using the ohm range. The major reason I don't have good Analog meters.
The smoke has gone.

 

[Ted in NE-OH]

Beware: The red cheepie Harbor Freight meters will read erroneously when the battery gets low and you won't know it. I was reading a 120 v outlet and that meter read 147 v, replaced the battery and then the meter read correctly.

 

[old]

Yes that can be a problem but if all you need is a ball reading there okay. But if you need to know if something is say 1-2 ohms you need a VOM that reads correctly. Back when I was in the Navy ALL of our electronic equipment got sent in fir calibration and if the date for calibration was not up to date we where told not to use it

 

[Geo-TH,In]

As a student in college I spent 2 semesters in the navy calibration lab. We checked most equipment every 6 months.

 

[Geo-TH,In]

To solve replacing expensive 9v batteries I bought 9v Li rechargeable from Amazon for about the price of throwaway 9v.
They have worked fine especially when I'm forgetful and leave the cheap EVOMs on

 

[old]

Being deployed something it could not be every 6 since sometime we would be out to sea for 9 plus month but I think they kept some equipment on hand that had not been used but was calibrated

 

[wore out]

(quoted from post at 07:48:17 11/28/22) +/-5% of WHAT?
If my memory is correct it's +/-5% of full scale for voltmeters.
+/-5% of a certain value when measuring ohms.
I'm guessing because I don't remember when measuring ohms, if the +/-5% was +/-5% the mid range value or something like that..

Some people want to measure the ohm of a 6 volt coil and ballast resistors.
We are talking about 1.5 ohms or something close to that value.

I want to calibrate an old analog meter. Not a biggie if an analog meter reads higher than it should when it's actually 2 ohms.
I'll know the higher reading is really 2 ohms.

As an alternative for your low resistance coil and ballast resistor work, get an accurate 10 Amp range analog Ammeter and note the Amps drawn and the voltage dropped across the components in question, then R = V / I.

 

[Geo-TH,In]

I'm surprised you didn't have a calibration lab on the ship.

 

[old]

We may have on the JFK CVA67 but on the sub they sure didn't but we where only out for 90 days on the sub then the crew was rotated

 

[tech4]

If you are just checking or verifying the accuracy of the ohmmeter, use some 10 ohm 1% resistors instead of 1 ohm. It is difficult to measure 1 ohm because of the meter probes as someone else mentioned. The ohms calibration in most of the analog meters inclucing the Simpson 260 are calibrated using precision resitors and not an internal calibration pot. The calibration pot internal the 260 is for the meter movement only and each scale is calibrated with fixed resistors.
I made a bad mistake with my old Simpson 260 when I had it on the R X 10,000 scale when I touched 2500 VDC. I thought the meter was gone but, it only blew the 1 A fuse and smoked 1 resistor. The resistor was a precision resistor and I found one very close in value in my resistor spares and installed it in the meter and it now works as well as it has for 45 years and checks very close to my Fluke 115. The reason your analog meter will not work on EFI is the input imedance is too low vs a digital meter that has a high impedance input.

 

[Geo-TH,In]

If I buy 1 ohm precision resistors and put 10 in series, I can check meter accuracy at any value from 1 to 10 ohms..
Someplace, I have a precision 10 ohm resistor.
I can't remember where I put it..
If I want to check for a 1.5 ohm coil resistance I can use 3 one ohm resistors. Put 2 in parallel (0.5ohme) and one in series = 1.5 ohms.

 

[560Dennis]

Test three different resistors at room temperature record reading .
Put the three same resistors in freezer with vom for and hour . Retest compare numbers

 

[caterpillar guy]

I just tes for continuity not a specific value. So Ohms works for me. I just want to know if it is open or shorted.

 

[sotxbill]

Yessir... its why all my analog meters are up on the high shelf. Dont let me get my hands on them... I keep them mainly for tuning radio circuits.. as its easier when tuning all day to see a meter swing up or down, than to having to interpret number values all day. Meter swing means muscle memory works fine, but digital means your brain must be 100% in the same room as your body. Hard to do for hours and hours, when I set up repeaters and cavities. But dont do that as much any more. Some digital meters will allow you to go to a quasi analog display now,, just for this purpose.

 

[tech4]

That will work.

 

[jimg.allentown]

If you set up 100 1% resistors in series, isn't there a possibility that the 1% tolerance can stack up and give you a less than accurate resistance value? Seems to me that a 1% variation times 100 could potentially give you a 100% error if each resistor was 1% off.
I would be more inclined to use ONE 100 ohm precision resistor rather than trying to string together that many 1-ohm resistors.

 

[Janicholson]

If the error value of each (in a package from one batch for example) were each 1% lower in resistance, you are correct. 100 resistors from 20 manufacturers from batches made months apart might average pretty close. But evn the leads of 100 resistors will shade the value. Jim

 

[Tony in SD]

I am a FIRM believer in quality test equipment. I ALWAYS buy the best test equipment available for the task at hand. I learned that lesson a long time ago. You can spend hours chasing a problem that isnt there if your test equipment gives you false readings. Additionally, I put my time in instrumentation and calibration lab in college. Now I just want to get the job done without experimentation. FWIW eBay has many Simpson 260s for sale for not much more than a HF VOM.

 

[guido]

Geo-TH,In,

Simply put the two meter leads together, and note the resistance. Now you have zeroed the meter. When you go and measure a resistance take away the meter total resistance.

Mine HF cheapy Has a 4 Ohms resistance with both leads touching on the lowet scale. Then , a 10 ohm resistor, will display a resistance reading of 14 ohm plus or minus the resistors tolerance.

Simple as that!

Guido.

 

[Hobo,NC]

I guess I am the lucky one in the bunch all my meters are dead nuttz are no more off than a 10th. My advice buy a good DVOM.

 

[MarkB_MI]

You seldom need much accuracy when making resistance measurements. And as I've said before, no handheld multimeter can make accurate, precise resistance measurements below a couple of ohms. For that you need a benchtop multimeter capable of four-wire measurements.

I don't understand the need to 'zero' a digital multimeter. The zero adjustment of the old VOMs was needed because of the limitations of an analog display, and the fact they had primitive circuitry and were battery powered. A digital multimeter will never read zero because there's always contact and lead resistance. Touch the leads together, read the resistance and simply subtract that value from your test measurements.

As others have said, you'll get major tolerance stack-up if you connect multiple resistors in series. Since they're probably all from the same batch, it's likely they're all off the same amount in the same direction. If they're off by, say 0.3 percent, you will exceed the 1 percent accuracy if you put more than three resistors in series. It's better to use precision resistors of various values to check out your ohmmeter rather than several in series.

On the subject of accuracy, let's look at one pretty-good DMM: The <a href=https://www.fluke.com/en-us/product/electrical-testing/digital-multimeters/fluke-179>Fluke 179</a>, which retails for 400 bucks. It has an accuracy of '&plusmn;(0.9% + 1)' or about one percent of whatever the measurement is. (The '+ 1' means you have to add one least significant digit to the result, since that's always rounded.) BUT, its RESOLUTION (precision) is only 0.1 ohms. Which means that if you're measuring around one ohm, its accuracy is actually only about TEN percent. And it's useless for measuring anything much below one ohm.

Back when I worked with automated test equipment, we kept a minimum ratio of 3:1 between the accuracy of the measurement we were making and the accuracy of the test instrument. So to measure something whose limits were, say &plusmn;3 percent, we needed a test instrument good to 1 percent. And we tried to keep that ratio better than 5:1.

 

[MarkB_MI]

Now, George, if you have about 1200 dollars lying around after your recent windfalls, you can shell it out on a REAL DMM: A <a href=https://www.fluke.com/en-us/product/precision-measurement/bench-instruments/fluke-8808a>Fluke 8808</a>. This baby can do four wire measurements. It can measure down to one milliohm! And it don't need no STINKING zero adjust knob.

 

[Texasmark]

How about the lead resistance that the zero pot used to remove? My $5 yellow HF and the newer red one have 0.4 Ohms of lead resistance. Course I just add it to the Ohm reading if it matters but its still there.

 

[Hobo,NC]

If their cheap Ohm meter is off no doubt the voltmeter is also so how do you trust them.

 

[MarkB_MI]

> How about the lead resistance that the zero pot used to remove? My $5 yellow HF and the newer red one have 0.4 Ohms of lead resistance. Course I just add it to the Ohm reading if it matters but its still there.

What lead resistance? That $1200 multimeter has four-wire ohms capability, which eliminates lead resistance from the measurement. As I said before, IT DON'T NEED NO STINKING ZERO ADJUST KNOB!

 

[Texasmark]

Ok , so how is it connected to the resistor to be tested since the resistor has 2 leads?