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 Post subject: Where to get multimeter calibrated
PostPosted: Feb Wed 28, 2018 9:18 am 
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Location: Spring Hill, FL
I just bought an Elenco mulitimeter kit. I've got it all together, but I'm hitting a roadblock on calibrating it. The AC and DC voltage scales seem to be good t go, but I'm not so sure about the current scales. The big problem here is that I have a digital multimeter, but it's an electrician's meter, not a technicians meter (which is why I got it, and I thought a kit would be fun). But I'm not certain if my other meter is 100% accurate in the first place.

As far as Elenco, looks like I have to call to get rates. Does anyone have a ballpark idea how much this might cost, whether from them or some other company that does calibrations?

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Feb Wed 28, 2018 9:48 am 
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Location: Stone Mountain, GA
You can get voltage reference boards using AD584 that can generate 2.5, 5 and 10V within 0.1%. Adafruit makes a 2.048 and 4.096V reference board.

Then you can check your other meter, or both meters.

Once voltage is calibrated use precision resistors and a DC supply to calibrate the current. I would think you would only need to calibrate 1 range.

Calibration services are not cheap.

Maybe some one nearby has a good meter you can borrow.

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Feb Wed 28, 2018 10:51 am 
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Greetings to Michael and the Forum:

This is just me, but I certainly wouldn't invest any money in getting the current scales calibrated. Again, just me, but I have yet to see a DVM that I trust for current measurement. I use D'Arsonval meters exclusively for current.

The problem is that DVM's can only measure voltage. To measure current, a small value resistor is placed in series with the current path and the voltage developed across this resistor is measured. The results are automatically calculated with Ohm's law and displayed as current. The problem is that in order to measure without upsetting the circuit under test, the series resistor inside the meter must be very small in value. This means that the voltages being measured are in the millivolt range. Anytime you are measuring millivolts, you are in a region where the measurement is extremely susceptible to noise. In my past life as a transmitter engineer, the ambient RF was enough to ensure that no two current measurements would ever agree. My body or even my hand position was enough to disturb the reading by a factor of 2 or more.

Even away from RF fields, a noisy fluorescent fixture or switch mode power supply or even a lead too close to a transformer will upset the accuracy.

Best to not bother with the current range.

However, if you must calibrate the current ranges, measure the resistance through the DVM with a good accurate ohm meter. If you have the schematic of the thing (and I assume you do, since it was a kit) then you should be able to locate the shunt resistor and determine its value even closer than measuring it.

Now, simply set up a variable current source and connect your meter to it to measure current. With a good accurate DVM, measure the voltage across the two probes at the Elenco. Apply Ohm's law... I=E/R. Since you know R and you have just measured E, you can then compute I. Adjust the Elenco current cal pot accordingly.

Good Luck,

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Feb Wed 28, 2018 1:10 pm 
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Location: Canandaigua, NY
You can buy an AD587 10 volt reference IC that's factory trimmed to 0.05%. Just a few bucks from any of the usual suspects. Put it in a box and power it with a wall wart. AC is a bit harder. Best bet there is to compare with another meter. If you have a scope and signal generator, calibrate the scope for 10Vpp using the reference chip, then use that to set the sig gen to 10Vpp. Read it with the meter and set to 3.536 Vrms. Do current as above with 1% or better resistors.


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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Feb Wed 28, 2018 8:22 pm 
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Analogue meters are rarely better than 2% FSD so with a suitable power supply and nice new 1% or better resistors with your DMM to read the voltage it should be relatively easy to calibrate the current scales. If you trust your DMM to read AC voltages then you can do the same with a low voltage transformer and a variac. Just don't overheat the 1% resistors!

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Thu 01, 2018 1:08 am 
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If this is the M-1250K analog VOM, there's only one calibration control in the unit which is set up on the 0.5-volt DC scale. Assuming this is done correctly, all the other ranges should fall into place except the 10-amp range. The shunt wire length can be changed to adjust that range.

If you do the check-out procedure in the manual, you should be able to see that each range is within spec. They do call for a beefy power supply and a hefty load resistor to check the 10-amp range, but the mA ranges are easily tested with common resistors and batteries. Rated accuracy is +/- 3% of full scale on the mA ranges, +/- 5% on the 10-A range. If the readings are off more than that, there may be a component in the wrong place on the board or a resistor that is out of spec. Another possibility is your test battery is dead or the load resistor is wrong, so the meter is telling the truth but the current is not what you think it is.

BTW, don't ever test an ammeter or milliammeter by connecting it directly across a power supply or a battery. It's a bad practice that can burn out a shunt, usually followed by immediate damage to the meter movement. Always put a suitable load of some sort (e.g. resistor) in series with the meter, and if possible bring the current up slowly.

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Sun 04, 2018 6:30 am 
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No, it's a digital handheld multimeter.

As an update, it looks like it's good to go. I was able to verify that my other meter is spot on for the ohms scale using a couple 1% resistors. I then used a regulated voltage source with a known value and a wirewound resistor, which I was able to see measure it's value. With that, I knew what my current reading should be, and the new meter is right on.

The only scale I can't seem to get to work right is the capacitor scale. It has an adjustment, but I tried three caps in various values, and when you adjust for one, the others are way off. But I already have a capacitor meter, so I'm not really going to invest much time trying to figure this one out. I did verify the components were installed correctly. But beyond that, maybe if I'm realllllllllly bored one day, and have no projects on the shelf, I might give that part another go.

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Sun 04, 2018 9:44 pm 
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Agree with Jthoruson, you can use the lowest scale of a good VOM or lab current meter to calibrate your DVM.

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Sun 04, 2018 11:00 pm 
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Greetings to Mike and the Forum:

Mike writes:
Quote:
Agree with Jthoruson, you can use the lowest scale of a good VOM or lab current meter to calibrate your DVM.


Yeah.... but that's not what I said. You can certainly do it that way, but you are dependent on the accuracy of the VOM or other current meter, which becomes your standard.

I suggested that you use an accurate DVM to measure the VOLTAGE developed across the internal shunt inside the DVM under test. If you know the value of that shunt, you can then calculate the voltage across it for any given current through the meter. From this, you can determine the accuracy of the DVM under test. Note that accurate voltage standards are easier to come by than accurate current standards. Therefore, the method I suggest (which is exactly what the DVM does - it computes a current from the voltage it measures across its internal shunt) will give you a value that is independent of actual current measurement. It only requires that you know the value of the shunt in the DVM under test with good accuracy. If the DVM is new, then we assume that the value for the shunt given in the schematic is accurate (to within the specified percentage) and then we can measure the voltage developed across it to calculate the current through it. We are also subject to error in the voltage measurement, but it is assumed that the standard voltmeter has been checked against an accurate voltage standard and that its error is known and can be allowed for.

Note that one must measure the voltage at the jacks of the DVM under test.... or better yet, internally across the shunt resistor. Measuring at the end of any test leads connecting the DVM under test will introduce errors due to the resistance of the test leads and jack connections.

Regards,

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Sun 04, 2018 11:55 pm 
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Location: Minneapolis, MN USA
Michael,
Your solution is good.
Note that you can also use a common 3 pin voltage regulator and put it in constant current mode.
It will do the job too.

http://www.bristolwatch.com/ccs/LM317.htm


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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Mon 05, 2018 2:42 am 
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Precision LM4040 Voltage Reference Breakout - 2.048V and 4.096V

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https://www.adafruit.com/product/2200

about... $10.00 ebay
Greg.


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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Mon 05, 2018 3:22 am 
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Don't set your sights too high.

The best you can hope for with a decent multimeter is 3% accuracy.
That's due to the calibration tolerance of the panel meter movement itself.

- Leigh

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 Post subject: Re: Where to get multimeter calibrated
PostPosted: Mar Mon 05, 2018 6:03 pm 
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Son of a gun. I downloaded info on the Elenco 1250 over the weekend. I have the 1150, 2nd hand with obviously a lot of use in it's history. Decent for what it is; not the nice Simpson 260s that I used as an ET in the Navy and as an I&C tech at a nuclear power plant afterwards.

The added features I like are the DC voltage null and the Hfe sockets. The 1150 needs home-built test leads for Hfe.

Ted, KX4OM


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