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Is This Simple Ohms Law Test for Calibration of a Multimeter Useful ????

**dimlow**:

I just got this Solatron 7150 Meter from ebay, and i want to see if there are any problems with it. With not having any references to validate the readings ,I came up with the idea of a simple test to see if the meter can at least follow Ohms Law.

What i mean is, if i setup say a simple circuit with regulated 5v, two resistors in series. Measure the Voltage of the regulator, the voltage drop over each resistor, the resistor values,and measured the current, all with the same meter to its best accuracy. Then do the math with the measured results to calculate for example, the current from the measured resistance and measured voltage and compared the calculated current with the measured current to see if i get the same result. Would this tell me anything ? Should i expect to get the same current from the calculated and measured results ?

What i was thinking, was that it would at least tell me if one of the meter ranges was out of calibration, for example the voltage range. What I'm getting at is if the voltage, current and resistance measurement were all in calibration, then Ohms law would work and the measured and calculated values for each of voltage, current and resistance ranges would be the same and i could be certain the meter is working and calibrated. But if say the current range of the meter was faulty the calculations would not match the measurements, then i would know the meter is faulty.

Does this sound reasonable or would i be wasting my time ?

If this idea does work then maybe i could take it further by starting the calculations using different a voltage for each of the voltage ranges in the meter. ie 2v,20,200v range. i would then be able to test if each range was in calibration. I would not be able to calibrate the meter, but at least i might know that a particular voltage range was off.

Umm Have i had to many coffees ? Better stop before you all think I'm mad.

**amspire**:

--- Quote from: dimlow on February 19, 2013, 01:25:42 am ---I just got this Solatron 7150 Meter from ebay, and i want to see if there are any problems with it. With not having any references to validate the readings ,I came up with the idea of a simple test to see if the meter can at least follow Ohms Law.

--- End quote ---

Ohm's Law is exact, so the idea is great. The problem with a meter with a 1ppm resolution is eliminating the errors. This includes 5V drift, resistor drift, DC EMF caused by connectors at different temperatures, lead resistance, etc.

Try this test. Put your best 10K resistor cross 5V and leave it there for 10 minutes. Quickly disconnect the resistor, and connect it to the meter to measure the resistance. See how much the resistance changes over the next 10 minutes. That will give you some idea into the problems of measuring to 1ppm.

Many resistors are 50ppm/C, so simply applying a voltage is enough to make them drift a fair bit.

--- Quote ---What i mean is, if i setup say a simple circuit with regulated 5v, two resistors in series. Measure the Voltage of the regulator, the voltage drop over each resistor, the resistor values,and measured the current, all with the same meter to its best accuracy. Then do the math with the measured results to calculate for example, the current from the measured resistance and measured voltage and compared the calculated current with the measured current to see if i get the same result. Would this tell me anything ? Should i expect to get the same current from the calculated and measured results ?

--- End quote ---

No, you will get different results. When you use the meter to measure the current, there will be a voltage drop across the meter that is significant. This will reduce the current in the resistor.

Your best first step is probably to buy one of the calibration boards from Geller Labs, or the other company (forgotten the name but search for Geller in the forum and you will find links to the alternative). One of the calibration boards available has multiple voltages and resistors measured to four digit resolution.

The meter will probably be more accurate then any cheap reference board, but it is a confidence test.

**BravoV**:

--- Quote from: amspire on February 19, 2013, 01:50:42 am ---Try this test. Put your best 10K resistor cross 5V and leave it there for 10 minutes. Quickly disconnect the resistor, and connect it to the meter to measure the resistance. See how much the resistance changes over the next 10 minutes.

That will give you some idea into the problems of measuring to 1ppm.

--- End quote ---

--- Quote from: amspire on February 19, 2013, 01:50:42 am ---The meter will probably be more accurate then any cheap reference board, but it is a confidence test.

--- End quote ---

As an enthusiast with limited "accurate" references to cross check, let alone doing official cert/cal, even just for confidence test, these above are excellent examples and made me happy with just 4 digits dmm for normal usage.

Sometimes even my HP 5.5 dmm makes me feel like I was going to have a nervous breakdown when seeing the least significant digit keep changing. :-DD

**dimlow**:

--- Quote ---Try this test. Put your best 10K resistor cross 5V and leave it there for 10 minutes. Quickly disconnect the resistor, and connect it to the meter to measure the resistance. See how much the resistance changes over the next 10 minutes. That will give you some idea into the problems of measuring to 1ppm.

Many resistors are 50ppm/C, so simply applying a voltage is enough to make them drift a fair bit.

--- End quote ---

Oh, yes this shocked me when i first got the meter, WTF ! Its drifting ?then i realized the resistor was cooling from when i was holding it. But checked a 1Ohm restistor and got this.

Geller SVN and Voltage Standards DMM Check/plus, yep looked at these, can't believe that postage prices to get one to the UK. But today, i found my son will be starting his Electronics section of his 3rd year Engineering course next month and me thinks the collage has some nice gear there. Maybe, just maybe they have something we can check the meter against.

Ah, i forgot about the voltage drop when measuring current. well i maybe when i get bored i will still give it a try.

thanks for putting me right on that ampire

**amspire**:

One of the things you can do extremely accurately is resistor ratio's.

Conrad, one of the forum regulars has some great metrology ideas on his site, and one of the really neat ideas is the Hamon divider.

http://conradhoffman.com/electronics01.htm

The thing is with a bunch of resistors and anything that can precisely measure 0V (like you 6 digit meter or an opamp circuit adjusted for 0V with a shorted input), you can make an extremely precise divide by 10 resistive divider. It is easy to adjust to better then 1ppm, but whether the resistors stay matching to 1ppm for long, that is another matter. With this divider, you can start to scale voltages (ie you can compare the 0.1V range to the 1V range, and then the 10V range to the 1v range, etc. The divider when calibrated ends up with a very precisely calibrated 10:1 ratio of resistors - and in the process you also get an equally accurate 2:1 divider. You can use the divider in a bridge to do relative calibrations for resistors from milliohms to gigohms if you are precise enough.

The trick with the Hamon divider is to start with at least 10 identical resistors - starting with a pack of 100 is even better as you can select the best matches. The value does not matter much, but around 10K would be good.

In each of the upper resistors of the Hamon divider use 3 resistors per leg, and one resistor down the bottom leg. Choose the resistors so that each of the upper legs is 3 times the resistance of the lower leg to within 0.01% if possible. One leg will have the adjustment pot so it has to have slightly less resistance. Since all the resistors are the same, they will have the same power dissipation and so hopefully when they drift, they will all drift equally and the ratio will stay the same.

If you see that the meter ranges are relatively precise, then it will show if anyone has been messing with the range calibration or not.

If you are using a 10MOhm input resistance meter to measure the divider output, you need to allow for this resistance. A 10MOhm meter will reduce a 10x divider that uses 10K resistors by 0.1%. I do not know about your 6 digit meter, but many high end meters have input impedances well over a gigohm for voltage ranges up to at least 20V and if so, there will be little error measuring the divider output.

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