Is This Simple Ohms Law Test for Calibration of a Multimeter Useful ???? - Page 1
Forum section: electronics / testgear

author: dimlow
author_rank: Frequent Contributor
timestamp: February 19, 2013, 01:25:42 am
content: I just got thisSolatron 7150 Meterfrom 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.

author: amspire
author_rank: Super Contributor
timestamp: February 19, 2013, 01:50:42 am
content: Quote from: dimlow on February 19, 2013, 01:25:42 amOhm'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.QuoteNo, 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.

author: BravoV
author_rank: Super Contributor
timestamp: February 19, 2013, 02:16:17 am
content: Quote from: amspire on February 19, 2013, 01:50:42 amQuote from: amspire on February 19, 2013, 01:50:42 amAs 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.

author: dimlow
author_rank: Frequent Contributor
timestamp: February 19, 2013, 02:21:16 am
content: QuoteOh, 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

author: amspire
author_rank: Super Contributor
timestamp: February 19, 2013, 03:02:46 am
content: 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.htmThe 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.

author: dimlow
author_rank: Frequent Contributor
timestamp: February 19, 2013, 03:29:05 am
content: it sounds very interesting but is my bedtime right now so I don't have much time to think about it. I had a quick look at conrads page and it looks like something I may be interested in. Right time to sleep. Its now 3.30 am here.

author: dimlow
author_rank: Frequent Contributor
timestamp: February 19, 2013, 05:14:49 pm
content: Ok, i did the simple test twice, once after just powering up the circuit and once after letting it settle for an hour and these are the results. I think i can say they it did work and that the meter can be confirmed as working well. Cant say that it is in calibration, but i bet its not far off.

author: dimlow
author_rank: Frequent Contributor
timestamp: February 21, 2013, 10:14:08 pm
content: I have had the meter calibrated but i have some questions about the validity of the calibration. If you know about these things could you please have a look at my posts herehttps://www.eevblog.com/forum/testgear/solartron-7150-replace-mains-filter-now-and-calibration/msg194472/#msg194472Thanks

author: dimlow
author_rank: Frequent Contributor
timestamp: February 23, 2013, 05:23:09 am
content: Quote from: amspire on February 19, 2013, 03:02:46 am@amspire I have had a good read now about dividers and ratios and think i will give this a try, not now to calibrate the meter but just for the fun of it. Thanks for pointing it out to me. It may be handy to have a Geller voltage reference to. So when i get some more cash i will order one and use that for the source of the divider. Could be an interesting little project for me as I'm more into micro controllers and software rather than analogue. It will be a learning exercise for me.Whilst i was reading about divers and references i came across a fluke page that referenced Conrads work. That was a surprising find. i would have linked it in here but have since closed the page and lost it.EDIT:FOUND IT HEREThe Solartrons Impedance is in fact 10Mohm, when building the Hamon divider could i allow for this by using lower value resistors?

author: amspire
author_rank: Super Contributor
timestamp: February 23, 2013, 07:03:50 am
content: I think you will find the input impedance is extremely high on the 7051 for DC ranges up to 2V. I am pretty sure it is not 10Mohm. So you probably do not need any correction for the low ranges.The other thing is to get a fet or mosfet input autozero opamp and make a voltage follower. You can get opamps with offsets around about 1uV. This way, you do not have to worry about the load.Many of the autozero opamps with the lowest offsets have a maximum supply of 5-6V, but there are still the older FET input ones that use a supply of 15V or more.Also, for the null detection when you are balancing divisors, input resistance does not matter that much. You could use a null detector with a 10K input resistance and it would still work as well, as when there is a balance, zero current flows into the null detector.

author: dimlow
author_rank: Frequent Contributor
timestamp: February 23, 2013, 07:37:40 am
content: I searched for input impedance of the meter in the manual and there was no mention of it. When i googled it all i could find wasthis.Its states the meter has 10E12 so, should be good then