Cheap DMM Accuracy Check with 0.01% Resistors - Page 1
Forum section: electronics / testgear

author: quantumvolt
author_rank: Frequent Contributor
timestamp: May 24, 2013, 03:14:37 am
content: Before I got a 6 1/2 digit DMM, I had 2-3 cheap 3 1/2 digit DMM's. So I ordered a 10 Volt cheap ref and these resistorshttp://www.ebay.co.uk/itm/Lot-of-5-536R00-Vishay-RS92N-Series-High-Precision-Foil-Resistor-0-01-536-/110982086772?pt=LH_DefaultDomain_0&hash=item19d70c4074. They are less than USD 20 for 5 pieces including shipping. The voltage ref has not arrived yet, but I got the resistors today.They are nominally 536R00 0.01% and measures 536,08 - 536,05 - 536,03 - 536,01 and 536,00 Ohms. The highest reading has a relative error of +0.0149%. Since this is out of spec, I suspect (know) that my meter is giving a too high value (bias error that can be adjusted away with calibration). In this range the meter - HP Agilent 34401A - has spec 0.02%.So given that I live in the jungle in Thailand, the calibration labs I have emailed do not answer and I do not want to go to Singapore where they have a decent (?) HP representative, I guess I can conclude that the meter is not to bad as it is uncalibrated. The measured values give no reason to distrust neither the resistor specs nor the meter.Also - measuring resistance includes both the DMM's ref voltage AND it's internal resistors used for constant current generation. When I get the voltage ref I can check the meter's voltage readings, but I cannot use the voltage ref to check it's current readings without having precision resistors and Ohms law for calculating the current.So, if I am not being circular myself - a handful of precision resistors should make a nice addition to a voltage reference for those of us who for some reason cannot or will not send the instruments away for calibration.Just wanted to share this. If there is nothing wrong with my thinking, others might like to do this. And if you search eBay for "precision resistor 0.01%" you will see that most of the hits are very expensive. BTW - my batch has temperature stability 2.5 ppm - I have checked the datasheet.My next step is to build 2-3 additional 10 Volt ref's from different IC's. The guy who sold me the purchased ref that I am waiting for said that he is willing to measure DIY ref's in NIST traceable newly calibrated equipment for USD 5 a piece. So I will first check my DIY ref's against his, and then send the whole bunch to the US for measurement (small box - just a few dollars in shipping).So what do you think? 3 voltage references based on modern precision voltage IC's and some precision resistors, a second hand quality DMM, a few chinesee DMM's and a few 9V and AA batteries (cells) - at least I will be within +-10%? Joking ...

author: Lightages
author_rank: Unknown
timestamp: May 24, 2013, 03:30:59 am
content: You might want to consider these two businesses for a way to check your meters:http://www.voltagestandard.comA great guy and fast serviceandhttp://www.gellerlabs.com/Voltage%20References.htmalthough they seem to not be building any for sale lately

author: quantumvolt
author_rank: Frequent Contributor
timestamp: May 24, 2013, 03:45:07 am
content: Yes, I just bought a ref from nice guy Doug in link #1 a few days ago. I think he produced, aged and calibrated it after order- and posted it yesterday. I look forward to get it. Thanks

author: mikes
author_rank: Regular Contributor
timestamp: May 24, 2013, 10:37:31 am
content: Are you measuring using Kelvin (4-wire) connections, and are the sense connections close to the body of the resistor?

author: quantumvolt
author_rank: Frequent Contributor
timestamp: May 24, 2013, 11:47:03 am
content: Thanks for your hint - I see what you are sayingI DO use 4W mode BUT1. The crocodile clip set was just put sloppily one set on top of the other so I do not know if the clip on the resistor was sense or currentAND2. I remember having read about measuring point distance on lead, ex.http://www.mouser.com/pdfdocs/VishaySseriesResistors.pdf,http://www.alpha-elec.co.jp/w2img/ww1212135820H19ww1128160635H19eM03.pdf. But I did a quick and dirty measuring session not thinking about it at all.If I get somewhat scientific about it maybe the mean at around 536,04 Ohm would fall 0,01 Ohm or 0,02??? If so I cannot any longer suspect the Agilent for a positive systematic error due to ou- of-adjustment. I will check it after the weekend (children weekend for me). Thanks.

author: nukie
author_rank: Frequent Contributor
timestamp: May 24, 2013, 11:44:53 pm
content: Remember to warm up your meter before taking the readings.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: May 28, 2013, 03:06:27 am
content: I did a new measurement of the Vishay 536R00 0.01% +-2.5 ppm resistors taking into consideration all aspects mentioned over: 4 wire measurement, position of sense leads on resistor wires, thermally stable resistors and DMM, etc.The results were (sorted in increasing order):536R015536R025536R044536R056536R090The last measurement has a positive error relative to R-nominal = 536.00 Ohms of: ((536.090-536)/536)x100%= 0.01679%.I double-checked the Agilent 34401A datasheet and found:Accuracy Specifications ± (% of reading + % of range)0.010 + 0.001 for range 1 kOhm / year.This allows a reading of (536.0 x 1.01/100) + (0.00001x1000) = 536.063(6). Since one of the resistors reads almost double the added error for the resistor (0.01%) and the instrument (0.01%), I can suspect that the resistor has a positive error (but within spec) and that the DMM has a positive (percent of reading) error (MAY BE also within spec).For example lets say that a given resistor has maximum positive error. True value will then be 536 x 1.0001 = 536.0536 Ohms. If also the meter adds 0.01% positive error to this, the resistor will read 535.0536 x 1.0001 = 536.107(2). My worst case read 536.090. , i.e. less than the maximum 0.01% x 2 cumulative error.Anyway - I operate as if the meter has p.t. spec 0.02%. Which is good enough for me

author: robrenz
author_rank: Super Contributor
timestamp: May 28, 2013, 04:16:27 am
content: Since the 34401A does not have offset compensated Ohms. You can get better accuracy in low ohms and 4 wire by averaging two readings with the source current reversed for one of the readings. Just switch the source banana jacks at the meter, you don't want to disturb the connections at the DUT. This will cancel out the Thermal EMF voltage errors. The errors won't be huge at 536 ohms but every little bit helps.This postis of my first current reversal switch adapter.This threadcontains a video of my improved offset compensated Ohms adapter that nulls out the thermal EMF errors. Much better because you get direct readings without having to any external math. Should work on the 34401 also.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 28, 2013, 05:11:46 am
content: Awesome idea

author: quantumvolt
author_rank: Frequent Contributor
timestamp: May 28, 2013, 06:05:37 am
content: Thanks. I have measured both ways already - I have made a small box with a quality relay that change the polarity of the current leads and also shorts the sense leads when there is no resistor under testing (I am tired of listening to the internal DMM relay going on-off-on-off all the time). With shorted sense leads the meter reads 0.0054 Ohm.As you say, for resistors in this range the effect is indeed very small - in fact so small that the meter can hardly detect it . Just for fun I found the worst cables that I ever use - rainbow-colored data flat cable terminated by twisting them around a chopstick and putting it into the DMM. Twisting the other side of the cables around the resistor leg and putting in a cheap DPDT toggle switch in the middle of the current leads length; the meter produced measurements (for one and the same resistor ) of 536, 085 and 536.083 Ohms repeatedly. So in this range - with a measuring current of of 1 milliAmpere - the EMF of a few microVolts doesn't interfere much with the 0.536 Volt over the resistor.So in order to learn more about my meter Iwill either have to calibrate it or get DUT's of higher nominal precision. But I am not that obsessed by (in)significant digits