Test gear accuracy - Page 1
Forum section: electronics / general

author: grumpydoc
author_rank: Super Contributor
timestamp: February 17, 2012, 12:00:17 pm
content: Just spotted the entry about the DMMCheck voltage reference, I'd seen these a while ago and would probably have bought one if it weren't for postage from the US.But it got me thinking - in the early 80's I felt fairly happy with an analogue meter which was 1% accurate at most (and that was 1% of full scale, not of reading) - something like a Fluke 17is only specified as 1% and most 'scopes are only about 1%.So - how accurate do youneedyour meter to be and, more importantly,why?: Edit: - though I believe this isn't considered a "real" Fluke.

author: amspire
author_rank: Super Contributor
timestamp: February 17, 2012, 12:43:43 pm
content: I managed with analog meters with about 3% accuracy pretty well but I do remember how painful it was to avoid parallax error, particularly with no mirror scale. Taking a large number of "accurate" readings with an analog meter was totally tedious, so I never regretted the coming of the digital meter.I like analog moving coil meters on power supplies. I find them accurate enough (most of the time) and more responsive then digital meters.I do remember with horror the first year Physics lab classes at University in the 70's, with the gear that looked like it was almost brand new in 1905. Even trying to prove ohms law ended up with a graph with the points scattered well off the ideal resistance plot line, and we then had to spend half an hour writing up a full error analysis to account for the variation.So I now love instruments accurate enough for calculations to just work. 0.1% is good. If it is a digital meter, I want better then 1% accuracy for DC volts and resistance at least.With scopes, most of the time I am looking at the wave shape, so I hardly ever mind the vertical accuracy of a few percent.Richard.

author: Rerouter
author_rank: Super Contributor
timestamp: February 17, 2012, 12:56:56 pm
content: for my job, i mainly have to deal with frequency, 0.1-4Khz range, standard automotive speed pulses, sadly i have no idea what accuracy the meter we trust is, but the accuracy allowed would be basically 0 to +7% of reading allowable without too hassle,Though on the flip side dealing with voltages, currents and pressures for digital instruments, aim for 100ppm or 0.5 count, whichever comes first, doesn't get much broader than that i would like to think :/

author: robrenz
author_rank: Super Contributor
timestamp: February 17, 2012, 01:29:41 pm
content: I am building some decade boxes to sell with 0.1% accuracy resistors. They will go down to 0.1 Ohm. So I buy resistors assuming they will be in tolerance, Wrong! If I didn't have a Fluke 6.5 digit meter at .0024% which is just barely capable of measuring that accurate at .1 Ohm I would have been selling out of spec junk. This is my post about out of spec resistors I recieved.https://www.eevblog.com/forum/projects-designs-and-technical-stuff/out-of-spec-resistors/msg68160/#msg68160Dave also had a similar situation with out of spec resistors.But with all that said, It depends on your situation and what you need for your expected uses. If all you are going to do is check continuity of fuses out of circuit and measure battery votages a $6.00 harbor freight meter will do just fine.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 17, 2012, 01:47:36 pm
content: QuoteI was thinking yupical "hobbyist"/simple repair type applications. My suspicion is that 0.5% would be fine e.g ATX PSU specs are mostly +/- 5% so if you're checking voltages in a PC 1% or 0.5% is good enough.Of course one of the big differences between digital and analogue multimeters is that digital has a much greater resolution - which means digital meters do need to be more accurate if the last several digits aren't to be meaningless. That said 0.1 to 0.5% is reasonable for a 2000-count meter (mostly the last digit which is unreliable) so I expect that's one of the reasons most are in that range.If yu need greater then you should probably have your meter regularly and traceably calibrated against a properly maintained standard.

author: Rufus
author_rank: Super Contributor
timestamp: February 17, 2012, 01:55:33 pm
content: Quote from: grumpydoc on February 17, 2012, 12:00:17 pmDepends entirely on what you need to do with the meter. Generally you need an accurate meter to verify equipment which is also in some way accurate.For example I have a display for a 4..20mA temperature sensor, I claim +/- 1% accuracy for the system, 0.5% accuracy for the sensor leaves 0.5% for the display and I need a meter with say 0.1% accuracy and verify the display is apparently within 0.4%. A 0.5% meter couldn't do it, a 0.4% meter means all the displays would have to apparently be 0.1% which mostly they are not.If you are just testing/fault finding 20% accuracy might often be good enough.

author: G7PSK
author_rank: Super Contributor
timestamp: February 17, 2012, 02:33:26 pm
content: You used to be able to get analogue meters with a vernier scale which were probably more accurate than many of the digital meters that are on the market.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: February 17, 2012, 05:48:48 pm
content: Accuracy or precision though? Most of the time I don't care how accurate my meter is, as it's really quite rare that it matters. Most of the time I care that the results are precise - i.e. repeatable. A reasonable degree of accuracy with good precision is a nice balance, but the way that multimeters are specced often means the two are hand in hand.

author: EEVblog
author_rank: Unknown
timestamp: February 18, 2012, 08:48:26 am
content: As other have have, depends entirely on what you are doing.For general use, 0.5% to 1% is just about good enough, but you'd likely probably be a bit miffed if your meter wasreally1% out on all rangesThat's the standard 2000-6000 count meter class, and general accuracy basically has to do with the counts.The next class of meter jumps to 0.1%-0.05% class for 20000 count +, and I for one feel more comfortable using one of those knowing that it's maybe an order of magnitude better than what I might typically be measuring.And you generally don't see many meters outside those basic DC spec ranges.Dave.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 18, 2012, 09:45:20 am
content: There are very few applications in electronics that actually need good accuracy. By far the most common is building other test gear!The only others I can think of are charging lithium batteries (1% cutoff voltage tolerance) and frequency measurement for real-time clocks and radio equipment.

author: SeanB
author_rank: Super Contributor
timestamp: February 19, 2012, 05:28:42 pm
content: I did build a 10.24V calibrator for my own multimeters, from an article in Silicon Chip. Could not get the Ad device they specced locally easily, but RS did have some AD591K with a similar accuracy. Built it in 2 boxes with a cable connecting them, one being a recycled wall wart housing a 15VAC transformer ( 2 secondaries of 6VAC and 9VAC from a dead DECT phone), a bridge, 2200uF/35V of capacitor. a 7815 and a pair of 0.1uF ceramics along with a green power LED. Reused the flat phone cable to power the other box, containing the reference on a small heatsink, a few decoupling caps and a set of banana sockets for the output. Heatsink is not needed but it was around anyway, so improves thermal response and effects of small loads. On initial switch and after 24 hours of warm up on was read as 10.24V by most of the DMM's I have on hand, so only those that were not reading the same were adjusted ( funny enough the older ones....... ) to match. At least allows me to check one voltage range, for resistance I have a 6 decade resistance bridge, which is sensitive enough to measure sheet resistance of copper. Bridge is hard to use, very sensitive to both movement ( moving coil balance indicator) and lead movement ( attach direct to screw down terminals).

author: wkb
author_rank: Super Contributor
timestamp: February 19, 2012, 05:52:58 pm
content: Quote from: mikeselectricstuff on February 18, 2012, 09:45:20 amHence your acquisition of the Rb-oscillator(Mine is working again btw)

author: saturation
author_rank: Super Contributor
timestamp: February 20, 2012, 04:46:42 pm
content: Absolutely accuracy is also needed when you work with natural phenomena: electrochemistry [ most people experience this with batteries ], piezo, pyro and thermoelectric effects, biological measurements etc.,. For resolution, it depends on the nature of the media. Reactions in fixed concentrations of material are often characteristic.For example, nerve cells are ~ 70.0 mV between in the inner and outer membrane, with minor variation for different specimens and conditions.For 1.25V NiMH batteries, the lower threshold for full discharge is 0.80V, below which increasingly damages the cell.

author: IanB
author_rank: Super Contributor
timestamp: February 20, 2012, 08:38:05 pm
content: Quote from: saturation on February 20, 2012, 04:46:42 pmMaybe that depends. For some modern cells like the eneloop, nobody has found any signs of deterioration after discharging a cell down to 0 V and shorting the terminals for a few days. You charge the cell back up again and it performs just the same as before. In this respect they seem to be just like NiCd cells.Of course they still don't like reverse polarization. Discharging a series pack down to zero rather than a single cell is a very bad idea.