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Meter Suggestions for Basic Alignment/Calibration
Hi,
Having just blown up (Schoolboy error) my only Meter capable of measuring down to 0.1uA, I thought I would put the Call out for any recommendations as to a suitable replacement Meter for basic Alignment and Calibration. My main interest is with repairing AVO Meters, so I need to be able to measure 37.5uA reasonably accurately. Most of the better Meters I've seen so far are 1% +- 2Digits. This means with a resolution of 0.1uA I could be more than 0.5uA out, or have I missed something? That is not really accurate enough for the Job! Does anybody have any recommendations for a Meter that doesn't need a Mortgage to buy? And if so, where can I get one? Thanks Ian |
Re: Meter Suggestions for Basic Alignment/Calibration
I will have a look at my own meters to check- but i don't think i have anything that goes that far down with good enough resolution for your purpose.
I have only been messing with avometers for maybe 3 yrs, and thus far have never actually measured the current consumption of any of the movements. My typical check is to approach the meter (say, an Avo 8) with a sensitive ohmmeter, and check the resistance between the terminals on the 10v DC range. If i get the desired result of around 200kohms, i then energise the avo with an accurate reference voltage of 10 volts DC, and if i get full scale deflection- job done. If there is a discrepancy then the magnetic shunt gets adjusted. This then tells me the current consumption is correct. I realise i haven't answered your question there, but you see what i'm driving at. (more than one way to get a result whilst you track down a new microammeter) Sooner or later i will need to measure the current consumption of one of my rarer meters, as there is no data trail to suggest which movement was fitted to it in the factory- and if it's unique to this model i will need to treat it carefully! Dave |
Re: Meter Suggestions for Basic Alignment/Calibration
Use another Avo, or something like a Fluke 87 or a bench top Agilent.
If your resolution is 0.1 then you won't be 0.5 out, assuming that's not a typo. |
Re: Meter Suggestions for Basic Alignment/Calibration
I have just looked at my cheapie DT830B and it actually has a 200uA range with resolution of 100nA!
Claimed accuracy +-1.0% (& 2 digits) Actual accuracy- who knows!? Dave |
Re: Meter Suggestions for Basic Alignment/Calibration
What meter where you using before?
Choice will depend upon what is more important, accuracy or precission! If you want accuracy and precission to 0.1uA you are into 4.5 digit UKAL calibrated meters, which will set you back several hundred pounds. I doubt this is what you are looking for though! I have several 4.5 digit bench multimeters which will give you the precision to 0.1uA but not the accuracy of a calibration to 0.1uA. Ask yourself how critical is your need to measure 37.5uA. That will dictate the standard of meter you will require. |
Re: Meter Suggestions for Basic Alignment/Calibration
For a tenner you can get a quite-decent digital meter from Screwfix/B&Q.
To be brutally honest, it'll probably be more accurate than you can repeatably read a typical analog meter to. Fed up with carting big bulky AVOs around with me, these days I use my cheapie orange-cased Screwfix meter - at least if I blow it up or lose it I've only lost a tenner, and can get a replacement next-day. Only time I use an analog meter is when I'm 'peaking' RF circuits, because for that it's a lot easier/quicker to watch a pointer move than it is to count digits. |
Re: Meter Suggestions for Basic Alignment/Calibration
Quote:
In your example 1% +/-2 digits, presumably on a 3.5 digit (1999 counts) dmm on V DC range. The 1% refers to the accuracy of the measured value, so if you were indicating 100V on the meter then the accuracy would be 100V +/- 1V, so displaying 100V you could have 99V to 101V at the probes. The +/-2 digits is the tolerance of the last significant displayed digit, so its value will depend upon the number of digits on the meter. So for a 3.5 digit meter the display for 100V input on the 200V range would be 100.0V. The least significant digit is 10ths of a volt (100mV) so +/- 2 digits would give an additional error of +/- 0.2V. The total tolerance will then be 1V + 0.2V or +/- 1.2V So for a displayed value of 100.0V the final accuracy would be from 98.8V to 101.2V. If you have a 3.5 digit meter on the 200uA range, measuring 37.5uA, then:- 1% of 37.5uA = +/- 0.37uA 2 counts = +/- 0.2uA Total tolerance will be +/- 0.57uA (just as you thought ;D) Therefore accuracy when displaying 37.5uA is from 36.9uA to 38.07uA That would be for a quality meter that has been calibrated and that meets its claimed accuracy, such as HP, Fluke, Keithley etc. 1% accuracy is pretty poor, and some of my vintage 4.5 digit DMM's claim "+/-0.1% +4 counts" (4 counts on 4.5 digit meter is 10x better than 4 counts on 3.5 digit meter) For example a fluke 8600 DMM (around £40) claims for DC current +/- 0.1% input reading + 0.01% of range (which is 2 counts on a 4.5 digit meter (19999 counts)!) (for voltage it is +/- 0.02V of input + 0.005% of range, which is not bad for a 30 year old design :o) so 37.5uA gives an accuracy of +/- 0.037uA + 0.02uA = +/- 0.039uA or 37.44uA to 37.55uA. Significantly better than the 1% 3.5 digit meter. However all this assumes the meter is calibrated to this level! However if the meter is not calibrated, you can still use it as a "transfare reference" by measuring a known good value on a good AVO, and then adjusting your target AVO's voltage to exactly the same displayed value due to "Precision". |
Re: Meter Suggestions for Basic Alignment/Calibration
Dave,
That's an interesting alternative to look at. Although, I don't have any reference Voltages, having always relied on a Meter instead to provide references when I needed them. Sinewave, I have a Fluke 89, but it only has a 1uA Resolution. Not a lot of use for AVO 8/9 Meter Movements. Ok for the older AVO's though. Actually, worse case senario with a stated Accuracy of 1% Measured +-2 Digits, with a 0.1uA resolution, you could be up to +- 0.575uA out. That's 1% (at 37.5uA) = 0.375uA + the Two Digits which is +- 0.2uA Totel = 0.575uA Going forward, I don't really want to be setting up my Meter Movements with that level of uncertainty. I wonder what equipment AVO used back in the Day? Peter, The 37.5uA is pretty Critical, and I would like to achieve an Accuracy of +-0.1uA if at all possible within my price Range. Bearing in Mind, this is for setting up accurate (Relatively speaking today) Test Equipment. It would also allow me to re-check some AVO's which have been set up with my old Meter, whos accuracy was, well probably uncertain. One option I was thinking of, would be a 0.1uA Meter and a Calibration Certificate, but what exactly does a Calibration Certificate tell you? If it only tells you the Meter is within Spec, it doesn't really tell you anything. and finally, G6Tanuki, :-) I think I would trust most of my old AVO 8/9's over any Screwfix offering! :-) Seriously though, the point here is the restoration to fully working order of Vintage Equipment. Very few people actually use AVO meters in anger these Days, although I try to use them where possible. I do occasionally lug a Model 9sx out to the Car for a bit of Fun. Ian |
Re: Meter Suggestions for Basic Alignment/Calibration
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It can start at a basic reference check on set points in each range, and a "look up" table to show the errors at these points. It could be a complete check and re-adjustment to bring into manufacturers specifications which would give fixed degrees of certainty. If your calibration said it was fully working to manufacturers specification then you can calculate the uncertainty (as you have done, and I did above) and give you the range that the actual voltage/current will lie between. The better the meter (both accuracy and precision) then the closer your certainty of measurement will be. So you need to find a meter that gives the resolution and accuracy to say +/- 0.1uA It would therefore need an accuracy and precision about 10X better than this to give you certainty that the indicated value is +/-0.05uA So you are looking for a 5.5 digit (minimum) dmm to full manufacturers specification. Therefore you are talking about new HP, Fluke quality meters in the £1000+ price range, and a re-calibration how often??? When you get to this level of accuracy and precision, then you need to start considering other issues such as lead resistance, noise, and temperature! So as I was suggesting in my original post you need to understand how much accuracy and what level of certainty you want, compared to how much you want to spend. If you want to precisely and accurately measure 37.5uA to +/-0.1uA then its going to cost! |
Re: Meter Suggestions for Basic Alignment/Calibration
Voltage references are quite cheap, i got one which supposedly came from china (the parcel label indicates it came VIA vietnam) It came with an accuracy certificate (to 3 decimal places) based on tests using an Agilent (The certificate indicated the Agilent hadn't been calibrated for 18 months, but then again what do you calibrate an Agilent with!?)
Anyhew, it only cost about £5, and gives 2.5v 5v 7.5v and 10vDC and can supply enough current to avoid the reference warming up too much even on an early AVO. (Particularly if the test is brief) The only downsides- it came without 100% adequate packaging and only survived due to luck....& it is a naked circuit board which needs setting into a project box. Voltage references from that part of the world tend to quote 23 or 25C as their best ambient, not too far off the 20C that i'm accustomed to seeing. Immensely satisfying to get an 8/9 avo as good as it was out of the factory. With the wartime military models you sometimes end up with it better than it left the factory. |
Re: Meter Suggestions for Basic Alignment/Calibration
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DC current accuracy is +-0.2% (+2 digits) with a resolution of 0.01uA. Your 1% is 0.000375uA, not 0.375uA, which alone is probably more accurate than our eyes can discriminate against, let alone the better accuracy of the 87V. |
Re: Meter Suggestions for Basic Alignment/Calibration
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What is 1% of 37.5uA? How do you get 0.000375uA? Also Fluke 87V is a 4.5 digit (19999 counts) quality DMM at a cost of £400 Accuracy will be +/- 0.075uA (0.2%) + 0.02uA (2 digits) = +/- 0.095uA, or near enough +/-0.1uA Resolution is not the sama as accuracy! I am sure a new Fluke 87V will meet the OP's specified requirements at only £400 plus a re-callibration every few months 8-\ |
Re: Meter Suggestions for Basic Alignment/Calibration
0.0375x1/100 = 0.000375.
I don't think it would need calibration every few months. Once a year at the most is more than enough for us with our Avos. |
Re: Meter Suggestions for Basic Alignment/Calibration
Hallo Jan,
Your 89 has a basic DC-voltage accuracy of 0,025%_that is really fine. All what you need is an precisions resistance of i.e. 1Kohm and you have to measure a voltage drop over that if it is serial connected with your (AVO) movement... An 0,01% resistor costs £ 5-20 typically, but for you-I think- is a precision of only 0,1% enough god too and their prices of £ 0,1--1,0 are really moderate :) Wish success, Karl |
Re: Meter Suggestions for Basic Alignment/Calibration
Calibration is an interesting minefield.
A general certificate of calibration simply means that an instrument was checked on a specified date and was found to comply with its manufacturer's specifications. Thereafter, several things go wrong: 1) The couriers bounce the thing down several flight of stairs on its way home from the cal lab. Maybe this affects the accuracy, maybe it doesn't. How lucky do you feel? 2) Once home, most users see the cal certificate and then treat the instrument as gospel, ignoring or unaware of the allowable amounts of error specified by the manufacturer. 3) Most users ignore or are unaware of the specified uncertainties of the cal lab they used. 4) Calibration certificates, like MOT certificates relate only to the condition at the time of the test. What if the couriers chucked it out of one van/plane too many? Everything you adjust using it from then on will be wrong. You may only find out at the next calibration time when you get notified that it failed. Calibration is BACKWARD LOOKING. You have to decide if you need to get things back from the field to correct them. Unfortunately, most people see a cal certificate as a prophecy that the instrument can be trusted for the next year. What it does do is justify what you did with it in the PREVIOUS year. Here is an application just ready for someone with a time machine, but they may not be interested in things less lucrative than cleaning up on the football pools, the stock market etc. Once you start digging into calibration and accuracy it comes as a surprise how nasty it gets. Individual correction tables can be useful, but they transfer the demand for accuracy into a need for repeatability and stability. I was involved in the HP/Agilent noise sources used for measuring noise figure. Each noise source has an individual sheet of Excess Noise Ratio and impedance figures for frequencies at 1GHz intervals across its range. Each of these figures has an associated value for uncertainty. They are all different because calibration at different frequencies may trace back to different thermal standard hardware, and different labs may trace back to different hardware sets as well. As time goes by, national standards labs revise the mean and uncertainty figures for their thermal standards hardware and uncertainty figures change from year to year. So, if you measure noise figure, the noise source you need as a standard doesn't have one global accuracy figure, each frequency point in its cal chart has a different uncertainty and the effect on noise fig results is very significant. Ian White and myself gave a presentation on errors in noise fig measurements at the EMC 2012 conference (Churchill College, Cambridge) and it caused a little bit of a stir. Maybe it's like what they say about sausages and laws; you really don't want to see how they are made? Avo probably had a Weston cell and a potentiometer/galvanometer as their root standard. Production instruments would have seen a transfer standard instrument checked against it. Perhaps they had several cells to compare against each other as a safety check. David |
Re: Meter Suggestions for Basic Alignment/Calibration
Guys,
Thanks for all your inputs. It's always interesting to get somebody else's take on things. I had looked at the Fluke 87V, but due to complete carelessness on my part, I didn't read the Spec properly! Didn't realize the DC Current accuracy was that high. So a Fluke 87V looks like the front runner, unless anyone has other suggestions? (Thanks for the recommendation Sinewave). Expensive, but you have to have at least one bit of good Equipment you can hopefully rely on. Ian |
Re: Meter Suggestions for Basic Alignment/Calibration
Quote:
I gather from this remark that what you have is some circuit where you want the current to be exactly (or very close) 37.5uA when its properly calibrated. One thing that could help is to be able to generate 37.5uA to make your own calibration standard. This could be done easily with a precision 10V reference, many accurate to 0.003 % and some precision resistors 0.01% or better. Then your meter just requires resolution only and its calibration accuracy is not as important. Also if you have the reference you can be confident your work is accurate and not rely on some meter's calibration which could well be off. Also, I'm not sure here what your test configuration is when you are measuring the 37.5uA, but it might be possible to use the 37.5uA current reference directly to perform the calibration task. One way to get around any series resistance in the circuit where the current is introduced ( I assume the meter movement ? and its resistance) is to set up a current mirror with a monolithic pair of matched transistors and use one arm for the generated current and the other arm for the meter movement. If this notion works then you would require no meter at all and it would be a cheap test jig to make. |
Re: Meter Suggestions for Basic Alignment/Calibration
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Ian,
if you have an Fluke 89-why do you need an 87? OK, both are practically same accurate and counts 50000/5000 but a 89 has lot more functions, is a usabler datalogger... If you search for 89 specifications: you can read values of cca 0,15-0,25% or so. In my opinion, if it would be only 0,5%=> far enough precise for a check/calibration of analog movements with readability of maximum 1-2%. Otherweise, as I wrote before; more simple and 5-10 times preciser would be a movement calibration wit DC-voltage drop measuring over good knowed precisions resistor as DC current measuring... rgds, Karl |
Re: Meter Suggestions for Basic Alignment/Calibration
Hi Karl,
My mistake, my Fluke is actually a 79. Only 1uA resolution and is quite Old now. I spoke with Fluke a few minutes ago regarding the 87V. Sinewave mentioned 0.01uA resolution and indeed, on the Fluke Website it also states 0.01uA for the 87V. However, on the Suppliers Website where I originally checked, the Spec says 0.1uA Res. The accuracy is also 0.2% + 4 Digits, not 2! Which I now remember is why I had originally ruled this Device out. I queried this and according to Fluke themselves, the resolution for the 87V is only 0.1uA! So I am pretty much back to Square One with the 87V, with an uncertainty of +-0.475uA Argus25, My main requirement (but not exclusively) is for measuring accurately 37.5uA so that I can Check/Set AVOmeter Analogue Meter Movements correctly. The AVO Meters spec is entirely dependent on the Movement being correct. The Meter Movement is precisely 37.5uA FSD in an ideal world. Now that I have to replace my damaged DMM I thought it was time to get a more accurate Meter. A calibrated current source would be ideal, and that is in the Plan eventually, but I would still need to measure it accurately after I built it! So still need an accurate measuring device. Catch 22! Although it is now starting to lean towards a solution that Karl and Dave have alluded to. Ian |
Re: Meter Suggestions for Basic Alignment/Calibration
1 Attachment(s)
Ian,
be carefull please with readings of accuracies_ their depends on ranges... Karl |
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