HP 3458 linearity vs the Fluke 752 and 845 combination? - Page 1
Forum section: electronics / general

author: acts238willy
author_rank: Regular Contributor
timestamp: October 05, 2020, 02:25:07 am
content: I may be missing something fundamental, since I'm not a metrologist.If I wanted to go from 0.1 volt to 1 volt and used the 3458 1V scale foreach end, would it be more accurate than using the 752/845 setup?How about all the way up the values from 0.1V to 1KV, doing a factorof 10 each time?I have some old 3458s... and I have a 752/845 pair. If the 3458sare as linear as the Fluke pair, I'll sell the Flukes.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 05, 2020, 03:04:15 am
content: The Flukes are fundamental and can be checked with more-or-less simple procedures. IMO, the 3458 has all manner of expensive things to go wrong. I'd certainly not sell anything! Not sure which is capable of better accuracy, nor about getting to 1kV, where thermal errors creep in.

author: acts238willy
author_rank: Regular Contributor
timestamp: October 05, 2020, 05:22:07 am
content: The old 4708 Datron I'm using may wander some, too.....The linearity thing was mentioned in a Fluke 8508 manual,which explained that the 8508 was linear enough to replacethe groups of tools that were used in the 1970s calibrationprocedures. That made the 3458 a possibility?

author: 2N3055
author_rank: Super Contributor
timestamp: October 05, 2020, 07:04:46 am
content: Please don't take this the wrong way but most of your errors will be from your technique, cabling, fact that you don't have controlled temp/humidity lab etc...Even on 6.5 digit meter, cabling thermal errors are clearly visible for instance...3458 in good shape is very linear, but like Conrad say (and he knows what he's talking about), old school Fluke method is made to be performed manually and is made to be able for you to verify results.To be honest, you should do BOTH, and if they agree to some extent, than you might have good measurement....That is in Voltnut world. In real world, both will be more than enough.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 05, 2020, 08:23:43 am
content: Hello,in your headline you write about linearity and 752 vs. 3458A.. that's the wrong comparison parameter.The 3458A is the most linear analogue comparator, that allows to make best transfers within an order of magnitude, e.g. from 10V reference down to 9, 8, 7, ..., 1V. Only the quantum based JJ array is better.Therefore concerning linearity you have to compare the 3458A with the Fluke 720A KV divider, where we have to compare < 0.02ppm vs. 0.1ppm INL.Transfer uncertainty is another aspect, where additionally other parameters come into play, what Conrad and 2N3055 have correctly suggested already.You obviously mean 10:1 and 100:1 transfers, when comparing 3458A and 752A.The 752A beats the 3458A in two aspects.At first, the 0.02ppm INL of the 3458A computes into 0.2ppm @ 1/10 F.S., plus other sources of uncertainty makes about 0.3ppm 10:1 transfer (see hpj 4/89)The 100:1 transfer uncertainty then accumulates to about 0.6.. 0.7ppm. But these are theoretical limits only, maybe only accessible with certain tricks.But if you look into the official specification of the 3458A, the range calibration is not better than 1.8ppm for 10:1, and not better than 2.5ppm for 100:1, where you only might substract a 0.5ppm reference uncertainty. The official Transfer accuracy of (0.05 + 0.05)ppm will yield 0.55ppm for e.g. a 10V => 1V transfer, and 0.55 + 3.1ppm down to 100mV.The 752A in contrast is officially specified 0.2 and 0.5ppm of output, i.e. 3 times and 7 times better, for this example.The 2nd aspect is the High Voltage case, where the 752A by design is far superior.These 0.5ppm for a 1kV => 10V transfer (or measurement at these levels) include its self heating effects, whereas the 3458A has up to 12ppm additional error.All other DVM and also calibrators like the 5440B or 57x0A show this effect, but mitigate it by calibration of this self heating effect, or by heating the divider.Therefore, please keep both instruments under all circumstances, especially if you ever want to measure/calibrate at voltages between 100V and 1kV.One last remark, Conrad stated that the Fluke 752A is a fundamental (standard) and can be checked with .. simple procedures.The first statement is true, i.e the 752A is a primary transfer standard 'by design', see elaborate error consideration in the 752A manual. All its uncertainty is derived from basic principles, and specification upper limits are derived from calculations only.Therefore, if e.g. the switch resistance deteriorates over the years (it's compensated in a tricky manner inside the 752A) then you will not notice that.. This error source and its mitigation is not explicitly mentioned in the manual, though.The 2nd statement is not that trivial.. as there really is no simple method to check (calibrate) its uncertainty.You would need an intrinsically better transfer standard for that.Only the JJ array can be used for that, for 10V or less.For High Voltages, there is no such quantum standard available, so you need to design a different divider, as well estimate its sources of error, and compare both.You also might check the self heating effect by dynamic methods.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: October 05, 2020, 10:12:55 am
content: The very good linearity specs for the 3458 are valid for the 10 V range only. Already the 1 V range can add some extra thermal error from the gain setting divider. The extra error can be quite large in the 1000 V range, but the lower ranges are also not as good as the 10 V range. Also don't forget thermal EMF / offsets in the 100 mV range.The Fluke 752 errors are also expected to be different for different voltage ranges: at high voltage there is also self heating and at low voltages thermal EMF can add errors that are less important at high voltage. Using the 752 also needs more care - a sloppy operator can make things worse.

author: notfaded1
author_rank: Frequent Contributor
timestamp: October 05, 2020, 05:00:24 pm
content: Question. How the 752A compare to the 720A KVD? If I have a good working 720A do I really need a 752A?

author: acts238willy
author_rank: Regular Contributor
timestamp: October 05, 2020, 08:43:15 pm
content: Also.... It took 3 tries before I finally got a 752 that could bejumpered enough to get the 10:1 and 100:1 ranges to calibrate.And 2 more to get another one that could be jumpered intousefulness. So I now have two 752/845 pairs that work!So it looks like the use of standards like the 732B and the 752are the most accurate way to check the individual 3458 voltageranges, since I'll never be able to afford to buy a Fluke 8508..

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 05, 2020, 11:19:41 pm
content: Quote from: notfaded1 on October 05, 2020, 05:00:24 pmLast question answered: Definitely yes!First question:The 720A is 0.1ppm INL of input, the 752A is 0.2 / 0.5ppm of output.That means, that a 10:1 transfer for the 720A is uncertain to 1ppm, compared to 0.2ppm for the 752A.A direct 100:1 transfer (e.g. needed for 1000V => 10V comparison) would be 10ppm for the 720A, but only 0.5ppm for the 752A.For 1000V the 720A consumes fat 10W, whereas the 752A consumes 250mW in its divider only.The self heating effect in the 720A adds 0.2ppm to the output uncertainty, which is already included in the 0.5ppm for the 752A.Frank

author: acts238willy
author_rank: Regular Contributor
timestamp: October 06, 2020, 04:52:36 am
content: So..... Is Fluke really right by claiming that the 8508 can replace the oldbut very good 1960s and 1970s gear? As was said earlier, you have to bevery careful in your wiring, watch for thermals, etc., to get the good resultswith the vintage gear.... which has a lot of items and wiring involved. Theinstructions with the old gear are very good but you have to use the latestupdates for the 752, for example.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 06, 2020, 01:56:36 pm
content: @acts238willy I'm glad I didn't know then what I know from your posts. When I got my 752 I had to re-jumper it to get it within calibration range. Made me nervous, but it was OK. They seem to have a lot of variation and I wonder how many are out there that need new ($$$) sealed resistors?

author: acts238willy
author_rank: Regular Contributor
timestamp: October 06, 2020, 03:05:59 pm
content: The 752 is half a century old and the components can show it.The manual has the jumper settings for error correction.Finding how far out of being able to balance, using the knobs,is your first clue on how much correction will be needed fromthe internal jumpers. Then you need to access the inside ofthe 752 to see what the jumpers are actually set at. The tableof jumper settings will tell you if you will be able to go fromthe settings you have now to actually make the unit balancewhen you're done.All of the fleabay units failed to balance when I got them.The two I have now were at the factory settings when I openedthem up and I was able to get the 10-turn calibrate knobsto be roughly centered by changing jumpers.The other failing 752s had already been jumpered to the maxand weren't fixable.

author: e61_phil
author_rank: Frequent Contributor
timestamp: October 09, 2020, 02:53:22 pm
content: Quote from: Dr. Frank on October 05, 2020, 08:23:43 amIn the Fluke calibration book they described briefly how they checked the transfer specifications of the 752. They used a chain of 732As. No JJA.

author: Bill158
author_rank: Regular Contributor
timestamp: October 09, 2020, 09:11:41 pm
content: In the Fluke calibration book they described briefly how they checked the transfer specifications of the 752. They used a chain of 732As. No JJA.For those of you who are interested, as I was, see"CALIBRATION: Philosophy in Practice, Second Edition", pg 9-9, Other Dividers, The Ring Reference Divider, 2nd paragraph, right hand side of page.It took me a while to find this as it wasn't located where I would have expected this to be.Bill

author: TiN
author_rank: Super Contributor
timestamp: October 10, 2020, 02:58:37 am
content: Quite obvious even from specification that no current DMM can replace 752A for high-voltage ratios. Fluke shows that themselvesin fancy 8588A promo papers.

author: acts238willy
author_rank: Regular Contributor
timestamp: October 11, 2020, 08:43:03 pm
content: That's the answer I was looking for! THANKS!!!Now.... is there a modern replacement for the 845?They're working now but they're old....

author: guenthert
author_rank: Frequent Contributor
timestamp: October 12, 2020, 04:10:54 pm
content: Quote from: acts238willy on October 11, 2020, 08:43:03 pmThat depends on the intended use of course. For low resistance sources, one might be better of with a nano voltmeter (they have high impedance inputs too, but also a 'pump out' current measuring in the order of tens of picoamps on average, peaks might be nanoamps).To adjust resistance bridges, one might prefer an analogue read-out, but many DMMs have an analogue output to which a MCM can be attached.The self-calibration procedure of the 752 calls for a null meter with which one can determine that the remaining error voltage is less than 0.5uV iirc. This can (barely) be achieved with a HP419A (as they are specified, individual units might perform better, if they still work after all those decades). The handbook calls (of course) for a (slightly less noisy) Fluke 845A. There have been null meters made in this millennium, but not many (they are obsolete for most purposes), the AVM 2000 is one.

author: TiN
author_rank: Super Contributor
timestamp: October 12, 2020, 11:27:31 pm
content: Keithley 155 is much better than both 419 and 845.

author: acts238willy
author_rank: Regular Contributor
timestamp: October 13, 2020, 06:13:09 am
content: It looks like the 155 and the 845 both have the same ranges?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 14, 2020, 01:49:04 pm
content: Quote from: acts238willy on October 11, 2020, 08:43:03 pmhttp://www.emelectronics.co.uk/low-level-voltage-measuring-instruments/p13/

author: Kleinstein
author_rank: Super Contributor
timestamp: October 14, 2020, 03:35:09 pm
content: The difficult part with the null-meter is having very low bias current - so the usual nV meters would likely not work with the Fluke 752, at least not without some special care to compensate for bias effects.The good point about the 845 is not it's noise, the good point is the bias current that normally is quite low. So a zero with 100 Ohm reads nearly the same as a 100 K or 1 M resistor.I know no good modern replacement. In some cases one of those nV meters (more the models for higher impedance, not the lowest noise ones) can work. However the use with the 752 is one of the more difficult cases - chances are many nV meter would not work well. There seem to be to small a market to get a good replacement.