Author Topic: T.C. measurements on precision resistors  (Read 271420 times)

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Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #100 on: July 11, 2014, 04:27:27 pm »
Hello Andreas,

After looking at your data for the UPWx resistors, in my opinion the flaws of the resistor design are revealed in the measurements to some degree.  The characteristics are not terribly flawed, at least within your measurement range nor are you subjecting them to any great stress, still the flaws are evident.

I would like to point out a couple of things here, first, they have a rather quaint way of specifying TCR, a 'typical' of 0 ±3 PPM/°C over a 0°C - 85°C embedded within the overall TCR spec of 0 ± 5 PPM/°C, by doing this they lead the customer into thinking that they will get the tighter TCR spec over the limited temperature range, which they very well may get if the definition of typical is adhered to (at least a majority of resistors fall within the spec), however a resistor can actually have a TCR anywhere within the 0 ±5 PPM/°C range and still be 'within spec', a bit tricky.

My second point concerns the tolerance of a resistor, in this case, we're looking at a ± 0.1% limit, technically, a resistor can fall anywhere within this range and still be considered good (note this measurement is considered valid at the time of manufacture only), usually the manufacturer will make use of a 'guard' band, this means they will 'pull in' the actual tolerance limits a little bit to compensate for whatever measurement errors exist on the production floor.  This 'guard' band varies with the equipment in use in production and usually can be as large as ±0.01%, so the manufactured limits may be ±0.09% for instance.  The measuring equipment at final QC should always be the most accurate on the production floor to insure the resistors going out the door are within the specified tolerance (±0.1% in this in instance).

Depending on the manufacturer, the resistors going out the door should be within the required tolerance and can be anywhere within that tolerance, even within the same batch.  Due to inherent variations in the manufacturing processes, whether the resistors are made by an automated machine or by hand calibration.  The distribution of values tend to be a bit different between machine and hand calibration, machine tends to be distributed around whatever the nominal setpoint of the machine is and the distribution tends to be more of a clump (note machine calibration in the case of PWW is only possible with larger wire sizes as the machines tend to be a bit more heavy handed than a human hand) while a human calibration tends to be more distributed within the given tolerance band.

Tight tolerances ?0.01% are always by hand and depending on the operator will have more of a scattered distribution of values within the tolerance band, some operators can be very good at 'hitting' the nominal value quite consistently but these people tend to be fairly rare.

TCR tolerance works somewhat similar to the value distribution, depending on the manufacturing process, of course the TCR should always be within the stated range, but the distribution of TCR varies somewhat with the manufacturing procedures, most manufacturers produce a wider variance within the tolerance band than others do, that is inherent to the process and design of the resistor.

In conclusion, very few manufacturers give any kind of numbers on how many resistors fall within those 'typical' (undefined) specs if they even mention a 'typical' specification at all.  Frankly, I think this is a bit deceptive, if you are going to give a 'typical' spec, then you should clearly state (even approximately) what percentage of resistors tend fall within this range.  Now to my favorite punching bag, Vishay, while these folks often do give a 'typical' spec on their TCRs, there is absolutely no statement as to how many of these resistors actually fall within the 'typical' range given.  According to many engineers I've talked to (direct or indirect), they have found few if any of these resistors even come close to the 'typical' TCR range.  That is not to say that these resistors are not within the stated overall TCR range, they are and that is perfectly fine, they are within the stated TCR spec but few are within the 'typical' spec and I find that misleading.....how many of these resistors are actually within your 'typical' spec?  Vishay should know exactly since they have to perform the measurements on these parts, time to come clean.  Oh, wait, Vishay doesn't intentionally do that.  Just to make sure I'm clear on this point, these VHP, et al, resistors are within the general TCR spec, I'm not saying they aren't, they just don't tend to hit the claimed 'typical' mark very often and they don't say how many do, it appears that their 'typical' is just a calculated statistic.  If Vishay has the actual numbers, they aren't sharing them, why is the question.

I do not accept any kind of statistical manipulation of the data, exact numbers are available which should give a good figure of what 'typical' is by simple averaging of batch results, of course there is going to be some variance to that number, that sort of thing is inherent, nothing is absolute.  Typical does not guarantee that you will receive more than the 'typical' yield in your resistor order, you just might get more than the 'typical' number of resistors within the 'typical' band, you might receive less than the 'typical' too, by nature, typical is an average figure and a good manufacturer tends to produce more consistent results, that means more customers should receive 'typical' more often than not, it just doesn't guarantee that.  No silly statistical manipulation, just simple actual results from actual measurements.
 

Offline Andreas

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Re: T.C. measurements on precision resistors
« Reply #101 on: July 11, 2014, 09:03:12 pm »
Hello Edwin,

if we remain at statistics:

What I as "customer" would expect is that typical means:
At least measured at room temperature one sigma (63%) of the units are within the spec.
(so from 2 measured devices at least one should fit; Ok 2 is not a statistical quantity).

For the "max" specs I would expect that 3 sigma (99.7%) of the parts are within the spec.

With best regards

Andreas

 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #102 on: July 11, 2014, 11:01:47 pm »
Hi Andreas,

Granted, what you state is possible, but not always probable, statistics also states that any given random sample does not necessarily conform to the 'norm'.  Your definition may hold true for some samples while others don't fit at all.

Basically, what I am saying is that if you don't use statistics, just using hard measurements, the numerical playing around pretty much goes away.  When I said about 60% of my resistors are within 0 ±1 PPM/°C, the yield stays close to that number all of the time, no matter what batch you might pick, the yield is close to 60% (in sufficient numbers, see below).  Now if I use statistics to come up with that yield, I could possibly come up with a number higher than that, 75% or 80% depending on the batch(es) that were used for sampling.  The only constant truth about statistics is the old joke; there are lies, damn lies and statistics.

I'll be the first one to agree that statistics can be a very useful branch of math, the problem is that it is much too easy for the data to become skewed or just plain nonsense unless the person doing it is very careful.  I've seen many instances of statistics used to skew data to make it look better than it was, sometimes it was unintentional, other times it was intentional.  If you don't have to use statistics, so much the better.

Even without statistics, the math may not work, for instance, if I make ten resistors are six of those resistors going to be within 0 ±1 PPM/°C, odds are, probably not.  They all might be within ±1 PPM or only 1 or 2 or 3 might be, but that doesn't mean the 60% statement is in error.  If I make 100 parts, it likely that ~60% are within ±1 PPM probably or some number close to it or possibly some number not very close to it (although the likely hood is that it will be close to 60% simply because the vast majority of them are because those figures were actually measured, not sampled). 

These preceding figures are not from selected parts, otherwise the specification might be different, if I was selecting parts for a given TCR result, say the 0 ±1 PPM/°C TCR, the 'typical' spec, if one was going to be used, might be something like ~45% is within ±0.75 PPM/°C, ~35% is within ±0.5 and ~20% is within ±0.25PPM/°C.  Statistically, those figures might be true or maybe not, not really much use to the customer.

« Last Edit: July 12, 2014, 12:03:22 am by Edwin G. Pettis »
 

Offline ltz2000

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Re: T.C. measurements on precision resistors
« Reply #103 on: July 12, 2014, 10:52:01 am »
Almost 20 years ago I measured the temperature coefficients of 10 or maybe 20 bulk metal foil resistors. If I remember correctly the resistors were the old yellow/orange Sfernice. I still haven't been able to find the documents, but I keep on searching...

Found something...

Numbers 1-20 are Sfernice RS92 10kohm 0.1% orange case. No selection made, just ordered 20 pcs the cheapest version the local distributor had in stock. Number 21 is a free sample from Vishay but I don't remember the type. It had the same datasheet specs but the price was higher.

As you can see the temperature sweep turned out to be way too fast. In the second test run the setup was moved away from the disturbing oil bath on/off thermostat, the sweep time was increased from 3 to 24 hours and the resistor pack protected by thin plastic foil was placed in a cup of mineral oil with the thermometer. The hysteresis practically disappeared. Except the Vishay and one Sfernice (which had hysteresis larger than others in the first measurement) still showed some hysteresis. Despite the obvious problems of the first test run the very rough TC slope approximations turned out to be surprisingly correct.  Unfortunately I haven't been able to locate the data of the second test run yet. The temperature cycle in both cases was 25C -> 15C -> 35C -> 20C.

The graph gives an idea of the absolute accurary too. The reference was a Guildline standard resistor in an oil bath. The data was corrected so that 0ppm in the graph represents 10kohm plus the Guildline calibration uncertainty (which was probably less than 5ppm). The resistor chain was fed with a constant current source and the approximate 1V across each resistor was measured with a 3458A.
 

Offline Andreas

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Re: T.C. measurements on precision resistors
« Reply #104 on: July 12, 2014, 03:39:46 pm »
Hello LTZ2000,

thanks for sharing.

so the Sfernice RS92 have roughly from +0.0 ppm/K to +1.5 ppm/K if I see it right.
Whereas the Vishay has around -0.6 ppm/K.

By the way: does anybody know how to get off the nasty thermal
grease from the resistors without destroying/ageing them.
It does not solve in alcohol. And also "Kontakt LR" PCB cleaner does not help.

With best regards

Andreas
 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #105 on: July 12, 2014, 03:51:13 pm »
Hi Itz2000,

Interesting graphs, by the way, the RS92 was discontinued by Vishay/Sfernice last year, they said demand was low and production costs were high.  It was interesting to see that there were some drift graphs on the data sheet showing the approximate distribution percentages, that at least gives an idea of some important performance parameters.  The data sheet did not have much in the way of specifics on TCR though, page one gives a list of TCR over specified temperature ranges but does not specifically say that these values are typical or maximum, it is only inferred.  On page three, there is a graph which shows TCR with a typical, minimum and maximum set of lines, note they are not linear. 

The apparent typical TCRS;  + 0.6ppm/°C (0°C to + 25°C, - 0.6ppm/°C (+ 25°C to 60°C), + 2.2ppm/°C (- 55°C to + 20°C), - 1.8ppm/°C (+ 20°C to + 125°C), notice there are curious overlapping of the outer TCR ranges which completely overlaps the ±0.6 PPM/°C ranges.  This in effect says that any resistor that is within +2.2PPM/°C to -1.8 PPM/°C is still within spec, isn't that cute?  Still, despite being rather non-linear, the TCR specs are quite respectable.

The plastic wrap around your resistors in the oil bath is responsible for the long temperature tails, there are no problems directly putting the resistors into the oil bath with their connections, that is a standard procedure when using an oil bath, insulating the resistors causes problems.  If your wires are soldered to the resistor leads, that is an oil tight connection, no problem.  In TCR testing that required the use of an oil bath, we used spring loaded clips to make the connections to the resistors and the entire board was submerged into the oil.  This did require cleaning the spring clips between each use with an approved cleaning agent, a standard electronics grade cleaning spray.
« Last Edit: July 12, 2014, 03:56:43 pm by Edwin G. Pettis »
 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #106 on: July 12, 2014, 04:14:02 pm »
Hello Andreas,

I don't know if this is available in the EU but I'm sure you probably have an equivalent:

http://www.miller-stephenson.com/contact_re-nu/

I have used it in the past to clean up thermal grease, a small soft brush (old toothbrush) works nicely if the grease has become gelled.

I checked, Miller-Stephenson is available in the EU.
« Last Edit: July 12, 2014, 05:42:37 pm by Edwin G. Pettis »
 

Offline Andreas

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Re: T.C. measurements on precision resistors
« Reply #107 on: July 12, 2014, 08:59:41 pm »
Hello Edwin,

which one of the 3 solvents do you use from the link?
I guess its the MS-730

with best regards

Andreas
 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #108 on: July 12, 2014, 10:20:48 pm »
Hi Andreas,

Either the MS-570 or the MS-730  should work, I've used both of them.

Cheers,

Edwin
 

Offline MisterDiodes

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Re: T.C. measurements on precision resistors
« Reply #109 on: July 12, 2014, 10:53:57 pm »
Another product that works well is CRC Brake Cleaner.  The stuff for car brakes.  Not sure if there is something like that in Europe. It is not an official safe solvent for electronics, but if all else fails that's what we use in the lab.  In combination with a small brush it is good degreaser and flux remover, and we've never had it ruin a PC board.  You will want to test on a small spot first, and use it outside with common sense.  It might work for you, maybe not.  For us it has worked well in a pinch too loosen up and flush mineral oil and vacuum grease.  Using the official stuff is better but sometimes you have to get work done.

If the parts are wash-safe, usually we follow up with a good soap wash & brush as required,,  rinse in De-I water, run thru dry nitrogen air knife at several angles to get all the corners blown out  and then bake for an hour or two to drive off any remaining moisture.  The boards / resistors will be very close to 100% clean at that point.  You can check with a Gig-ohm meter to look for any remaining board residue, and usually its fine - otherwise it may need more attention.
« Last Edit: July 13, 2014, 12:12:08 am by MisterDiodes »
 

Offline MisterDiodes

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Re: T.C. measurements on precision resistors
« Reply #110 on: July 12, 2014, 11:35:36 pm »
Edwin,

Good to see you here!

I'm not sure if this was in another post, but I loved the piece in EDN - and I'm sure others would enjoy it also if they haven't seen it:

http://edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-one

http://edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two

I would like to point out again to all the guys building LTZ1000(a) refs and chasing after resistor TC:  At least in my experience and dealing with voltage refs used in a semiconductor process clean-room:  Its not the resistors that cause any measurable drift, at least not in the hundreds I've built in production.  I always used the basic App Note circuit for resistor values, and we used 13k/1k for the heater circuit without issue. We built some with 5ppm WW, 3ppm WW and some with Vishay Foil.  We never used any slots or any special construction, just cover up the circuit and keep the wind away.  We tried some with copper tape wrapped around the resistors and some we potted in Tfe + Foam.  It all worked, but the basic circuit construction guidelines and doing an initial burn-in helped the most.  The things that had absolutely zero effect on the final outcome were expensive resistors, slots, weird copper-fill patterns, putting the board in at crazy angles etc.

The beauty of that circuit is that the resistor change in PPM is attenuated at least 100 times.  The circuits we built with expensive Vishays were performing no better after 10 or 20 years than with good wirewounds - in fact on a cost / benefit ratio the wirewounds are MUCH more cost effective over time.   Even the units that we built with 5ppm WW and even 5ppm PTF56's were running fine after years of service.

I'm not saying the Vishays were bad - but at least in our experience they were never, ever worth the price for use in an LTZ1000a circuit.  I would say that from what I've seen over longer time frames is the Vishay datasheets have proven to be a bit on the optimistic side, especially when it comes to the more esoteric varieties.  Especially the oil-filled magical varieties.

The point being is that if the LTZ1000 ref is drifting, that means the strain in the crystal lattice of the Zener die is still changing and it hasn't settled out yet - and you need to burn it in a few more weeks / months and recheck.  They will settle down.   Some Vrefs take longer than others to settle down and get stable.  Be patient.

If you're looking at resistor stability on a unit that's only a few months old, you want to also look at stability at WW that are decades old.  I have a Dial-A-Vider 4107C that is is a work of art - still in calibration, and it hasn't been opened since I think late 1990's to clean up one switch contact.  Nothing but WW's inside, and good switches.  Very useful even today.

Also, on a production line, nobody cares if the Vref used in the quality control room last a year between calibrations - nobody is going to take a chance on that, nor does anyone need that on a production environment.  Typically the production line test gear is swapped out and goes to recal every 30 or 90 days anyway.  The good voltage standards stay in the temperature-controlled calib lab, and then the small boards with LTZ1000's go out onto the production line, where the temperature might vary a couple degrees at most.

I get it that the hobby Volt-Nut types want to chase down the drift.  Its kind of fun and addictive.  But really, outside of a cal - lab I have never seen a "huge" need for low drift over 365 days.  Usually on the factory floor the standard is to swap in fresh re-cal'ed units much more often. 




« Last Edit: July 12, 2014, 11:54:31 pm by MisterDiodes »
 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #111 on: July 13, 2014, 12:57:07 am »
Hi MisterDiodes,

Thank you, yes someone posted the History article links in the Ultra Precision Reference LTZ1000 a couple of months back.  Unfortunately, a few people misunderstood the point of the article; how difficult it can be to fix basic design flaws in a resistor design, particularly when they are being masked by other flaws (even admitting that there are flaws!).  The trials and tribulations involved were frustrating but in the end, they were fixed and a new design cleared up the long lasting flaws.  It wasn't about how bad precision wire wound resistors were compared to Vishay's imperfect resistors at all.  Vishay's resistors went through very similar design flaws and fixes over a period of 55 years, despite what some idiots may think, Felix Zandman did not pull a perfect resistor out of his butt 55 years ago.  Truth be told, Felix had a lot of help, a very lot of help, over the years tweaking the Vishay designs, he sure didn't do it all on his own by any length of the imagination.  Yes, Vishay does make some really good resistors, I'm not saying that (again) but Vishay has always buttered the specifications over the years and they haven't stopped yet.

By the way, EDN also published an article (that's a laugh) after mine from Vishay on their resistor 'history', if you'd like to read that I can post the link here as well.  Be forewarned though, the published article was somewhat modified from the original before it was published.  The original was put together by a Wall Street PR firm and was freely handed out among employees and at shows.  Frankly it was full of cock and bull hot air and very little fact, it was exceptionally thin on the history of how the Vishay resistors came to be and the long trek of re-engineering them (time and again) to the point that they are at today.  I know, I was around for most of it; I can post a copy of the original for comparison if anyone would like to see it.  It is good for a laugh or two and is also as effective as ipecac syrup.

Welcome aboard, MisterDiodes, the postings are often informative and sometimes controversial and sometimes just nonsense like other forums, enjoy.  I haven't been here all that long myself, having been pointed here by somebody that posts here.
 

Offline Galaxyrise

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Re: T.C. measurements on precision resistors
« Reply #112 on: July 13, 2014, 01:53:36 am »
The beauty of that circuit is that the resistor change in PPM is attenuated at least 100 times.
For the ~7V portion, yes.  But if you're looking for 10V, there's still a problem to solve that isn't answered in the datasheet.
I am but an egg
 

Offline MisterDiodes

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Re: T.C. measurements on precision resistors
« Reply #113 on: July 13, 2014, 02:37:13 am »
A precision boost from 7V to 10V?  There are several proven, precision, virtually "no-drift" ways around that one.  I can't show you, because the design I use is still owned by my customers.

Someday over a beer...




 

Offline MisterDiodes

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Re: T.C. measurements on precision resistors
« Reply #114 on: July 13, 2014, 03:04:57 am »

....
By the way, EDN also published an article (that's a laugh) after mine from Vishay on their resistor 'history', if you'd like to read that I can post the link here as well.  Be forewarned though, the published article was somewhat modified from the original before it was published.  The original was put together by a Wall Street PR firm and was freely handed out among employees and at shows.  Frankly it was full of cock and bull hot air and very little fact, it was exceptionally thin on the history of how the Vishay resistors came to be and the long trek of re-engineering them (time and again) to the point that they are at today.  I know, I was around for most of it; I can post a copy of the original for comparison if anyone would like to see it.  It is good for a laugh or two and is also as effective as ipecac syrup.
...

I am quite familiar with the follow-up "article" which was more a thinly disguised Vishay ad. I used it to light that evening's fire in the wood stove.  It has no place here, it was mostly smoke & oakum.

As far as I can tell, the more expensive the Vishay resistor is, and the more "magic" there is inside, the more goofy tricks are on the datasheet.  Again, I'm not saying they are bad - its just I've never seen a need for it that couldn't be solved with a better, more robust design that doesn't need such a high precision resistor.  The datasheets are (diplomatically) just plain optimistic.

What Vishay (/ or Bournes) does make that is fairly useful is a good 20ppm low-noise trimmer pot at say 10ohms - if used properly with a 3ppm Wirewound to trim out the last few ohms of a 1k /  3ppm WW for instance, sometimes you don't have to buy one of the custom laser-trimmed foil units they sell (Look at the stability of one of those laser trimmed units after 4~5 years - or after a small pulse surge - and tell me what happened).  You will still have a cost-effective, fairly low-noise 3ppm Tempco resistance even when an "old school" - but good - pot is attached.

Sometimes a pot isn't wanted or seems old-fashioned (or can be replaced with a DAC if you're careful), but on occasion they are just the ticket.  The good benchtop test voltage refs / dividers were made this way, and in general they work fine after decades - at least mine do.  Unlike a lot of "modern" equipment.  Was it a perfect design?  No, but it was very clever, and I always admired those designers.  Some of the old stuff just keeps on working long after the new stuff is tossed out.

A bad design using bad components will still be bad no matter what you do.



 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #115 on: July 13, 2014, 03:32:54 am »
MisterDiodes,

Oh you didn't see anything with that published article, you really should read what the original pap was!
 

Offline Andreas

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Re: T.C. measurements on precision resistors
« Reply #116 on: July 13, 2014, 08:38:57 pm »
Hello,

I updated first values for S102JT on page 1 with surprising results.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303

Seems to be similar to the RS92 values: low T.C. but high hysteresis.

With best regards

Andreas
 

Offline ltz2000

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Re: T.C. measurements on precision resistors
« Reply #117 on: July 14, 2014, 01:13:06 pm »
low T.C. but high hysteresis.

Even if the thermal resistance from the metal block to the resistor housing is low,  it still can be quite high from the housing to the chip. Seems possible that most of the hysteresis comes from the long thermal time constant and not the resistor itself. In the second test run the hysteresis practically disappeared when I increased the sweep time from 3 to 24 hours. You can estimate the time constant by introducing a temperature step and monitoring the resistance value until it stabilises.
 

Offline ltz2000

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Re: T.C. measurements on precision resistors
« Reply #118 on: July 14, 2014, 01:59:16 pm »
there are no problems directly putting the resistors into the oil bath with their connections, that is a standard procedure when using an oil bath, insulating the resistors causes problems.

I decided to use the kitchen plastic foil because I was worried that the epoxy housing could absorb oil. For example some integrated circuits are quite sensitive to humidity due to the mechanical expansion of the epoxy housing. It seemed logical that the metal foil originally designed for straing gauges would suffer the same problem.

I actually sacrified one of the resistors and tested my assumption. First measured it 24 hours in air just above the oil bath surface (very stable temperature) and after that another 24 hours in the oil. The first 24 hours were more or less a straight line, but when in oil I noticed a few stepwise changes.

The process took some time to start, so it is probably safe to dip the resistors in oil for a short period of time. Unfortunately I had other things to do and not enough time to find out if the resistor would eventually find balance in the oil. Probably, when the housing has absorbed all the oil it can.

« Last Edit: July 14, 2014, 02:02:18 pm by ltz2000 »
 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #119 on: July 14, 2014, 03:15:06 pm »
Hello Itz2000,

Yes, IC manufacturers found out a long time ago that water did indeed wick up the leads past the mold compound into the IC and caused problems.  The main problem was with the aluminum wires used to connect the outside pins to the IC, aluminum corroded fairly quickly and that was the end of the connections.  There was two solutions, keep it powered up and the heat kept the aluminum dry or use much more expensive methods to seal the insides from the outsides.  Over the years the molded types were improved to the point where wicking was considerably reduced (but not eliminated).

You are also correct in that early strain gauge / 'precision' film/foil resistors also had some sensitivity to water wicking.  Precision wire wounds (PWW) also had some sensitivity to water wicking, mainly because of the non-welded crimped joints between the wire and the lead terminations, this was not a severe problem however, it was more of a gremlin that caused short-term variations in the resistor's resistance (looked like TCR) when the part had been unpowered for some time and then powered up.  The effect was small enough that parts routinely passed the military humidity tests.

Generally, precision film/foil resistors are not particularly affected by water wicking as the interconnections between the leads and internal resistor are welded and the materials used are also not particularly sensitive to water molecules.  That being said, the lower cost types are not as well protected against wicking as the more expensive types.  If water does manage to get inside film/foil types, they are rather sensitive to corrosion as everything is very thin.

In PWW, their main weakness has always been the lack of welded terminations, the enameled wire is coated the entire length and unless there are tiny pin holes (they do occur) the water has no way of getting at the wire alloy, water in the crimped joint was and still is a weak point but it tends to be a long term exposure weakness as the terminations usually are not particularly sensitive to corrosion, somewhat similar to the better film/foil types.

Welded terminations eliminate, for all intents and purposes, the problem of water wicking, what tiny amounts of water may be absorbed has an insignificant effect on the resistance, I've never seen any measurements higher than sub-PPM in mine.

Oil, a very good grade of heavy mineral oil, is what is used in oil baths and hermetically sealed resistors (yup, those magical Vishay hermetics are full of oil).  It should not have any effect on the resistors, if it does, that resistor is bad.  One note here, in oil baths, the oil is changed periodically, it gradually becomes contaminated from the air, resistors and anything else it is in contact with.  In hermetically sealed resistors, as long as things are kept clean when making them, there is no long term problems with the oil.  So you should not see any changes in your resistors by putting them in the oil, the epoxies used for resistors should not absorb the oil.

By the way, oil does vaporize, just like water, over time, particularly when in a stirred oil bath and even more so if it is heated so the oil can get into everything around the bath.  I have a story about that I'll have to relate sometime, caused a lot of problems on a military contract at Ultronix.
 

Offline Andreas

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Re: T.C. measurements on precision resistors
« Reply #120 on: July 14, 2014, 07:17:14 pm »

Even if the thermal resistance from the metal block to the resistor housing is low,  it still can be quite high from the housing to the chip. Seems possible that most of the hysteresis comes from the long thermal time constant and not the resistor itself.

Hello,

if I look closer at the first picture then I see that it cannot be a thermal time constant.
For a thermal time constant the restistor change would lag all the time behind the temperature.
In the diagram the resistor change leads a long time (minute 160 - 300) and then reverses direction and then lags from minute 500 to 800.

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462303/#msg462303

https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/?action=dlattach;attach=101983;image

For me no thermal insulator can achieve such long thermal isolation (cycle time is ~11 hours).
So this effect would never be a thermal time constant. -> this effect has mechanical root cause.

The bad thing with such effects is that you never know from which point you are starting off. So the resistance value is unknown by a value of +/- 5-6 ppm even if you stay within the temperature range.

With best regards

Andreas
« Last Edit: July 14, 2014, 07:32:28 pm by Andreas »
 

Offline ltz2000

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Re: T.C. measurements on precision resistors
« Reply #121 on: July 15, 2014, 09:39:07 am »
You are correct.

Increasing the sweep time from 3 to 24 hours remarkably reduced the hysteresis of the RS92 resistors, but didn't much affect the Vishay. The speed of the temperature change seems to be important. Interesting...

 

Offline ltz2000

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Re: T.C. measurements on precision resistors
« Reply #122 on: July 15, 2014, 10:37:50 am »
I took a few pictures of the internals of an ESI SR1 10kohm standard resistor. This specific unit has measured TC less than 1 ppm/C in room temperature and predictable drift approximately +0.3 ppm/year. Evanohm wire on mica card, but it looks like the wire was not welded but soldered...
 

Offline Edwin G. Pettis

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Re: T.C. measurements on precision resistors
« Reply #123 on: July 15, 2014, 02:27:42 pm »
Hi Itz2000,

No , it isn't Evanohm, I reiterate, Evanohm cannot be soldered, it must be welded.  That looks like 0.001" diameter Manganin, 290 ohms/foot, so there is approximately 34.483 feet of wire on there.  An equivalent Evanohm size wire would only need approximately 12.5 feet.  Any of the ESI series standards with resistors like the one pictured here is made with Manganin.

By the way, Manganin requires years to achieve 'predictable' aging, even with the newer versions such as Zeranin, it does not 'completely stabilize for at least 2 or 3 years under careful use.  Evanohm does tend to stabilize quicker and remain very stable for many years, primary standards have been made with Evanohm for some years now and have replaced nearly all of the Manganin standards in primary labs.

Cheers,

Edwin
« Last Edit: July 15, 2014, 03:08:35 pm by Edwin G. Pettis »
 

Offline ltz2000

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Re: T.C. measurements on precision resistors
« Reply #124 on: July 15, 2014, 04:07:00 pm »
The ESI SR1 datasheet from the 90s says "modern alloy with excellent stability, an extremely low temperature coefficient over a wide range of temperatures, and very low thermal EMF to copper".

The same decription can still be found on the IET Labs website:
http://www.ietlabs.com/esi-sr1-calibration-resistor.html

Manganin ... modern alloy? Well, at least ESI had a good adman.

By the way, the exact same type resistors were used in the ESI 242D bridge (240C, RS825D).
 


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