Does anyone else a see an ad below the embedded youtube video above?
I've never seen that before. I don't know if that's youtube or the forum plugin?
Here is what I mean
Does anyone else a see an ad below the embedded youtube video above?
I've never seen that before. I don't know if that's youtube or the forum plugin?
Really surprising to see that new "branded" benchtop DMM just can't do auto ranging decently, isn't that strange ?
Here is what I mean
Those 1 ohm and 1.9 ohm resistors look like they are wrapped on mica sheets
I'm betting those cost a very pretty penny each.
Wiki snippet about Mica Sheets...
These sheets are chemically inert, dielectric, elastic, flexible, hydrophilic, insulating, lightweight, platy, reflective, refractive, resilient, and range in opacity from transparent to opaque. Mica is stable when exposed to electricity, light, moisture, and extreme temperatures. It has superior electrical properties as an insulator and as a dielectric, and can support an electrostatic field while dissipating minimal energy in the form of heat; it can be split very thin (0.025 to 0.125 millimeters or thinner) while maintaining its electrical properties, has a high dielectric breakdown, is thermally stable to 500 °C, and is resistant to corona discharge.
Sheet mica is used in electrical components, electronics, isinglass, and atomic force microscopy.
Mica is one of the few materials that can provide a physical barrier, yet allow alpha radiation through. It's pretty much the exclusive material used in high sensitivity geiger probes.
Why aren't the resistors mounted inside a thermal oven to reduce the effect of ambient temperature and sealed in a hermetic chamber with desiccant to prevent humidity from changing the values?
Really surprising to see that new "branded" benchtop DMM just can't do auto ranging decently, isn't that strange ?No. What's the problem? Any decent autoranging system will be designed with some hysteresis to prevent the meter from nervously switching between two ranges. In general a stable reading with one digit less is preferable to the meter switching range every few seconds due to noise. If you want to force the meter to choose one particular range, use manual ranging. Calibration procedures will always call for manual ranging.
So we just can not use this expensive benchtop DMM say like in logging mode at a DUT that it's resistance may wildly jumps across ranges ?
Along the same lines, the relays on the top and bottom of that schematic aren't the rows and columns of a matrix (without a link to that earlier video you mentioned, I can't verify what you meant by "matrix"
The boards most likely were made, tested and then stored in a CA store until needed, and then they were pulled out of storage, cleaned and assembled.
The Mica used to make those low value resistors was likely mined in South Africa, funnily enough in a mine situated in the Lowveldt right next to a small village called appropriately enough Mica. Nice town, with some beautiful scenery and some lovely road cuttings through the actual mica bands.
I'm thinking the shielded box around the low ohm wirewound resistors is not so much to prevent electrical noise but to decrease thermal differentials caused by drafts within the enclosure. The noise suseptability of this low impedance part of the circuit would be minimal. Temperature gradients however, would be an important consideration.
Here is what I mean
Here is what I mean
This gets into experimental physics and metrology. ...they will make a (supposedly) very stable standard resistor and characterize its value and stability using the quantum Hall effect. They will then use this resistor to calibrate other resistance standards.
Dave said something that interested me. There were four 40 ohm resistors in parallel to make 10 ohms. Each resistor was at 0.05% tolerance and he said having four of them makes that better than 0.05%. Why would that be? Obviously later in the video he discovers they are all specially matched which could make this true but he said this before knowing that detail.
So each 40 ohm resistor could have a resistance between 39.98 and 40.02 ohms and still be within tolerance. If each resistor just happened to be 39.98, you would have a total resistance of 9.995. That is still 10 ohms at 0.05%.
And the fun thing is that current is currently defined in terms of Ohm's law and the definition of resistance (in terms of the QHE) and voltage (defined based on the Josephson effect).
The resistors will have something like a "bell curve" normal distribution. If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim. The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them. But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.
The SI ampere is still defined via the force between two conductors. But that is impractical to realize, so standard labs derive it via Ohm's law. There is some work going on to redefine the SI ampere based on the charge of a proton. But the BIPM members are discussing that for years now, and they don't seem to be in a hurry to do the change.
The resistors will have something like a "bell curve" normal distribution. If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim. The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them. But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.You can't assume a normal distribution (although the Fluke engineers might have had additional information), especially not one centered around the nominal value, so worst case is that you should assume a uniform distribution within the stated tolerance. Still, the standard deviation (and any confidence interval you care to calculate) of the parallel (or series) combination will be smaller than the single resistor. I believe the freely available book 'Analog SEEKrets' by Leslie Green contains a decent discussion of this topic.
Dave said something that interested me. There were four 40 ohm resistors in parallel to make 10 ohms. Each resistor was at 0.05% tolerance and he said having four of them makes that better than 0.05%. Why would that be? Obviously later in the video he discovers they are all specially matched which could make this true but he said this before knowing that detail.
So each 40 ohm resistor could have a resistance between 39.98 and 40.02 ohms and still be within tolerance. If each resistor just happened to be 39.98, you would have a total resistance of 9.995. That is still 10 ohms at 0.05%.
The resistors will have something like a "bell curve" normal distribution. If they've got a standard normal distribution, the chance of getting all four of them at the very bottom or very top of the range is very slim. The very worst case would be that the percentage error in the combined resistance would be as bad as the percentage error on one of them. But the average case is much better than that, even if you're not specially selecting the resistors in matched sets.
If you're randomly selecting the resistors, you can only put a statistical confidence level on the likelihood of the worst case not happening. But if you individually select the resistors, combining high ones with low ones and medium ones with each other, you can guarantee that the worst possible case doesn't happen.
When fluke describes the resistors as matched sets, they're doing something far better than relying on reducing standard deviations. As mentioned in the video, they'll be binned, that is, very accurately measured and grouped ("binned") according to measured value. Take a resistor from the 50.004 ohm bin and pair it with a resistor from the 49.996 bin, and hey presto, you're doing much, much better than the root-4 improvement you can expect to get from random chance.
I'm asking for help understanding the math on that aspect because I can see how if you select a random set of four, on average the resistance should be on a normal distribution but each resistor is probably on a normal distribution too. Only a few would be on the extreme value but that's why there's a normal curve instead of them all being an exact value. However, can it be stated mathematically that if you combine four items on a normal curve, the final product will have a non-normal curve but will be more narrow? How does that math work?
I believe the freely available book 'Analog SEEKrets' by Leslie Green contains a decent discussion of this topic.
See chapter 3. Google should give you a link to the (legal) PDF version within a few seconds, hosted on this very site. It explains when and when not to use statistical tolerances, and gives some rules of thumb.
But I'll explain the basic case with the assumption that the resistors follow a normal distribution, which is not necessarily a fair assumption (see the book for reasons why and how to handle that). To keep it simple I'll first explain the series case. Assume the resistance of the 40 Ohm resistors is normally distributed with a mean of 40 Ohm and a standard deviation (or any multiple of the standard deviation, like a 95% confidence interval) of 0.05% * 40 Ohm, or 0.02 Ohm. The probability distribution of the total resistance of the four resistors in series is the sum of four normal distribution, which is another normal distribution with mean = sum of the means and variance (which is the standard deviation squared) = sum of the variances. If the total variance is the sum of the variances, then the total standard deviation is the root of the sum of squared standard deviations. So in this case the mean total resistance would be 160 Ohm with a standard deviation of sqrt(4*0.02^2) = 0.04 Ohm. This is 0.025% of the mean, i.e. half that the tolerance of the original resistors. A quicker way to calculate this for identical standard deviations is sd / sqrt(n), where sd represents the standard deviation of the individual resistor and n the number of resistors.
For the parallel case you can show that for normal distributions which are well above zero, the distribution of the reciprocal value is another normal distribution with mean 1/40 Ohm and standard deviation 0.05% * 1/40 Ohm. You can then add conductances to get the conductance of the four resistors in parallel, and again get a factor sqrt(4) improvement in standard deviation.
Note that this is about what the majority (eg. 99%) of the circuits is doing, as you note the worst case does not improve.
If you were to design a resistance reference box like this today, wouldn't it be better to use solid state relays instead? You wouldn't have to worry about contact resistance issues and the leakage is very low like 0.006 pA at 80 degrees C. And only 0.003 pA at 40 degrees C. And if you're concerned about that small leakage, putting a couple in series couldn't hurt because they are only around $1 each.
Without doing much research about the best ones to use, I picked a random one and looked at the specs and they are pretty good compared to a conventional relay. http://www.clare.com/home/pdfs.nsf/www/CPC1016N.pdf
With conventional relays, the contact resistance can be low in the beginning but increase in time as the contacts become worn and seems that can mess with the calibration as you use it. With the solid state relays, I would think the resistance would be consistent throughout the life of the product. The internal resistance of the IC could be compensated with different value resistors if that is important.
Without doing much research about the best ones to use, I picked a random one and looked at the specs and they are pretty good compared to a conventional relay. http://www.clare.com/home/pdfs.nsf/www/CPC1016N.pdf
Where are these 0.006pA leakage current solid state relays? The one you linked has 1uA leakage, which is certainly unacceptable. I'd be pleasantly surprised if you could find a reasonably priced solid state relay that had a reasonably low on-state resistance (all the ones on digikey are basically immediately too high for 2-wire measurement; as I mentioned in an earlier post, though, on-state resistance is irrelevant in four-wire mode unless the relays are burning out or loading down the current source) and low leakage current (beware the distinction between control-to-relay leakage, and off-state leakage).
One point that comes to mind, at least for two-wire measurement, is that the on-state resistance would be a function of relay drive voltage. If you want the on-state resistance to be very repeatable (a far greater concern than absolute value), this is a very bad trait. At least a relay is very much either on or off.
It has an on-resistance of max. 16 ohm. A mechanical replay can get easily 16 milliohm. That's 3 orders of magnitude better. And the resistance of the solid state relay changes with temperature. That's unacceptable for a calibrator.
It says it in the graph titled "Typical Leakage vs. Temperature Measured Across Pins 3&4" The pA was too small on the pdf so I had to copy and paste it into a text editor to confirm it was pA and not uA or mA.
It says it in the graph titled "Typical Leakage vs. Temperature Measured Across Pins 3&4" The pA was too small on the pdf so I had to copy and paste it into a text editor to confirm it was pA and not uA or mA.
On that graph I read :
- 0.006µA @ 80°C
- 0.002µA @ 20°C
@cengland0: I see uA too
graph attached.
The SI ampere is still defined via the force between two conductors. But that is impractical to realize, so standard labs derive it via Ohm's law. There is some work going on to redefine the SI ampere based on the charge of a proton. But the BIPM members are discussing that for years now, and they don't seem to be in a hurry to do the change.
Naa, that's not correct. The Proton experiment was too unprecise so it's not followed up.
... then defining fixed (exact) values of e, ...
But that is the charge of a proton, isn't it? I thought the idea was fixing e (one value in Coulomb = As), and then "simply" deriving the Ampere from the charge.
Great video!! @ 27min I'm guessing the low resistor are wound on mica card.
how much this little jewels cost?
Yes, the group is still going. It seems they're still considering whether they should buy another spool of the nichrome wire... That's assuming they can get a new crop of nude virgins, that is. The hermetically-sealed laser-trimmed resistors are still made in house at Fluke in Everett (and also Scotland, I think).
Yes, the group is still going. It seems they're still considering whether they should buy another spool of the nichrome wire... That's assuming they can get a new crop of nude virgins, that is. The hermetically-sealed laser-trimmed resistors are still made in house at Fluke in Everett (and also Scotland, I think).
I'm very surprised the group hasn't been given the arse by the "The Danaher System", as equivalent resistors could be outsourced to Vishay.
very nice info dr Frank, did you need to physically apply IPA to clean anything inside?
Interesting!
That 90M part looks like it's a standard Caddock device; probably TF series.
Does any know where to get the 4.5 MOhm resistors or a replacement?
Has anybody of you, lymex and quarks, measured the 5450A high Ohm resistors with a Fluke 8508A or similar, using its high precision high Ohm mode, which sources up to 200V as compliance voltage?
This 8508A is really superior over the 3458A, concerning such resistor measurements...
Hello Frank,
no I did not use "HiVOhm" of the 8508A because max. allowed voltage is 50V (at least that is stated in the 5450 manual).
But I am glad you asked, because of that I meassured the Fluke 5450A with my Fluke 8508A and the Fluke 5440A-7002 cables again and found all values to be in spec.
My guess is, although I used the very same cable, most likely my setup (1 or 2 years ago) was just not clean enough. Because today I cleaned my 5450 inside and also the cables.
bye
quarks
2W Ohm, guard connected, open at the 3458A, NPLC 100, and just measure the 10M, 19M and 100M range, and compare these to the values you measured with your 8508A.
I guess your front/rear solution my suitable, but has disadvanmtages (I personally prever to stay as close as possible to original function to prevent mishandling).
How is the idea of following solution/fixing:
- use low leakage COTO relays mounted in a fly over the board solution
There should be enough room/volume in the 5450A
BR
PeLuLe