Resistance standard shielding - how important is it? - Page 1
Forum section: electronics / general

author: kj7e
author_rank: Frequent Contributor
timestamp: May 09, 2017, 04:09:03 pm
content: I've read the resistance standard teardown and searched for other related topics and I'm left with the question of the importance and effect a shielded housing has. I understand the effects of temperature, humidity, tempco, and aging but I'm not sure at what level stray noise plays with various values of measuring precision resistors or at what level it will effect the stability of the measurement.So, for a DIY resistance standard for a home hobbyist, how important is a shielded housing?I will be using VPG Z Foil resistors;1, Y14531R00000B9L10, Y000710R0000A9L100, Y1453100R000V9L1K, Y14531K00000T9L10K, Y145310K0000V9L100K, Y0062100K000T9L1M, USF340-1.00M- 0.01%-5PPM10M, USF340-10.0M- 0.01%-5PPM

author: CalMachine
author_rank: Frequent Contributor
timestamp: May 09, 2017, 04:31:16 pm
content: The higher the resistance, the more important shielding and guarding becomes. This is because the stray leakage currents / parasitic errors become a larger portion of the total current as the current level drops(with increasing resistance values). If you want precision measurements, all values should be housed in a shielded enclosure. Most DIY standards, I've seen, are house in die-cast aluminum boxes. That should be well sufficient shielding. I've seen a few more experienced members on here measure pretty high resistances (GOhm - TOhms) with a thin disposable cooking tray as the shielding around the resistor and contact points and it worked very well.If you are going to be housing resistors with accuracy/stability of Z-foil resistors, you will definitely want a shielding enclosure.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 09, 2017, 04:56:40 pm
content: As a rule of thumb, about 100k and higher resistors benefit from proper shielding, i.e. a shielded box, but also shielded cables, both shields attached, and connected to GUARD on the measuring instrument .10M and higher would even benefit from an additional driven guard, but that's not so common.Likewise important, from 10k onwards, are highly insulating connectors and PTFE cables.Pomona jacks are great, whereas mc jacks already create 1ppm error @ 10k, when a conducting box is used.Frank

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 09, 2017, 05:07:04 pm
content: Part of the answer depends on what you are using to do the measurements with, a DVM has different characteristics from a resistance bridge. To some degree, the values of resistance also come into play, on average high values tends to be much more susceptible to noise pickup than low values. The two main sources I contend with using my resistance bridge is static fields and small emf values and with high values, particularly >1Meg, some types of noise can cause (such as lighting) wavering nulls. With a resistance bridge most effects that interfere with a reading tend to be self evident and as it is a differential device, it tends to reject a lot of noise. That said, in making very precise measurements, you still have to be careful to pay attention to any possible sources of error, some can be very 'sneaky' and not so easy to see.While DVMs are sampling devices that 'average' the input over a period of time, that can help with certain types of noise (Gaussian) but not all noise sources can be averaged out. While higher resolution DVMs have 4-terminal resistance measurement which can minimize certain error sources, it is not free of errors by any means. It is also susceptible to outside error sources such as static fields, relatively high EMI fields and even EMF to some degree. A DVM can be even more sensitive to noise pickup with high value resistors than a resistance bridge, this may appear as 'dancing' digits or perhaps just an offset from the actual value.In any case, external noise fields should be eliminated or minimized for best measurement accuracy, it is easier to eliminate noise at the source than to try and filter it out, most DVMs will have a guard terminal as do high end resistance bridges, this is very helpful to use for shielding purposes. Leads should be kept as short as possible, long leads invite trouble, copper leads should be used (not plated) and for high accuracy, banana plugs should be avoided as they introduce unknown contact resistance and possible noise into the circuit. Keep all terminals at the same temperature to minimize EMF and keep them the same length. Kelvin clips can make very good connections to the unknown resistor if done correctly. There are a lot of sources of error in precision measurements, any number of them can bite you and you might not even know they are there, experience is a good teacher.Be aware that shielding increases capacitance and can affect settling time of the measurement, sometimes noise can even be injected into the measurement from the shielding, nothing is perfect, you can only minimize at best, with practice you can achieve very good accuracy and find your limits....don't make assumptions, that is the path to making errors, don't assume your measurements are as accurate as you think they are unless verified by a credible source, calibration is very important for accuracy.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 09, 2017, 05:20:19 pm
content: Okay, looks like I will revise my plans, I have a nice plastic enclosure laying around that's the perfect size. Once all the parts come in this week, I'll have to make some mock layouts and order a nice die cast Bud enclosure. The terminal resistance is not something I would have thought of off the bat, thanks Frank.Once built, I was going to offer it up to pass around like in the DMM Calibration club thread.

author: Gyro
author_rank: Super Contributor
timestamp: May 09, 2017, 05:46:23 pm
content: QuoteYou could always construct some internal shielding (guarding), maybe copper foil, tin plate, pcb etc. Then you would have the advantage of a guarded resistor with the guard insulated from ground. That would be a useful feature.

author: VintageNut
author_rank: Frequent Contributor
timestamp: May 09, 2017, 07:13:16 pm
content: I constructed three units earlier this year of what you describe; 100M, 10G and 100G.They were calibrated at a primary lab that I have access to once a year.My intended use for these is to calibrate a KE487 picoammeter. In a picoammeter, the current measure is on the LO side so any guard would be at virtual ground. Any leakage from HI to LO at the HI input would not be measured. Only leakage from HI to LO in the air around the resistor would be measured. So, I chose to not make these guarded.If your intended use is with a sourcemeter, then its a different story. A sourcemeter measures current on the HI side and any leakage betwen HI and LO will cause an error.So, you have to decide how the resistors will be used and make your choice for guarding appropriately.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 09, 2017, 08:52:06 pm
content: I'm going to model mine with a common Hisense with increasing values down the line, as in the photo below. I'm just going to size it and pick up a die cast Bud box.

author: The Soulman
author_rank: Super Contributor
timestamp: May 09, 2017, 09:07:17 pm
content: This was pointed out before elsewhere but there is a flaw in the design in the picture, move one of common binding postsfrom the top right corner to the left corner on the other end of the bar.Otherwise if you measure the last resistor ( or any other) you will also measure the length of bar with it in series, on a high value giving perhaps a sub ppm error,on low values such as 1 Ohm this additional resistance, lets say 0,001 Ohm gives a 0,1% error.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 09, 2017, 09:41:13 pm
content: Quote from: The Soulman on May 09, 2017, 09:07:17 pmThe photo above is the view from the underside. The Hi (sense) and Hi (current) are really on the upper left and the lowest value resistors are closest to the binding posts. The first reference is 1 Ohm, then 10 Ohm, then 100 Ohm and so on down the length of the common Hi bus. As you point out, the added series resistance is less and less important as the reference resistors increase in value and virtually insignificant on the higher kOhm and mOhm measurements.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 11, 2017, 12:54:19 am
content: Since I have this topic started, I may as well just continue here. More parts arrived, just need the enclosure which will be here tomorrow. Impressed with the accuracy of the initial values (I know its the tempco and aging that really matter).

author: ap
author_rank: Frequent Contributor
timestamp: May 11, 2017, 07:36:04 am
content: You did not mention what you are expecting in terms of aging drift of these. While the Caddock ceramic (hermetic) parts once settled after a year drift only slowly (see data sheet), the VPG parts, having epoxy cases, drift quite a bid. You can expect 10ppm and more a year (measured). So for real precision use, they are not a good fit.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 12, 2017, 01:55:58 am
content: Quote from: ap on May 11, 2017, 07:36:04 amNo real expectation, I just wanted a little project this week and this seem to fit the bill. Maybe Ill look into oil filled hermetically sealed resistors in the future:http://www.vishaypg.com/docs/63006/hmetlab.pdfAnyway, built it this afternoon;

author: babysitter
author_rank: Frequent Contributor
timestamp: May 12, 2017, 04:01:13 pm
content: ... and you cloned the glowing-purpleVISHAYmistake of putting both high connectors on the same side!You rather cut the connection between the paralleled HI connectors and wire the upper one to the open end of the HI wire.

author: Nuno_pt
author_rank: Frequent Contributor
timestamp: May 12, 2017, 04:29:09 pm
content: Look at the replay #5 from Dr. Frank also regarding that Vishay connection box, here <https://www.eevblog.com/forum/metrology/precision-resistor-box-project/>.There is a mistake in that connection.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 12, 2017, 04:40:48 pm
content: I'll read the notes from Frank in the link above and think about this some. Not sure I follow the reasoning yet, have to give it some thought to understand why the change is recommended.

author: iainwhite
author_rank: Unknown
timestamp: May 12, 2017, 04:50:51 pm
content: Do you mind sharing the type of binding posts you used? Are they Pomona model 6883 ?

author: babysitter
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:11:35 pm
content: Four wire connection is done simply to separate the "force" current from the voltage drop over the resistor.Every "shared" conducting material adds its own albeit small resistance to the resistance you actually want to measure. R+Rwire.To prevent this wire sharing, each of these pairs (above and below the resistor) shall meet just at the desired location.When you open the connection between the "high" posts and connect the higher one at the other end of the fat cooper wire at the holder, the cooper wire will not be shared and the force and sense currents go in different directions.(Also, put cooling to the resistor wires when soldering so you do not heat the resistor body, use forceps when bending wire to save the resistor body from stress. Also, there is a certain reference location at the resistor wires - too short or too long and you have a "wrong" wire resistance.)Vy 73Hendrik

author: kj7e
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:24:27 pm
content: I read it over and thought about it, makes sense now. Thanks guys!Fixed;

author: kj7e
author_rank: Frequent Contributor
timestamp: May 12, 2017, 06:36:26 pm
content: Quote from: babysitter on May 12, 2017, 06:11:35 pmYes, that makes sense. And yes, I am very attentive to sinking the heat away from the resistor and quickly cooling the heavy copper wire after each solder. I tested the resistors at various points on the leads with my kelvin probes, only on the 1did I see any difference and it was maybe one or two digits at the extremes of the leads. Its reads 1.0000-.9999 on my meter as its wired now.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 23, 2017, 10:30:00 pm
content: Results on my new 34465a;

author: kj7e
author_rank: Frequent Contributor
timestamp: August 05, 2017, 05:54:50 pm
content: Friend with a recently calibrated 3458a was kind enough to take some measurements as well, same test leads used;

author: babysitter
author_rank: Frequent Contributor
timestamp: August 05, 2017, 07:49:15 pm
content: Satisfying.