Author Topic: EEVblog #834 - Wekomm Resistance Standard  (Read 16745 times)

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Offline Vgkid

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #25 on: December 31, 2015, 09:45:24 am »
Thanks for the video, I enjoyed it.
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Offline VK5RC

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #26 on: December 31, 2015, 09:10:22 pm »
Interesting videos.
For these high accuracy voltage measurements, I understand that a solid copper conductor may be best, I note DrFranks advice re Teflon/PTFE insulation, some shielding also advised.
Some cables come with silver coating, I don't know what that would do at DC (i.e. not RF) and it may introduce some thermal EMF I wonder?
Most banana plugs are coated brass, good ones seem to be gold, ? true or not, is soldering better or screw clamped? I suspect there may be pros and cons for each way but I suspect this road has been trodden before!
Whoah! Watch where that landed we might need it later.
 

Offline plesa

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #27 on: December 31, 2015, 09:19:31 pm »
The difference between copper gold and silver plated are not big
Junction                    µV/°C
Copper-Copper           <0.3
Copper-Gold              0.5
Copper-Silver              0.5

What I learned from one member here is the fact that PFTE insulation is not compatible with bare copper, that why you can find on market only silver plated PTFE wires.
Surprisingly good is TiN patented solution :)
https://dev.xdevs.com/projects/kx/repository/entry/img/vref_a01/ltz1000_initial.jpg
« Last Edit: December 31, 2015, 09:26:40 pm by plesa »
 

Offline VK5RC

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #28 on: December 31, 2015, 09:33:06 pm »
@plesa, thanks for some real data. I should have looked myself, a quick search revealed this paper, interesting copper-copper oxide >1000uV/C ! A bit of silver plating might be useful, also this paper suggests a low thermal mass to allow more rapid thermal equilibration.
http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&ved=0ahUKEwjVg7Py8YXKAhUDJKYKHXlND3sQFggoMAI&url=http%3A%2F%2Fwww.minerva.pro%2Fdownloads%2Fproduct%2F172%2Fpapers_articles%2FWatch_Out_for_Those_Thermoelectric_Voltages_Cal_Lab_Journal_reprint_182_MB.pdf&usg=AFQjCNHCMy9LQOmXT8lo4T8OEg3SyEAJ0w&sig2=x7LtiRau4irXT9RrkBXGqg
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Offline HighVoltage

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #29 on: December 31, 2015, 11:02:07 pm »
I bought an 'identical' ReptiPro 6000 a couple of years ago.  I replaced its on-off-on temperature control with a PWM controller with a tiny thermistor placed in front of the internal recirculating fan intake.  It now modulates the temperature to within +-0.05 C.

The PWM temperature controller I bought does require me to swap the output leads to the Peltier thermometric module. but it still cost 2x what the incubator/test chamber cost.
  Controller = TE Technology Model TC-48-20.     http://tetech.com/product/tc-48-20/
Interesting setup you have there
Can you post us a picture?
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Offline plesa

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #30 on: January 01, 2016, 12:54:08 am »
I bought an 'identical' ReptiPro 6000 a couple of years ago.  I replaced its on-off-on temperature control with a PWM controller with a tiny thermistor placed in front of the internal recirculating fan intake.  It now modulates the temperature to within +-0.05 C.

The PWM temperature controller I bought does require me to swap the output leads to the Peltier thermometric module. but it still cost 2x what the incubator/test chamber cost.
  Controller = TE Technology Model TC-48-20.     http://tetech.com/product/tc-48-20/
Interesting setup you have there
Can you post us a picture?

Another way can be using LabView and bipolar power supply and some DMM as temperature controller.
Modification of attached .vi is piece of cake.
http://www.tek.com/sites/tek.com/files/media/document/resources/BuildingBenchtopPID_WPaper.pdf
 

Online dr.diesel

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #31 on: January 01, 2016, 12:57:30 am »
I can't seem to find a price on the fancy Wekomm standard, any rough guesses?

Offline plesa

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #32 on: January 01, 2016, 01:07:08 am »
I can't seem to find a price on the fancy Wekomm standard, any rough guesses?
3750 EUR
http://www.datatec.de/wekomm.html
 

Offline HighVoltage

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #33 on: January 01, 2016, 01:24:17 am »
Another way can be using LabView and bipolar power supply and some DMM as temperature controller.
Modification of attached .vi is piece of cake.
http://www.tek.com/sites/tek.com/files/media/document/resources/BuildingBenchtopPID_WPaper.pdf
That is a great paper.
Thanks.
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Online dr.diesel

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #34 on: January 01, 2016, 02:03:22 am »
3750 EUR

 :o

Guess I won't be buying one of those to play with.

Offline Fungus

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #35 on: January 01, 2016, 03:23:35 am »
 

Offline 3roomlab

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #36 on: January 01, 2016, 03:55:27 am »
if they plot the wekomm TC curve like in http://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/
it is suppose to be the "good" well behaved banana curve?
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Offline MisterDiodes

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #37 on: January 01, 2016, 05:57:03 am »
Dave,
Looking at your test setup, you might not be looking at all meter drift...Some other areas to look at when you're making measurements in the ppm area - just based on experience and some "gotchas" that we have discovered the hard way sometimes:

1.  As others have pointed out, you've got un-shielded cables from meter to ref resistor.  I would use a good quality shielded twisted wire paired assembly (as Dr Frank pointed out), and get a meter with a Guard or Shield  connection - and make use of it.  I use 4ea RG cables with guard amp driven from highest potential driver on your 4-wire setup, or just connect as basic grounded shields.  If the DMM doesn't have a Guard point, get one that does - especially if you're going to be doing a lot of PPM-resolution measures.

2.  You want another rag or iso thermal shield on those front meter connections.  It's easy to get drift of some ppm just on those cheap banana plugs out there flapping in the breeze. Clean Tellurium copper spade connections where you can tighten a nut down to a known torque work well when you're chasing down the last half ppm drift, otherwise gold test clips are good.  Good crimped / welded, gas-tight connections are much better than soldered joints at this point.  Any crud or tarnish on the connections is a bad thing, and a clean silvered joint is better than a dirty gold or copper joint any day.

3. Turn off all switching power supplies in the vicinity.  I've seen many a precision resistor - especially some non-bifilar wirewounds - easily pick up local switching noise.  That includes unshielded flouro / LED lamp drivers.  We use politically incorrect, electrically quiet incandescent lamps everywhere in the test lab here for illumination for that very reason.  Plain old sunlight thru an IR-filtered glass window works too, if available.  As long as it doesn't cause bad thermal effects.

4.  If this were a '3458a meter, you'd want to run Auto Cal if your lab temp changes more than 1°C, or at least once every 24hrs when you're trying to play in the PPM accuracy pool.

5.  You also want to flip your leads to the ref resistor periodically.  It is unknown what thermal and diode effects are inside that box, especially when flying leads are present..  We even flip the ref resistor around physically occasionally to check for gravity-force stress effects at different orientations.  You'd be surprised at what shows up sometimes - especially on cheap resistors.

6.  Keep an eye on lab humidity and baro pressure also. Sometimes we've seen a drift pop up that is only seen during a local heavy storm or exceptionally sunny / rainy weather.



« Last Edit: January 01, 2016, 09:34:04 am by MisterDiodes »
 

Offline joeyjr

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #38 on: January 23, 2016, 08:41:23 pm »
Hopefully this is not too late to post.

  When I use to calibrate decade boxes many moons ago in 1989, I had to place the standard resistor in a mineral oil bath. This will keep it at a more stable temp in the controlled environment of the lab from the changes of the air around it. Also, using shielded leads and getting rid of power line noise is another factor to consider. Finally, keeping your body and other noisy equipment away from the test will also help with errors. Anyway thats my two cents worth.  :-+
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Offline EEVblog

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #39 on: January 23, 2016, 09:21:37 pm »
Re the shielded leads and all the other stuff to get better precision, yes, as explained in the video this was not meant to be a proper controlled test setup, I was just mucking around with no intention of doing a video showcasing the standard, but it turned into one  ;D
 

Offline TomB19

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #40 on: January 30, 2016, 05:50:22 am »
This is my second post on this forum and I'm not an EE but I enjoyed the videos and have a couple of questions.

1) Couldn't a sealed container of water, perhaps a couple of litres, be placed in the thermal chamber to reduce the control authority of the peltier module and smooth the thermal transitions?

2) Would a meter of that caliber have an isolated and thermally controlled analog front end inside the instrument?
 

Offline zlymex

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #41 on: March 06, 2016, 12:07:23 pm »
Curiosity driven, I toredown the same Vishay resistor(VHA518-7) five years ago.
Because it is oil filled, it was very hard to de-solder both for outer ring and for inner Kovar tube, I had to tore the case apart.
As the name suggested, there are seven identical foil elements in the container and they are arranged in parallel probably owning to the fact that medium value around 1k has the best property.Therefore, I bet that that 10k resistor inside Wekomm standard consist of seven 1.42857k elements in series.
 
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Offline Macbeth

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #42 on: March 06, 2016, 12:56:00 pm »
zlymex, welcome to eevblog!  :-+

Every single post you have made so far is 10/10, top drawer!

You purport to be from China, are you active on bbs.38hot.net at all?
 

Offline VK5RC

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #43 on: March 06, 2016, 04:10:35 pm »
I thought they put them in parallel to 'RMS average' out the errors ????????
Whoah! Watch where that landed we might need it later.
 

Offline zlymex

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #44 on: March 06, 2016, 07:34:55 pm »
zlymex, welcome to eevblog!  :-+

Every single post you have made so far is 10/10, top drawer!

You purport to be from China, are you active on bbs.38hot.net at all?
Thanks very much Macbeth. Yes I'm from China, and I'm one of the oldest members of 38hot.net
 

Offline zlymex

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Re: EEVblog #834 - Wekomm Resistance Standard
« Reply #45 on: March 06, 2016, 07:45:56 pm »
I thought they put them in parallel to 'RMS average' out the errors ????????
Well, in a sense yes. When 7 elements connected together in series or in parallel or mixed, any single element has an error say 100ppm, the combined value will change by only 1/7, that is 14ppm, provided that all 7 element are equally weighted.  On the other hand, foil resistors are easy to adjust when manufacture, even a single element can be trimmed to within 0.005% of nominal value.

When more resistors are parallel or series connected, they have more thermal mass and heat dissipation ability thus rated at more power than single element, this is an important factor for standard resistor. Employ foil technology into standard resistors is a relative new thing, there are people(including myself) suspect the performance compare to traditional WW mainly because of its small effective conduction mass. When I DIY standard resistors, I often put a lot of elements together to increase the thermal mass and the conduction mass. And yes, when more elements put together, they make adjustment of deviation easier.
 


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