• Intro
• Disclaimer
• User manuals and datasheets
• Exterior of the MI 4010M scanner
• Interior design of the scanner
  • Minor repairs and replacement for post caps
• Modification and automation
• Test results
  • Manual operation as original design
  • Automated operation with xDevs RSCAN
• Summary and conclusion

Introduction

For resistance metrology with the best precision various ratiometric measurement methods are utilized. Typical instruments to perform resistance measurements with best uncertainty better than 10^-8 are resistance bridges. Such bridges were developed back in 1960’s and evolved a lot since then. Even very old systems like ESI 242D are still in use today, sometimes even beating the more common and user-friendly 8½-digit DMMs like 3458A or 1281. Good calibrated IET/ESI 242D can do transfers with uncertainty down to 1 part in 10^-6 for middle range of the resistances.

These systems later evolved into self-automated instruments and bridges such as models developed by national metrology institutes around the world. NRC from Canada is one of the leaders in resistance metrology and their designs later evolved into instruments commercialized by Guildline Instruments and Measurements International. I’ve looked at older Guildline Model 6675 DCC bridge before, as well as fixed MI 6010B bridge after a mishap with overcurrent.

Measurements below 10 kΩ usually utilize DC Current comparator method with special magnetic transformer and sensitive voltage null detector to obtain very stable and low noise transfers between unknown RX resistance and known RS standard. Typical examples of this type instruments are Measurements International 6020A, older 6010 variants such as our own in-house 6010B. Our 6010B is a grandfather from 1997 of the today’s modern Model 6020C and good to do measurements from 0.1 Ω to 10 kΩ.

Another method, typically for above 10 kΩ ranges is the use of ratiometric voltage ratio measurement with voltage source, very stable Cutkosky/Binary Voltage Divider and sensitive detector. These bridges can perform accurate transfers for resistances over 100 GΩ and at higher voltages. Typical example is MI 6000C or older versions like 6000B or 6000A. As showcased in my IET/Tegam/ESI SR104 10000 Ω measurements before such bridges can provide excellent performance with relative uncertainties to 1 part in 10^-8.

All these bridges are designed to measure ratio between standards and operate in a specific parameters envelope with programmed test current and range. But for practical metrology applications there is a need to calibrate whole range of different nominal resistances. For example when testing performance of multi-function calibrator or verifying accuracy of 8½-digit DMM we would often need to obtain good measurements for all nominal decade resistances from 1 Ω to 100 MΩ. Performance of different bridges is also quite different and great resistance laboratory may have whole set of systems to have good confidence in results.

And most laboratories would have only few “golden” resistance standards that they would maintain carefully calibrated, such as 10000 Ω or 1 Ω. Since resistance bridges provide best performance with a limited ratio range such as 1:14 for DCC lot of re configurations are needed. This renders a need to have whole lab with additional transfer resistance standards as a temporary units for the buildup chain as lot of connection changes.

Human interaction with these re-connections and terminals at standards is a source of additional errors and possibility of mishaps. Sometimes changes in connections also are not practical when resistance standards are submerged in large oil baths. To reduce these problems automation with some scanner/switcher system is highly desirable. Such scanner would allow setting up somewhat permanent connection to each resistance element and routing detector bridge to connect to desired channels in a sequence. Additional functions such as sides reversal or low leakage four-wire connectivity are important to provide additional methods for measurement uncertainty analysis and improved transfer performance results.

I could also see such system in action during my visit to Taiwan’s National Measurements Laboratory in 2019 shortly after SI redefinition. Be sure to click on the photo below to read more details about that visit. With such scanner/detector setup everything can now be programmed and automated and full set of ranges can be calibrated without the operator intervention.

Automated system would be reliable and repeatable if such scanner does not introduce additional large errors that would affect the operation of the bridge. And indeed such scanner products are widely used at many laboratories and sold by key manufacturers mentioned earlier. Typical example are current modern MI 4210, 4216, 4220. There are multiple variants of these scanners, denoted by the letter in model and number of channels:

• Model 4210A – 10-channel automated scanner system with low-thermal copper binding posts.
• Model 4210B – 10-channel automated scanner system with direct PTFE-insulated shielded cable leads.
• Model 4210C – 10-channel automated scanner system with LEMO 4-pin connector ports.
• Model 4216A – 16-channel automated scanner system with direct PTFE-insulated shielded cable leads.
• Model 4216B – 16-channel automated scanner system with low-thermal copper binding posts.
• Model 4216C – 16-channel automated scanner system with LEMO 4-pin connector ports.
• Model 4220A – 20-channel automated scanner system with low-thermal copper binding posts.
• Model 4220B – 20-channel automated scanner system with direct PTFE-insulated shielded cable leads.
• Model 4220C – 20-channel automated scanner system with LEMO 4-pin connector ports.

These scanners designed to switch resistance channels with two coil latching relays and tested for parasitic thermal EMF under 50 nV and overall error contribution below 20 nV per channel, while being rated at 1000 V potential. Insulation resistance for channels is over 1 TΩ and contact resistance below 50 mΩ. Because of these parameters and rather difficult specifications such switch boxes can be quite costly and large. To explore some of the these switching challenges today in this article we’ll look at much older historical variant of resistance 4-wire scanner from Measurements International, model 4010M. It was built 30+ years ago and acquired from secondary market for this article study.

Disclaimer

Redistribution and use of this article, any part of it or any images or files referenced in it, in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Re-distributions of article must retain the above copyright notice, this list of conditions, link to this page (https://xdevs.com/article/mi4010m/) and the following disclaimer.
• Re-distributions of files in binary form must reproduce the above copyright notice, this list of conditions, link to this page (https://xdevs.com/article/mi4010m/), and the following disclaimer in the documentation and/or other materials provided with the distribution, for example Readme file.

All information posted here is hosted just for education purposes and provided AS IS. In no event shall the author, xDevs.com site, Measurements International International or any other 3rd party be liable for any special, direct, indirect, or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortuous action, arising out of or in connection with the use or performance of information published here.

If you willing to contribute or add your experience regarding instrument repairs or provide extra information, you can do so following these simple instructions.

User manuals and datasheets

Sadly there is no much mention of this old scanner online and search engines unable to show any examples where one was used. Most of metrology was very analog back in 1990s and computers were only starting to take firm hold in this field. I was not able to source user manual for 4010M but got the datasheet for same generation 20-channel version, called 4020M. I’d expect it’s the same design and construction, but just with twice less channels to switch.

Exterior of the MI 4010M scanner

Unlike modern automated scanners filled with relays this MI 4010M S/N 890102 is a manual scanner with rotary switch. Interesting to note that color of the enclosure is plain grey, opposite to cheerful ocean blue that all the typical Measurements International equipment has. There are no power, no interface and no automation parts in this box.

This model is 10-channel version with low thermal tellurium-copper binding posts at the back for channel connections and RX/RS ports for bridge interface at the front. Modern functional replacement for this scanner would be MI 4210A.

On the front face we got two large switch dials at the each arm of the scanner for manual selection of the desired channel. Switch in the middle is provided to alter the RX/RS pathway to switched arms either in normal or reverse connection. This is very useful to test pathway errors with resistance bridge and to have ability of flipping resistance sides without the need to change channel connections to actual resistors.

On the back we can see all the channels from both switches 10 per each bank side. Each channel has four terminals for separate current and potential routes to utilize kelvin-connection between bridge and each connected resistor. Shield/guard terminal is shared between banks. Many of the posts were damaged over the years and lost their plastic housing, leaving bare brass metal nut on the post instead. Some of the posts missing the nut altogether.

Overall scanner is not deep and definitely designed for primary use in the equipment rack, close to the bridge. I’d probably prefer location of the primary port RX/RS connections to bridge at the back, just to isolate wires from possible human interaction when reaching to the knobs to switch channel. Modern MI switches already implemented that improvement, but it’s interesting difference here.

On each of the sides there are four screws of different type. Perhaps they were replaced at the some point?

Decades of time didn’t pass unnoticed for this instrument and I can clearly see some nasty looking green goo and salts around few of the connectors at the back. Perhaps the best way to rejuvenate this scanner would be to remove all the binding posts, clean them and insulators extensively and then reassembly everything back together.

Maintaining low leakage is very important for higher resistance measurements especially given the fact that MI 6010B is able to resolve 9 digits of resolution during the ratio transfers. Example leakage error effect of 0.1 µΩ/Ω that would be buried in the noise on something like HP3458A would be clearly visible with such bridge system. And I think such scanner would be quite interesting to test in it’s original state as it was designed to do prior to any wild experiments I might do on it later.

Interior design of the scanner

Dull solder joints hint for cadmium solder joints as it is hard to imagine RoHS back in the years when this scanner was developed and built. Cadmium solder, just like any other soldering/flux fume is toxic and additional care is required when working with it. It is a good idea to use dedicated tool and soldering tip when working with such alloys so the daily projects wouldn’t be contaminated with Cd.

Minor repairs and replacement for post caps

To restore functionality to some more channels and posts I’ve 3D-printed the new knurled caps. Metal brass cores are still present on some of the terminals and it’s really easy just to CAD and print the matching cap. Cap cutout is slightly smaller than outer diameter of the brass core so it could be press-fit with force.

This worked out pretty good for a first try. From far away it looks almost like a real deal knobs too :).

Modification and automation

Back in 2023 I’ve started yet another project with idea to add more automation for resistance bench. Project involved in-house 16-channel scanner with modern latching relays, drivers and Raspberry Pi as a controller. I wanted to use 4 pole DS4E-ML2 relays to experiment with resistance scans and automate TCR measurements of large number of resistors. Plus integrating some of the clever switching and internal foil standard I could couple such device with a bridge to even provide two-box absolute resistance measurement system with performance better than any 8½-digit DMM capabilities. Some parts and circuits were designed and tested but then other projects took more priority and everything was shelved. This 4010M box gives second-life to that concept build and we can continue with a slightly revised goal here:

• Reuse 4010M chassis for the new automated 4-wire resistance scanner unit
• Reuse rear binding posts for the interconnect to the devices under test
• Implement all 20 channels on two banks of programmable latching relays
• Provide galvanic isolation on interface link for the host controller
• Add shield/covers for both front and rear ports to prevent airflow around connectors
• Design custom front panel with selected channel indicator and interface link
• Integrate temperature and humidity sensors for monitoring purposes
• Provide option for active external guarding
• Hopefully achieve thermal EMF performance better than 50 nV for all channels

To make it easily repeatable all this functionality is implemented on the single PCBA, called xDevs RSCAN. Front panel will also need to be changed, which should be pretty easy with a flat sheet reusing existing 425 × 129 × 2.5 mm dimensions.

I’ve opted to implement all the switching hardware like relays and pathways on a single four-layer FR4 board with areas for thermal isolation and shielding. Below is a prototype sketch with footprints for now obsolete DS4E type 4PDT latching relays, but this concept can be easily modified for different relays. Currently I’m validating various candidate relays for this application with my old but good MI 6010B bridge.

PCB size is 400 × 180 mm and to be mounted in the chassis on soft electrically isolated mount.

Test results

Manual operation as original design

Automated operation with xDevs RSCAN

Summary and conclusion

This was an interesting deep dive into resistance scanners and metrology switches in general. In the past we looked at the scanners for voltage, in case of Dataproof 160A unit that is utilized daily for automated zener array voltage measurements with nanovoltmeter. This time resistance scanning capability is the most welcome to the lab to increase the amount of interesting tests I could do while saving lot of time and finger wear doing them.

Feel free to contact us for questions and comments. Discussion about this article and related stuff is also welcome at our own IRC-chat server: xdevs.com (port 4808, channel: #xDevs.com) if you like more real-time interaction with electronics engineering enthusiasts like the author of this article.

Projects like this are born from passion and a desire to share how things work. Education is the foundation of a healthy society - especially important in today's volatile world. xDevs began as a personal project notepad in Kherson, Ukraine back in 2008 and has grown with support of passionate readers just like you. There are no (and never will be) any ads, sponsors or shareholders behind xDevs.com, just a commitment to inspire and help learning. If you are in a position to help others like us, please consider supporting xDevs.com’s home-country Ukraine in its defense of freedom to speak, freedom to live in peace and freedom to choose their way. You can use official site to support Ukraine – United24 or Help99. Every cent counts.